The AM Forum

Decided to do something less strenuous this afternoon (after yesterday chucking 5500 lb of firewood down my cellar hatch, and splitting about 1/4 of it with a hydraulic log splitter, and replacing the washing machine water valve this morning), so made the typical ham modification to my 40m Command Set receiver. This was a very common mod back in the day, but some were not done so neatly if I do say so myself  8)
Controls are the usual gain, BFO switch and phone jack and I added a 2nd IF regen pot.

There were a couple of big AM signals up near 7290 a few minutes ago... don't remember the callsigns but I bet they're AMFoners  :D

The "ARC-5" receivers seem to work very well for listening to those who strap on AM. Lower down the band the very wide IF makes it tough to listen to just one QSO at a time especially with the KW's blasting in from 5-land. If I could find an affordable source of 2830 KHz crystals I might add a crystal filter since it has good sensitivity and excellent stability...

Anyway it looks neat sitting there  ::)

-Charles

Looks good!   So far I"ve managed to avoid accumulating any ARC-5 gear....yet.  :)

In the last several months there have been good discussions about homebrewing Q-multipliers and adding them to radios.  One, maybe two tubes and at most a dozen components. 

It's pretty simple.  Basically you add a regenerative function to the the IF stage.

I'll see if I can find a reference.

bill

"Using the Q-Multiplier with Military Receivers" Donald Stoner, CQ Magazine, Feb. 1957.


http://www.antiqueradios.com/forums/viewtopic.php?t=96230
One discussion including comments from our 'very own W3JN"

Here is a compilation of info on homebrewing a Q multiplier. I don't know the source to give credit.   b.

I have an 80 meter version that was given to me several years ago. Actually sounds quite good but it is very broad. One Class E station transmitting and you think the tuning dial stopped working.

ARC-5 receivers. Fun!

In educating myself on the subject of add-on selectivity possibilities, a major challenge is modifying existing circuits to work with non-455khz IF's.

It appears the ARC-5 series radios used a number of different IF"s, and not being sure which your radio uses, it is impossible to provide an instant solution.

The other alternative is to use an Audio filter such as the National Select-O-Ject device introduced in the early 1950's.  It was offered both as an external box, and the circuit was incorporated in several National receivers.   

The manual and schematic are available at the BAMA Mirror archive.   bill

http://bama.edebris.com/manuals/national/s-o-ject/

Sorry, I didn't explicitly state that this 7-9.1 MHz rx has the 2830 KHz IF... although those IF transformers have pretty decent Q, the BW is inevitably much greater than 3 KHz at that frequency.

The 3-6 MHz unit has a 1415 KHz IF so it can be tuned to be not quite so broad but given the crowding on 75m, ultimately no more useful. (I do have the 3-6 transmitter and a homebrew screen modulator. About 10W of PW carrier)  ;)

I did install a plate-grid gimmick and a cathode pot (the 2nd IF regen mod that I alluded to). It helps some, but again, not enough in today's crowded bands.

An audio filter won't help (except for CW). Once the undesired signals have been detected, they are now in the audio passband along with the one you're trying to listen to. It really needs a narrower IF, and I am leaning toward a minimally invasive mod (like a crystal filter or perhaps a Q-multiplier if I can get it sharp enough) rather than redesign the entire innards.

Otherwise it ends up like my grandfather's axe... it has a new head, and a new handle, but it's my grandfather's axe  ;D

Edit: searching for surplus crystals near 2.830 MHz... so far, only finding lots of cheap 2.4576 and 3.000 MHz xtals but not sure the IF cans (or the LO) can be tuned that much. $urplu$ $ale$ has 2.850 in the HC-6/U can for $4 each. That'll work but $20 for crystals. Yeah, I'm cheap :)

Charles,

Check out the videos I did on the command receiver on Youtube. I compare the regen mod with a q-multiplier, and do the crystal filter mod you describe on a 40M.

https://www.youtube.com/watch?v=XlrPCR1IKlo

Good Luck! Mike

Nice work! The Q-multiplier looks like it would do the job, but requires an outboard chassis and another knob to constantly diddle...

I'm impressed with how effective the (single) crystal filter was. The ARC-5's audio quality isn't anything to write home about anyway, nor am I trying to drive a good speaker, so I'm not too worried about it being a little narrow.

 I might just try it before building a 4- or 5-crystal Cohn (minimum loss) filter. You wouldn't happen to have a spare 2830 KHz crystal in the junkbox, would you? ;)

I have a AM Arc5 station. A Bc454for listening on 80M and a couple of transmitters , one I moved to 80m and one i moved to 160M. 160m in any mode has proved a waste of energy here however 80M has yielded several QSOs. Sometimes I hear my audio isn't this or that and is a bit light however I modulate them with a homebrew series cathode modulator (or sometimes the 56 ARRL so called simple grid modulator which even though it is inserted directly in series with the cathode and ground  (because it indirectly affects the grid they insist on calling it that even though a proper grid modulator it isn't) I don't use the rigs or the rx regularly but just for fun when conditions are good so I am happy just to be able to communicate and be understood with such a simple solution. The Rx does leave a bit to be desired but it does work and is stable so I just enjoy them for what they are. I am glad you are having fun with yours.
don

I am always favorably impressed with the capability and performance in such a compact package (except for the selectivity). My first ARC-5 was also a 7-9.1 Mc brand-new unit I bought at the Gaithersburg MD hamfest in '78 for $5.00, and built a power supply onto the back deck. Wish I knew what happened to it...

Anyhow I emailed JAN Crystals and got a reply back (via Monique Whan at calebscrusade.com!) saying "sorry, we no longer manufacture crystals" so will see what ICM wants for a 2.830 MHz xtal.

I have a 2830 KHz crystal if you still need it. Drop me a PM.

Thanks Pete!  Much appreciated. :D

Today I modified the IF can per Mike WU2D's video. I used some Kapton tape for interwinding insulation since the remaining original primary has plate voltage on it. Now all I need is for the crystal to arrive!

Incidentally, to manufacture new 2.830 MHz crystals, ICM wants $36.25. Each.  :o

If you believe USPS, it will be there Saturday.

Surprisingly, it did arrive as scheduled. I put it in and fired up the rx. Smoke from under the chassis. Shit!  ???
Turned out I'd left a connected piece of (the original) bus wire in the IF can and hadn't cut it off and had shorted to ground, frying the 200 ohm resistor providing plate voltage to the mixer and RF stages.

Fixed that. Then it started to work but was deaf as a post. I was getting worried until I realized that a very sharp xtal filter at the input to the first IF would make realigning all three IF cans critical. Tuned them up to 2830.0 KHz but the modified can still wasn't peaking even at max trimmer capacitance. I added a 33 pf silver-mica across the 110 pf NPO cap (that replaced the original 180 pf can), found a peak. Sensitivity definitely better!

Then I adjusted the new crystal phasing trimmer to peak at 2828.5 KHz for LSB (Do I have that backwards? I'm assuming the LO is lower than the RF, not higher), and finally set the BFO for intelligible SSB signals.

Now it works but is still down on sensitivity somewhat (probably because the new IF transformer winding driving the crystal has far fewer turns than the original pie-wound coil). The background noise is quieter too, so perhaps it'd be easier to add some AF gain rather than IF? Is there a tube I can plug in to replace the 12A6 that'll have more gain?

Anyhow it's MUCH sharper now, requiring a delicate touch on the tuning knob, although (with just one crystal) the skirt selectivity isn't that great. But it's a big improvement from the stock (20 KHz?) BW and it's almost usable on 40m now - don't have to listen to that one W5 whether I want to or not  ;D
It still works for listening to the AMers on 7290 also!

40 meters just died tonight! I thought something was wrong with the receiver since I could only hear a couple of stations on my OCF dipole, whereas an hour or two ago the "Winter Field Day" contest was really crowding the band.

So I hooked it back up to my hp 8640B signal generator and re-tweaked the IF and the crystal phasing trimmer... everything OK. So I fired up the SB-102 and reconnected the dipole to it... STILL can't hear anybody. Amazing the bottom really fell out quick.  ::)

On the subject of reduced sensitivity, I think some matching from the output of the crystal circuit (which is probably on the order of 1-2k ohms impedance) to the grid of the 1st IF stage (likely to be considerably higher Z) is required. I wonder if a tiny toroid (T37 size) unun, say 9:1, would be worth trying?

Edit: a little research shows that a FT50-43 toroid with 11 turns is 1000 ohms, so if I trifilar-wind some small (say, #32 AWG) wire on such a core, that'll be a better match and provide 3:1 voltage stepup too. That will fit in the IF can with ease.

I wound an FT50-43 toroid with 12 turns of quadrifilar #32 as a 1:4 step-up. Checked to make sure it did work at 2.830 MHz and was wired correctly with my signal generator and HP 400D VTVM - (yep - 0.1 volts RMS in, 0.4 volts out), and tucked it into the IF can between the crystal filter and the output to the 1st IF grid. When retuning I noticed I was at max on the trimmer so I had to add another NPO capacitor.

The sensitivity is considerably improved (experimenting with various loading resistors on the transformer output looking for a better match to the crystal didn't seem to make much difference)... now I can hear quite a few more stations, so the modest skirt selectivity from one crystal is again at issue. It's much narrower than the 20+ KHz bandwidth as initially designed though. The broadcasters are really destroying 40m tonight as well as the alligators in 5-land... Anyhow I'm done trying to make a silk purse out of a sow's ear. It's as good as it's gonna get without major redesign 8)

Besides, I also picked up a 40m transmitter, missing the tubes and cover for a couple of bucks; I have lots of 1625's in the junkbox and my homebrewed screen modulator, currently connected to the 80m transmitter. Maybe I'll get on the air with the PW'er soon ;D

Im good for an arc5 to arc5 80M anytime
don

That'd be neat to have an all-ARC-5 QSO  ;D Someday.

My receiver is powered as designed by 24-28 volts DC and the original dynamotor, too! The whirr will probably be heard in the mike. Transmitter supplied by the AC line, I don't have near enough 28 DC to run the big dynamotors (plus they are really noisy).

I don't have a 3-6 Mc. receiver (at the moment), and I'm waiting for a 1626 (oscillator tube) so I can get the 40m transmitter on the air. Wonder if I can reach VE-land on 10 watts of carrier?

I only have 80M ARC5 with both Rx and TX. I have one tx  on 160M but never had a contact. I don't have a 40m (yet). If you use another RX on 80M I,ll never tell. We are only 281 miles apart so we should be able to pull it off on a good day. I have worked into Maine and Virginia with my one tubers on phone (300 to 400 miles) and they are less then 5 watts out. Early morning some weekends I used to hear some Hams on AM from out your way. Ie 7 to 8 AM EST. If your receiver can hear CHU time signal well (3330kcs) the propagation should be right for us at the same time. That is how I decide when to try . I check for CHU and WWV . If I can hear CHU I know east west is probably not bad  and if I can hear WWV North south is likely to be good. You are mostly west of me I think in a southerly direction if you know what I mean. IE more west then south. We could find each other on CW (full carrier) then switch to phone. I often use a set of VVV VVV VVV de ve3LYX as it is quick and easy for one to to pick out when hunting for another station.
don

281 miles? I think your map is just a little "off"... that distance northeast is still in Missouri, and there are a few other states to traverse before reaching Ontario!  ;D Or are you somewhere else...

I wasn't even looking for the 7-9.1 Mc receiver when I found this one - just stumbled across it at a flea market. I'm keeping an eye out for a 3-6 Mc. unit too. Not on ebay, those prices are outrageous and tend to be for carcasses "missing tubes and covers" and looking like a deuce & a half ran over them.

I haven't operated CW since the 70's (got my Novice in 1976). I doubt I could pass that test  ::)

Meanwhile, cleaning up the 7-9.1 Mc transmitter. Although it looks like it may never have been used, someone dropped it on the right front corner of the panel and severely bent it. I straightened it as best I could (luckily the roller inductor was intact once its bearing support wires were realigned), put in a couple of 1625's, now waiting for the remaining two tubes to arrive.

Here's a pic of my 3-6 Mc. transmitter and screen modulator/power supply setup.

That was from QRZ .com so your comment made me go back and look. They have your address in MO Howell Cty but on the map your antenna is in Pittburgh PA. That is a log feedline!  Ur right! Probably more like 600 miles. My antenna is about 30 miles north of where I live too on their map but I guess that is no real big deal. Can you give me another ham call in the area and I will check his details.   I am interested in how you made your screen modulator and would like to know more. Forget the CW then we can just holler. On a good day we might still do it.
don

Weird! I just looked at my own entry on QRZ and those lat. & long. coordinates are completely wrong. I'm more like 36 deg N, not 40... Although I did live in Pittsburgh for 2 years while going to school there from '96-'98. Wonder if QRZ can fix that.

The screen mod is a simple design mostly lifted from mid-1960's ARRL Handbooks. A typical 12AX7 speech amp driving a 6AQ5 in class A, coupled (I think capacitively, will try and dig up the schematic) to the screens of the 1625's. The rest is just power supply, using a 480:120 volt control transformer wired backwards in the "economy" circuit, with VR tubes for oscillator regulated B+, and a hefty transformer/bridge/filter cap to supply 28 DC for filaments, receiver dynamotor, and relays.  8)

My PS is thhe "economy" from the handbooks. I am familiar with your mod circuit. I have an earlier 50s version 6sl7 6v6 used a bit differently but in inch in more or less the same.
don

Strange goings-on... in crowded band conditions with the sensitivity restored, I noticed it did not seem adequately selective. After much frustrating experimentation last night and this morning, initially assuming the crystal was broken, I finally discovered that with the IF shield can removed from the assembly, the filter is quite sharp over the nose although (not surprisingly with a single crystal) the skirts and stop-band aren't very good. Still better than original.

But when I reinstall the shield can, the selectivity goes away considerably as I slide it over the frame!  ??? It's gradual, not "switched" so I'm positive there is no short circuit to the can. Perhaps the loss of Q wasn't an issue with the designed 20+ KHz IF BW (and that's with all the stages in series, so each can is probably even wider) but it's a big problem with a narrow crystal filter.

I think I may just remove the original coil and replace it with another T50-43 toroid. To resonate the original fixed 180 pf (plus 10 pf nominal from the air trimmer), my calculator tells me 16.7 uH are needed and that's only six turns. A trifilar winding is easy to make and keeps the phasing trimmer too.

Removed the original ceramic form air-core coil, and replaced it with the proposed trifilar-wound toroid. That took care of the shield-can sensitivity! It's still interestingly less selective with the can in the radio,  although the effect is less pronounced now, and I can't figure it out.

This is a simple circuit - the output comes from the plate of the 12K8 mixer to the IF can, and its small plate current goes through the primary. Then the output from the can just goes directly to the grid of the 1st IF stage. Nothing there to load it down, and with a crystal filter the load impedance is the primary determinant of selectivity (which is in the can). I'm not putting in enough signal to cause the primitive AVC to affect the gain, either.  ???

After pondering further it appears that the Z presented to the mixer plate was probably far too low, given an estimated ESR of the 2.830 MHz crystal of 300 ohms and a 680 ohm load resistor through the new 1:1 input transformer. A quick use of an online pi-network calculator showed that 17 pf input and 169 uH series (20 turns on a FT50-43 toroid) would match 10k(?) ohms down to 1k ohm at 2.830 MHz. So I installed that under the chassis by simply lifting the plate lead from the 12K8 (pin 3) and using a handy 8.2 pf NPO disc (to allow for tube + wiring capacitance).

I didn't sweep it on the signal generator or check the sensitivity again (that 8640B is heavy and it's on another bench!). However, subjectively it appears quite adequate when connected to my OCF dipole. Selectivity will never be close to "battle mode"-ready without a four- or five-pole ladder filter, but that's a project for another day. I listened in on a few QSO's, checked the time on CHU (7.850 MHz AM), and called it good.  8)

Here's the current version of the circuit in a .pdf document. I think it could be improved with the addition of a second crystal at 2831.5 or 2828.5 KHz instead of the "phasing" trimmer, but probably wouldn't be worth the effort since the skirts still would not be very steep and the filter nose doesn't seem excessively narrow as it is.

There's plenty of room inside the IF can for more circuitry once the original bulky ceramic coil form is removed (T50 toroids are very compact). However, four or more HC-6/U or HC-33/U crystals would be hard if not impossible to fit, and I'm not sure if 2.83 MHz crystals come in the small HC-49/U package.

Any suggestions from those more skilled in RF circuitry would be welcome.  :)

Started on the transmitter since I got the missing 1626 tube in the mail today. I made a temporary wiring harness to my screen mod/power supply. While tracing out the schematic which I'd misplaced sometime since '04, I discovered that I'd made a PC board for the power supply components! (see pics). Somewhere I've got the Eagle files if anyone needs them. Basically it's 210 volts regulated for the oscillator, 150 volts regulated for the screens (fed through an improvised modulation transformer, actually a Triad N-48X 115v 15VA isolation transformer). Standard '60's ARRL Handbook 12AX7 speech amp driving a 6AQ5.

I put a 50 pf doorknob in series with the output and it loaded up nicely into my Bird dummy load at about 70% of the roller inductor. Dial is only off a KHz or two, not bad after 72 years. It's overmodulating a bit on the scope (displaying 20 v/div into 50 ohm load), but it's hard to take a cell phone pic and get the audio gain adjusted right at the same time  ;)

Regulating the oscillator voltage near 200V (two OB2's is about 210V) is the prescription for stability. There's amazingly little drift even from cold, and the eye tube opens at 7000.314 KHz with a FT-243 crystal marked 7000.00 in the socket (which could be the error of a 40 year old crystal itself, there's no trimmer and I haven't put the counter on it).

The Bird is only accurate at VHF which is why the needle is barely moving. Actual unmodulated carrier power is around 10 watts  8)

Anyone got a (black) cover and a bottom plate to finish it off?  :)

I am missing a bottom plate for my RX but then it was give to me at Durham hamfest  so I better not whine! Was pretty banged up but works fine now. gladyou gotthis tx up and running.
don

Having resumed work on my '04 screen modulator design, I'm having to fine-tune it a bit.
First, there seemed to be a lot of hum on the output for 50 uf of HV filter capacitance. About 20 volts of ripple on the +600 key-down. Hmm... something I forgot about the "economy" configured power supply. The effective filter capacitance for the HV output is not only the HV output filter, since it's effectively in series with the LV filter (center-tap) capacitor  :P
So I really had 50 uf in series with 22 uf, or about 15 uf! Adding 33 uf across the LV cap took care of much of the ripple.

I also found that, although I was regulating the PA screen voltage with an OA2 at 150v, that basically gives full carrier output with 600v on the plates. I must have tried to "rig" that back in '04 with a 56K dropping resistor and a 10 uF electrolytic cap feeding the modulation transformer. Doesn't give a very stable carrier output. Jumpering out the resistor (with a milliammeter) showed about 4 ma resting screen current, increasing on voice peaks, and a larger carrier output - but no upward modulation at all  ???

I think this is telling me to use a lower screen voltage... If I can find a spare 0B2 (108 volts) I'll just plug it in place of the 0A2 and see, but that may be too low. Hope I don't have to resort to those newfangled Zeners :D

Edit: found an OB2, plugged it in. 2 ma resting screen current, 13W unmodulated carrier with decent looking upward modulation only if the variometer is set to full coupling. At lower couplings, although the unmod carrier does decrease, so does the upward modulation and the waveform really deteriorates. I don't remember enough about tetrodes and screen modulators to figure this out, but I'll run it the way it looks best on the scope.

I also found enough room behind the transmitter front panel to mount the Centralab 50 pF doorknob once the old antenna relay was removed. Rather than hack up the panel for a coax connector, I rigged up an RG-58 pigtail to the binding post with a PL-259 on the other end.

Now I just have to dig out my homemade 28V T/R relay and maybe I can put the station on the air!  8)

For screen modulation, you need to over load it a lot, way past the point of maximum output.
For good audio, its an adjustment of the resting carrier power (screen voltage) and loading.
One way to do it is to put full screen voltage on the tube, tune up at full power output, then turn the screen voltage down to 1/2 the power output or less, then modulate.
On my rigs, I just increase loading till I get between 100 and 150% positive modulation.
A peak reading watt meter is your friend.
Power output with screen modulation will be very low, between 25% and 50% of full power ratings.

You will also need enough voltage swing to modulate fully.
I never had luck with transformer type screen modulation, but I suspect the transformer has to be a good match...

Thanks, that confirms what I have been learning by experiment  ;)
A tune/operate switch to apply full screen voltage might be a handy addition in future.

There's definitely enough voltage swing, since I can easily see overmodulation in the negative direction with the audio gain turned up. Not sure if a "real" modulation transformer would be any help, except maybe to improve the highs, compared to the 1:1  (115v 60 Hz isolation) transformer I used that was in the junkbox  ;D

I may need to add a few turns to the output link to get overcoupling since I can't go beyond 100% on the variometer. But that requires taking apart the variometer assembly... So maybe I'll just leave it alone.  8)

I had mine on air today twice. I have been wondering about eliminating (bypassing perhaps) the roller and feeding the antenna directly off the variometer coil alone. 
don

Yes, positive modulation is all about loading.
I have a bypass switch but never use it except when I plate modulate the rig.
I just tune everything for maximum power output then increase the loading till power falls off lot and I get lots of positive peaks.
I had worked out the static screen voltage in the past, and to some degree it works as a power output adjustment.

Its not unusual to run 250 watts carrier and have 1200 to 1300 watts pep.
4X would be 1000 watts and 200 watts over that is not unusual at 80% negative modulation.
If I turn the screen voltage up (that feeds the modulator) I get mostly more carrier power past about 300 watts.

So tubes (they are all different) have an operating point they like to get good modulation.
Most tubes seem to like a lot of plate voltage to screen modulate well at a good power level.

Don, from what I've been able to figure out from many ARC-5 articles (especially those by ARC-5 guru Dave Stinson), the transmitter is designed to match a very short wire which has considerable capacitive reactance and a Z of about 5-12 ohms. This is a long way from the holy amateur grail of 50 + j0.

The simplest solution to minimize harmonics and other undesirable output is to put a 75 pf (for 80m) or 50 pf (for 40m) transmitting cap in series with the output post and your feedline. That gives the highest power output at around 60% of roller inductor turns. It seems that a 1:4 unun is more trouble for only very slightly cleaner results and possibly a few watts less output.

It occurred to me that I didn't have to take the variometer apart since I'd never run it at 0 coupling anyway... so I added two turns around the middle of the tank coil and hooked it up in series (trial and error to find out which direction to wind in). That proved to be too much. So, one extra turn works just right. Now I get the cleanest waveform at about 5/10 on the variometer.

Meanwhile, I've been tweaking the modulator circuit to reduce hum pickup, increase low frequency response, etc.  I must have picked some of my parts values based on what was handy from the junkbox at the time.  ;) I drove the mic input from a good GR audio oscillator and to my surprise discovered (although my speech amp is flat as far as the 6AQ5 driver's grid) the output at the mod transformer (6AQ5 plate) has a peak response at 2 KHz (falling off sharply above that, which is OK) but steadily decreasing as I lowered the frequency and below 400 Hz was way off and starting to show distortion. Naturally the RF modulated waveform was the same. I'm not going to put this thing on the air yet!

I was surprised because my improvised modulation transformer is a small 60 Hz power transformer, so if anything I would have expected poor response above 500 Hz or so, not the exact opposite... I may need to try some swamping resistors for impedance matching, or a different transformer, or maybe couple the 6AQ5 to the screens another way. Choke? RC?   ???

You could try the DX60/QIX modulator, no transformer, hifi, and it works like a champ for only 2 tubes from D104 to screen.

Thanks for the tip. If I can't get acceptable results with the setup I have, I might just rip out the 6AQ5 and transformer, and build that circuit in the same space!

That circuit really does work fantastic as long as you do not try to push it too hard.
With a line input, its broadcast quality, and it souns good with the 1st stage set up for a D104.
I built it as a standalone modulator to use on anything up to a pair of 4X150's at 100 watts out.

I've been in the shack experimenting some more... thought the problem was with the "modulation" xfmr, but when driving it directly with the GR oscillator, although I could not achieve 100% mod, the response was perfectly flat from below 100 to at least 3 KHz. I also experimented with a resistance load for the 6AQ5 and coupling it to the transformer with a large capacitor (thinking the DC component in the primary, about 25 mA, was causing trouble). No change. So it's not the transformer.

I got out a '66 Handbook and started reading the "Audio Amplifiers and Double Sideband Phone" chapter and the screen modulation section. My lights came on - "strongly recommended" negative feedback in the modulator is the answer! A simple .047 capacitor from the 6AQ5 plate back to the cathode resistor of the preceding 12AX7 stage flattened the response nicely, and nearly eliminated the <400 Hz distortion. But then I couldn't get 100% mod due to the reduced gain.

Anyway, now it's just a matter of cut and try until I find the right combination of two R's and one or two C's for the feedback path to give a reasonably flat speech frequency curve with enough gain to fully modulate the screens!  ::)

I dont use the holy grail 50 ohms. I use it on a long wire and it works quite well I think. I did build a tuner and have used that to match the unit to my home brew 811A pair linear. I get sufficient power out to use it which is all I want from it. I myself don't desire super audio. I want it to sound like it did in 1943. I cathode modulate mine and have thre e units i use. Just plug in the key hole and hook up filament supply. Works well. Been watching a parallel discussion on the method here and don't understand all the fuss. It s very easy to do and works well. A morning with a soldering iron will get it done. No schematic should be required and there is nothing to discuss. It works period. Your screen mod I found interesting too. (I have build two rigs from scratch with screen. ) Two of my cathode modulators are built to use carbon mics and one is for a crystal mic. I have been using it lately because some folks don't like that bandits a 12 oclock audio. I do! However trying to fit in but sometimes it is hard to put a square head in a round hole. As long as my radio work reasonably well, aren't hurting themselves and aren't interfering with others I am content. I have enjoyed very much reading your thread here and admire you project whatever the outcome. Building is a lot more fun and instructional then BS and postulating. I see you also understand that. Anytime you want to try a contact give me a time a freq. It might be impossible for us but who knows. On a low noise day with good prop it might just happen. So far I have gotten out about 200 miles in normal average conditions on 80M . Anyway keep at it and keep posting. Tis most interesting.
don

Feedback really helps in screen modulators, I added it to the DX60/QIX modulator.
There is no phase shift in that circuit so you can use a lot.
Maybe just change some values in the 1st stage to get more gain, higher plate resistor...

Going to play with it some more this morning, time permitting...

Here is the current circuit. (The odd-looking tube symbols are from a user-created library "radiotubes" for Eagle which I downloaded from Cadsoft's website). I used large interstage coupling caps to avoid too much low-frequency rolloff. Would you recommend different values of plate resistors for the 12AX7?

Well the higher the resistor (220K now) the more gain.
330k, or 470K. Try a 330 K.

The higher the resistance, the more voltage swing.

I tried another 220K in series with it... made no difference! I think the gain of the 12AX7 itself is the limiting factor.

Anyhow I determined with a substitution box that the optimal value of the feedback resistor is near 100K.
What I find strange is that the voltage at the grid of the 6AQ5 gets smaller with increasing frequency when the feedback loop is closed, but the plate swing is more or less constant. There is no high-frequency rolloff within the circuit except perhaps the inductance of the transformer primary.

The modulator output (and the RF envelope) shows some distortion above 75% mod when supplied with a sine wave of 250 Hz or less. Clean at smaller amplitudes all the way down to 100 Hz. Looks fine all the way to 100% from 350 Hz on up to over 2 KHz. I think maybe this is limitations of the transformer.

Speech with a hand-held "communications" mic, which won't have much output below 250 Hz anyway, looks good on the scope. I got it done just in time for the AM "Noon Forum" on 7290 to break up! Tried to make contact in case anyone was still on freq but with only 13 watts and at noon I doubt anyone could have heard me.

Now to get back to work on the 4x4-125A class AB1 amplifier I also built back in 2004 (in my profile pic) to go with this PWer. 250W of carrier should be a bit more audible (13 dB increase).

I used the first stage of this circuit:

http://amfone.net/Amforum/index.php?topic=32889.0

Seems to give loads of gain out of a D104.
I did not use the 10 meg input resistor but went with 3 meg to cut the lows somewhat.
What is the voltage on the tubes?

If you think about the grid changing the current flow through the tube, a higher plate load resistor
should give more swing on the output as the voltage drop through a higher resistor should be more if the plate voltage is high enough...

That's pretty much the standard speech amp circuit... looks about like mine, too!   ;)

Regarding the increased voltage swing, if increasing the plate resistor from 220K to 470K, won't the 1M grid resistor of the next stage load it down proportionally more, so there's less swing across the next grid? Anyhow I have plenty of audio gain, driven from an old Shure CB10E hand-held mic.

I am surprised at how critical the adjustment of loading (both the variometer and the roller inductor) is. If tuned for maximum power output, the modulation is severely distorted with barely any upward peaks. With just a little more loading, which only drops the carrier slightly, the scope shows a normal pattern.

Is this true in general of screen-modulated transmitters? I wouldn't dare put this one on the air without a monitor scope!

This afternoon I tried again to join a couple of ongoing QSOs barefoot, but no luck. Maybe tomorrow morning...

Yes, that is normal, and even more loading will give a lot of positive peaks.
Scope is nice, mod monitor is nice, pep watt meter is nice, all three allows one to fine tune things very nicely.
You could use any ONE method.

I love the way screen modulation works, it can be VERY HI FI (if no transformers are used) along with nice positive peaks.
Since I liked the way it worked, I just built up something that would run it at 300 watts carrier output.
14 watts should do ok on 40 meters with a good antenna...
I have made contacts with 5 watts.

Yes, I also have made a couple of 250-mile QSO's with my 3-6 Mc. transmitter barefoot, so I agree it's possible. If there's propagation, if the alligators in 5-land and the foreign broadcasters aren't wiping out the band, and perhaps most importantly, if someone's listening  ;D

I've been trying without success to find out what Bristol spline sizes are needed for the various setscrews in the receiver and transmitter. Any ideas? The wrenches are rather expensive (at least compared to Allen hex wrenches) and I don't know if the "SS-508" set sold on ebay has the right sizes.

Here's some pics of the transmitter output with a sine wave applied to the mic jack.
Audio: 1 KHz

1st pic: approx 50% mod.
2nd: 100% mod. There's a tiny "kink" that appears as the envelope approaches zero, otherwise looks good.
3rd: 100% mod, undercoupled final tank (loading too low), other settings the same.

Now with a sine wave input of 250 Hz.

1st pic: approx. 50% mod. Looks reasonably clean.
2nd pic: distortion visible at 100% mod

And at 100 Hz:

3rd pic: looks crappy at 100% mod  :P

I couldn't capture a good trace while speaking into the mic but it looks much better than this.

If I can make a QSO, it'd be interesting to hear reports of what the low-freq distortion actually sounds like... above 400 Hz where most of the speech energy and intelligibility is, it's clean.

   That low frequency distortion will likely go away at a lower level, such as at 20% modulation. The idea is to keep the lows, but at a level such that they are heard, but the level is low enough to not distort. Many folks EQ this way anyway while accentuating to 1-2 khz region. I was playing around with this idea with the Retro-75 transmitter since its overloaded modulation transformer could not pass much below 300 hz unless the level was cut way back.

   You might consider making both RF and AF taps such as was done at this thread:

http://amfone.net/Amforum/index.php?topic=37653.0

    This way you can easily observe a trapezoid OR modulator envelope pattern on the scope. The trapezoid is key to measuring RF linearity independent from modulator distortion. You can have a perfect trapezoid when the modulator is heavily overloaded. Then once you get a decent trapezoid, then look at the modulator. All too often we skip the trap and try to clean up the modulator when the main culprit is poor RF linearity of the final modulated stage.

   Most modulated class C amplifiers can get improvement to the RF linearity by juggling the drive level, screen modulation level, and phase of the screen modulation. For example, with the 6146 plate modulated, the amount of screen modulation is often excessive, and there can be appreciable phase shift at the higher audio frequencies above 1 Khz. This phase shift is from the screen dropping resistor and the screen bypass capacitor. Some or all of this might apply to the ARC-5.

Jim
Wd5JKO

I bet most of that is in the transformer you are using to modulate the screens with.
Without a transformer, there is nothing to distort the signal except the power supply and the way the screen modulates the carrier (tube type).
Not that an arc 5 has to sound like a broadcast radio, but it CAN.
 

This is my rig at 30 Hz with screen modulation:

Yeah, I can never stop with good enough... I think I'm going to eliminate the transformer.
The neat thing is that I can make the change in situ, using many of the same components.
This is what I'm proposing to do (see attachment) - anyone see obvious "gotchas"?

After looking at the GE data sheet, the existing 6AQ5 driver can be triode-connected instead of changing to a 6DE7 like the DX-60 circuit. The OB2 will now be used to provide a regulated -108V, and I can either use the existing 115v isolation transformer with a bridge/cap to feed it, or just rip it out entirely and tap a half-wave rectifier off one leg of the 480CT HV trans. It only needs to supply keep-alive current (5 ma).

It looks like 200V p-p will be more than enough screen voltage to get 100% mod at 560V on the plates. So the plate of the 6AQ5 will rest at a nominal 100V, which is about half RF power output, and can swing to ground since its cathode will be tied to the regulated -108. There will be 5 ma quiescent plate current, dissipating 1 watt in the tube. If the PA screens ever show negative current, this circuit can absorb it.

The 6AQ5 grid bias, which according to the triode curves needs to be about -40V to conduct 5 ma, can be provided by a pot from the -108 since there is no grid current.

It looks like I may actually have too much voltage gain in the speech amp... the 6AQ5 should only need a few volts of grid swing to provide full swing to the PA screens. Any thoughts before I start messing with it?  :)

Edit: Jim, the low-freq distortion does indeed go away at lower modulation levels, as shown in the pics. I don't see a problem with the communications mic which tends to emphasize the midrange anyway.

I suspect that might work well.
Two things you need for good operation, enough voltage swing and the ability to set the resting carrier level.
Then it all comes down to how the tube works with screen modulation, some work much better then others.

I do wonder what the ideal transformer would look like.
The right impedances, the ability to handle DC on the screen side, maybe push pull on the audio side, push pull off the cathodes? and HI FI.

Single ended would have dc on both sides...

The nice thing about no transformer is no phase shift so you can use wide band feedback to clean things up a lot.
You should add it to that circuit, it helps a LOT.

Sure is fun to play with after years of plate modulation...

I spent the evening rewiring the modulator for transformerless operation, and making stupid mistakes. It's been a long time since I designed any tube circuits  ::)

First (after removing the transformer) I discovered that I can't use rectifiers to create negative bias in the economy PS circuit since the CT is not ground! Dumb mistake, I should have known better. So I put the transformer back in, and wired up a bridge rectifier and capacitor for the OB2 supply.

Smoke test good - but no output. I had made an elementary mistake - since the cathode of the 6AQ5 is at -105 volts, setting the grid at -68 volts is in the wrong direction (grid positive w.r.t. cathode!)... should have been -145. -:-X For a temporary fix, I wired the grid pot to the unregulated supply feeding the OB2 (about -145 volts). Now it works and the frequency-related distortion is completely gone, as expected.

I can't get the peak screen voltage to go over about 160 volts. I can push it to 200 volts but the waveform is beginning to flatten). I think the peak current required is in excess of what the plate resistor can flow, but making it smaller will increase the dissipation in the driver. Compromises in a simple design, as usual... anyhow with the loading controls optimized for my 50 ohm dummy load,  there is just 7 watts carrier and 16W peak (75v p-p into 50 ohms). The 1625's are certainly capable of more carrier and PEP without overheating, but the screens need more drive! I was getting that with the transformer but as we have seen, although the 6AQ5 has plenty of drive power available, the transformer causes distortion at low frequencies.

I need to play with bias voltages and the plate resistor to get more peak screen voltage (and current) while minimizing the wasted heat in the 6AQ5. In triode configuration I probably don't need 100 volts across it to be able to pull the screens to ground. I may have to resort to a zener diode or two, although I was hoping to do it with hollow-state devices other than the rectifiers ;) an NE-2 is the right voltage but won't handle the required current (10 ma or so).

Also, there is still a little "kink" in the bottom of the envelope as it approaches 100% modulation. Some investigation with my scope (one channel on the screen voltage, the other on the RF output) and I discovered there is still output from the 1625's even at a screen voltage of zero!  :o I didn't think this is even possible.

In order to achieve full 100% negative mod, the screen voltage actually has to go negative about -10 volts. I'm only running about 560 volts on the plates. Can someone tell me what is going on? Screen emission perhaps?  ???

Edit: another thought... the ARC-5 has a 15k grid-leak shunted by a .05 uf can capacitor. Those caps are known to become leaky after only 70+ years, so maybe the 1625's are underbiased. Still surprised to see output with zero screen voltage though.

On most tubes, you have to modulate the screen negative, not just down to zero.
Also adjust the grid drive and bias.

I built the dx60/qix modulator and it did not work great unless I lowered the grid drive a lot.
I then added the feedback and then the grid drive did not matter much at all!

Look at the KS3K dx60 mods in the transmitter section and add that feedback from the screen to an early stage, amazing things may happen.

Oh, I just noticed you are taking the screen modulation off the plate side of the tube.
Most screen modulators are cathode followers to get the low impedance drive to the screens, they are a variable load, just like class B grids, no load at all then the screen draws current and there IS a load.
Another thing I found that helps a lot is a load on the modulator, 100K resistor from the modulator output to ground.
 

Great minds think alike.  ;D I just figured that out independently a few minutes ago, recalling that screens take more current the more positive they get, and reexamined the DX-60 circuit... sure enough, it's a cathode follower!

Time for version 3.0 after I get back from a business trip, on Wed. evening. Watch this space for updates :)

Mods, should this thread be in the Technical section? Feel free to move it ;)

What are the wave form like at 90%?
I ask because the Dx60 circuit has been mentioned several times. I have one and seriously doubt it could do 100%.
Anything above 70% should sound decent.
As for someone needs to be listening. = Email me when you want to try. My call letters plus @outlook.com and I will be there if at all possible
don

This is the dx60 circuit with some simple mods:
http://n2dts.smugmug.com/Ham-radio/i-XjX39zC/A

100% is easy with screen modulation, simple circuits can give well over 100% in both directions.

I reconfigured the circuit as a cathode follower and made a few other minor changes (attached).
It looks very clean now, still requiring about -15V on the screens (at 560-580V plate) to cut off completely.
Looks just as good at 50 Hz as at 2 KHz :) I think that is "good enough for now".

I can get 56V pp carrier and 128V pp fully modulated. According to my trusty calculator into 50 ohms that is 8 watts carrier and 40 watts PEP. It's a bit cleaner on the scope when driven less vigorously, say 6 watts carrier and 30W PEP.

There is some AM activity going on now around 7290 and I tried to join... someone said "I heard a little carrier out there but couldn't make out any audio"  :( Not a great propagation day from central MO it seems.

Time to head for work until Wed. evening anyway... when I get back I may hook it up to the 4x4-125A amp and try that on for size!

Another problem just occurred to me while driving to my hotel... now I'm exceeding the heater-cathode rating of the 6AQ5 by quite a bit, since the cathode is swinging several hundred volts above ground  :-[ I haven't designed with tubes in so many years I've forgotten to check stuff like this.

There is absolutely no room in that compact chassis (selected because it's just about the footprint of the transmitter) for even a small filament transformer. The easiest solution is to run the 6AQ5 off the 6.3 volt filament trans already tucked underneath (with cathode tied to filament), and power the 12AX7 with its filaments in series from the +28 DC supply via a dropping resistor. Only requires 0.15 amp so the resistor is dissipating 2.5 watts.

What is the filament to cathode rating?
The tube used in the dx60 is ok, I have been running it quite hard without problems.
Was on today with it, 650 volts on the plate, 200 volts resting carrier voltage plus the swing which must be quite large...
6AQ5 is rated at 200 volts.

The 6DE7 and the 6AQ5 both have 100 volts DC, 200 volts DC+peak ratings. In this design the PA screen resting voltage is currently about 125V and the total swing currently goes from -15V to 260V, which is in excess of the ratings with the heater grounded. I don't have a complete DX-60 schematic so I can't tell if they used a separate, elevated filament winding for the mod tube in the Heathkit. It may be ok even in the long term but I don't like exceeding manufacturer's breakdown ratings, when a short will destroy the tube...

Are two 1625's at 560V capable of significantly more than 40W PEP? Something's running out of breath when I try to push them harder (envelope starts to show a soft flat-topping above 125V p-p at the 50 ohm load). The PS is not sagging. Do I need even more screen swing? Perhaps the 1900 ohm plate resistance of the 6AQ5 in triode connection is getting in the way (the 6DE7 is 925 ohms and only driving one 6146).
Or maybe not enough drive from the 1626 running at 280V? I'm not looking for legal-limit from an ARC-5  :D but would like a little more output without frying the tubes if it's possible.

It varies from tube type to tube type, but it will never be higher then half the power the tubes would run at in plate modulated service.
More voltage on the plates often helps.

You have about 50 watts of plate dissipation to play with, about twice the power output of a DX60 which is about 12 watts, so 24 watts carrier would be expected, about 100 watts pep.
In order to get to 100 watts pep, you would need to be able to get there in say CW service, and may need 750 volts on the plates.
If you want say 50 watts of carrier, you would need to plate modulate it if the parts can handle the peak voltages.
At under 600 volts on the plates power output in any mode is going to be much lower then it could be.

Thanks. Will see what I have for transformers that'll give me higher plate voltage. (Have you seen the price of new Hammond plate transformers??)  :o

This project originally started as a junkbox special and I had available a large (250VA) control transformer with a 480V primary, 120V secondary, which I run "backwards" in the economy PS circuit. That gives about 580V on the plates and half that for the rest of the tube circuitry.

I think the transmitter originally did have a 600V dynamotor providing plate voltage, another reason I built it that way. From the ARC-5 manual, it was rated for 40W CW output and 15W "voice carrier" (but that was with plate+screen modulation from the MD-7 modulator unit). So I'm probably getting about all I can expect with screen modulation at 580V.

I would bet that in the space taken up by that big control transformer, there is room for two smaller ones. A similar 50VA unit, which I have, will handle the small load from the speech amp, VR tubes and oscillator B+. Then all I need is something like 600V secondary at 100VA or less, to supply the 75 watts of 750V for the plates and screens. While of course staying under 900V key-up, because that's what two 450V electrolytics in series can stand off ;)

This is turning into the restoration of my grandfather's axe... I installed a new head and a new handle, but it's still his axe  ;D

It's time to leave well enough alone, and get back to work on the Class AB1 linear, which I designed to take the PW output from this transmitter and bump it up to 200+ watts of carrier!

This beast consists of four 4-125A's in parallel with 3.5 KV on the plates from a beefy 900VA (CCS) Thordarson plate transformer, a spare 4-65A to shunt-regulate the screens, vacuum variables for the pi-net, and grid-driven (50 ohm non-inductive swamping resistor followed by a step-up unun, (don't remember if it's 4:1 or 9:1) so it should be fine with just a few watts of drive and be easy to neutralize despite the high power gain.

Last time I messed with it (a few years ago), it seemed to be amplifying OK, but the modulation from the ARC-5 looked so bad I gave up on the project for a while. Now that I know more about screen modulation and tube PA loading than I did, and have a decent looking "exciter", it's time to have another try at it  8)

Well, the rule of thumb (if the power supply is good for it) is an amplifier will do about one third carrier output of the plate dissipation of the tubes in the amp.
A pair of 3-500z's is good for about 300 watts carrier.
A single one will be good for about 150 watts, same as four 4-125's.

I think 4-125's screen modulate very well, you could run the same 150 watts out with direct screen modulation.
That has the advantages of needing no screen voltage regulation/protection, very hi fi, and a very small modulator.

15 or 20 watts out is enough to make contacts on 40, I have made contacts on my ten tec at 5 watts carrier, and 100 watts is a nice power level.
I just removed my 811 modulator and put the DX60/QIX modulator in its place.
100 watts out instead of 250, but it sounds so much better, and modulates so much better.

Only 1/3? I thought that the overall efficiency of an AM linear was 1/3 (at least according to the Handbooks), so the carrier output should be 1/2 the plate dissipation. 750W DC input, 500W plates, 250W carrier. What did I miss?

I used to have a 4x813 grounded grid amp I ran with a flex 5000.
Same rating per tube I think, 125 watts, at 150 watts carrier out it was putting out a LOT of heat.
200 watts carrier was really pushing it (it seemed).

In amplifier service you will likely want to run the 4-125's at 2500 volts or more.
For ssb service, I could put 3kv on the 813 plates and get over 2kw pep out.

Grounded grid or grid driven?
Not sure you will have enough power in grounded grid?

I went whole hog. A pair of 811A GG is what I use if I need to "Reach" with my ARC5. 12v heater supply fed down the middle. My own concoction.
don

4-125A's should be OK up to orange plates as long as they get plenty of air... will see how much power that actually is.



Already running them at 3500 volts, but I have the ability to put a Variac on the plate transformer and reduce it if needed.


Grid driven, for exactly that reason. In AB1 effectively no drive power is required except that for the swamping resistor. Even 6 watts of carrier should be able to drive this amp to full output. I'm not sure my PW'er would have enough "scrote" to drive a big GG amp.

3500 volts will make those tubes sing!

How did you match the very low-Z output of the ARC-5 to the input of the 811A's?
What plate voltage/current is your supply, and how much power are you getting out of that configuration?

I made a simple antenna tuner for matching other stuff in the shack and used to to make the arc5 happy feeding the twin 811As which have an untuned input BTW. Power out appears to be 100watts or so. I don't trust in shack meters because stray RF can throw you. I use the old school dummy loads you can read by. No power no light , period. (And no I have never not once ever had a QSO on one of them.) I tune for max and then switch to antenna (longwire) and only trim if absolutely required. Usually it isnt. I forget the HV. Probably 1100 to 12oo volt keydown. I use indicator bulbs rather then meters cause I can barely read the meters anyway. This station is old school so perhaps may not interest most but it works and that to me is very interesting.  Normally I use the 811A deal with my DX60B without the tuner.
don
The twin 811A build was on here somewhere and was a struggle at first but is a very robust trustworthy addition to my shack. It was my own design based on others I had seen over the years. China tubes and I have pushed them into the orange several times but not with the ARC5. Many here who have worked me have worked me on this combo either the arc5 or more likely the Dx60B on this linear. I use it in marginal conditions . I really do not care for big power. Why I don't know. I just dont. Of course I ran a six cyl dragster for many years too. Must be an underdog thing.

Don, sometimes it's just fun to do more with less  8)

Got back from my trip and tinkered with the PW'er some more. The modulation looks good, but when listening to my carrier (into the dummy load) on the receiver with the antenna disconnected, there is an unacceptable level of 120 Hz hum with the audio gain at zero. It's not very noticeable on the scope but it sure is audible... The problem is that the 6AQ5 screen modulator tube is fed from the plate supply so any ripple on that supply also directly modulates the PA screens  :P
 
So I either need to improve the power supply (which I had been thinking of redesigning anyway, as posted earlier), or put a good filter between the B+ and the plate of the 6AQ5. I don't have a small low-current 5 or 10H choke, or any room to mount one. Perhaps an RC filter since the current draw is only a few milliamps. Either way I'd need another electrolytic capable of withstanding the full B+(near 600V currently). Hummm...  ???

I found I need a good stiff voltage feed to the screen modulator.
Plenty of capacitance.

The existing plate supply would only drop from 600V idling to 580V but there was something like 20V p-p ripple on it at full carrier.

I found a decent Stancor power transformer in the junkbox labeled 270-0-270 @ 120 ma DC. It's more like 600 VAC unloaded with my relatively high line voltage, since it was designed in the days of 117 VAC line. I used the 5 VAC rectifier filament winding to buck the line, which helped some. That should give me plenty of plate voltage with the existing bridge rectifier - maybe too much!

Anyway today I removed the unnecessarily large 250VA control transformer and installed the Stancor on my compact chassis with barely enough room for it and a small 50VA control transformer also wired backwards in FW CT with a couple of 1N4007's to supply the oscillator regulated B+.  The Stancor's 6.3V filament winding is now feeding the 6AQ5, which allowed me to remove another transformer from under the chassis.

No more "economy" power supply circuit. Now I can, if need be, increase the filter caps on the plate/screen supply without affecting anything else. Stopping to eat, then smoke test... with a Variac  ;)

Passed the smoke test (except I'd forgotten to hook up the primary of one transformer)  ::)

Now I have about 700V on the plates, and considerably less hum, since the HV filter caps (two 100 uf in series, or 50 uf) are not dividing across the LV side caps of the former "economy" supply any more. My effective capacitance was only 26 uf, and now of course it's 50 uf. Not surprisingly it reduced the ripple! I may tinker with an RC filter to the 6AQ5 since there's more than enough headroom for voltage drop in the R.

Tuned for largest cleanest peaks into 50 ohms, there is now about 9 watts carrier, and 64W PEP on voice peaks  8)
That's enough work for a pair of 1625's I think. They aren't very hot, either.

Nobody on 40m this afternoon to try it out on  :( Sometimes there's AM activity on 7160. There's a strange broadcast station around 7290 - in English but with an indefinable accent I can't identify. Chinese broadcasters?

Too much audio, unload the final some to get more carrier.
If you get 64 watts pep you can run 15 watts of carrier.

Sure, I can load it (and adjust the screen mod bias) to run more carrier, but do I want more carrier? The modulation is what carries the information, in the sidebands... and at the current settings I get the biggest peaks on the scope.

Won't I have a more audible, intelligible signal with the highest upward modulation possible without clipping at baseline?

4 times carrier is about 100% modulation, and if you go past that you can sound distorted in most receivers.
Unless its an sdr with a sync detector, most receivers distort badly over about 150% positive modulation.
Many normal transmitters will not do 4 times carrier without a lot of work, plenty of audio power, separate power supply for the modulator, tweaks to the screen modulation in tetrodes, etc.

I can adjust my screen modulated rigs to run silly low carrier power and super high peak power, say 100 or 200 watts of carrier and 1500 watts pep, but it does not sound good, and can start being dsb with reduced carrier which is what it actually is...

When you say "four times carrier" you do mean four times the power, right?

Edit: obviously... 100% mod goes from 0 to twice carrier amplitude which is four times the power  ::)

I will try adjusting the audio gain, loading and screen bias for twice the carrier amplitude on peaks... until I get this thing on the air, it's pretty hard to tell what it's going to sound like (ever tried to listen to yourself while you talk?)   ;)

I listen to myself in the mod monitor all the time, I want to know if something happens to the audio.
Once you start doing that, its like an SDR bandscope, its hard to stop using it.

AM modulation: 100% modulation is 0 and 2X the plate voltage on the final tube.
When you double the plate voltage, you get 4x the output power.
So if you ran 15 watts of carrier power, you should have 60 watts PEP.

In many plate modulated transmitters, the modulator power supply sags a bit on peaks and can not deliver full peak power, or the final tubes run out of emission to handle the peak current you would have with 2x the plate voltage, sometimes the screen gets in the way in tetrodes, sometimes its a grid drive/bias problem, but many rigs may only do 3x the power output.
My stock 32V's would only do 3 times the carrier power, mostly because of the screen voltage setup on the final tube.

Anyway, its great you are able to get such nice peaks, but more is not always better.
The loading control sets the carrier to audio level, you can adjust it to give 60 watts carrier out with only downward modulation or 5 watts carrier with 60 watts pep output.
If the values in the output were right, you might be able to adjust things to get one watt carrier and 60 watts pep, it comes down to the tank coil and loading cap..
For the lowest distortion, you would want about 4 times the carrier power as a pep reading.
Measuring average power, it should stay steady, not go up or down.
That is another way to tell things are adjusted right, you get 4X the carrier power and the average power does not change. Not enough loading and the average power goes down, too much and it goes up.

So, if you start from the point of maximum power output, more loading (loading cap more open) gives less carrier power output and more peak power.

4 times carrier power only exists for sinewave program. If there is sufficient bandwidth that rule of thumb can be exceeded.

I set it for more carrier (maybe 12W) and still 60W PEP on voice peaks, and was able to get into the Noon Forum on 7295 with my "All ARC-5" (both the rx and tx) and a homemade TR switch ;D

Jon AC0OQ in Iowa, today's NCS, reported I was coming in S9, loud and clear, but with slight audio distortion, although it looked OK on the scope. My mic is an old Shure hand-held so definitely not hi-fi... He was also using a notch filter for heterodynes and a couple other guys got the same audio report, so maybe it's on both ends.

Right now the "leenyar" has the filaments (only) lit, cooking for half an hour or so (I've read about power tubes flashing over if they've been dormant for a long time, and it's been close to ten years since I fired it up).

That amp looks nice, just plug in some 4-400's.

If you get reports of being distorted, you need to find a way to listen to the audio and adjust things for the best sound.

Once you get the screen voltage and audio levels in the ballpark, you can adjust the grid drive/bias and the loading control.
Did you add the feedback from the screen modulatior output to an early stage?
That helps a LOT. Makes things less dependant on grid drive/bias.
 

Don't have enough iron (plate or filament)... my Thordarson plate transformer is rated for 900W CCS and there are two 5V, 14 amp filament transformers. I designed this amp with the ARC-5 in mind, and to use 4-125A's because they are cheap and readily available (I think four of them cost less than one 4-400). At least they were ten years ago!

I let the filaments and fan run for an hour, then switched on the HV. 3.4 KV with a light bleeder, 3.2 KV with 100 ma cathode current (all four tubes) with 400V on the screens, some orange on the plates at 80 watts per tube which is normal. No magic smoke release.
 
Couldn't remember how I wired the 24V T-R relay and tired of playing with radio today - not good to be tired or impatient when working near 3 KV supplies with 56 uf filter caps. :o Maybe over the weekend I'll get back to it.

Although, after looking at the specs, I could plug 4-400A's into two of the sockets and leave the other two empty. Still don't have enough plate transformer to support full legal output.

Anyhow, I sorted out the TR switch mess and got everything to key up with the mic button on the ARC-5. Unfortunately the amp is not working (well, it's putting 0.75 RF amps into the 50 ohm dummy load, and the ARC-5 by itself makes 0.5 amp)  :P Meters say the voltages are normal (no screen current on the panel meter, but that's not unusual in AB1 with only 450V on the screens).

Cathode current in "Tune" with no input is about 90 ma with everything warm, and 120 ma with the ARC-5 driving it. Plate tuning is very peaky, and most strangely, with the loading padding caps switched out, the output power is still rising as I reach max capacitance on the loading vacuum variable. But the output drops with the first padding cap in, no matter what I do with the variable, and even worse with more caps!

There should be. as usual, a broader peak of plate tuning with increasing loading, and of course more output. So something is definitely screwed up. Maybe a bad solder joint somewhere. Got to scope it and see if the drive through the TR relay, swamping resistor, and 4:1 unun is actually making it to the grids, to begin with. Working anywhere near that HV supply still scares me though...

Edit: After studying the 4-125A data sheets again, there should be a few ma of screen current with the input swinging the grid about 100V p-p. Screen current could be zero or even negative with no signal or severe underloading. Also 120 ma cathode current isn't near enough, should be more like 200 ma at full output. I'm betting there's an open in the tank circuit. We shall see...

I remember fighting through some similar deals with my twin 811A amp. I could orange the plates but got next to zip out. I had to revamp my plate choke and even remake my parasitics. It took very little difference to cure. I also experimented with link output vrs PI and settled on PI . Getting enough C in the load cap was a big deal. 1000pf just didn't cut it. But you are a doer and I know and can see you will sort it out. Till then,
bTW picked up a pair of MkII no 19 wireless sets. One is supposedly NOS unused. No cables but I didnt have any for the Arc5s either. Output is less then an ARC5 as only one 807 as opposed to two (1625s) however it should be able to excite the amp enough to enjoy it. They are coming on the bus from Northern Ontario sometime today. Meanwhile to your success!
don   

yeah, some of it was operator error (have three taps on my toroid tank coil and only the middle one gives decent output). Also it seems as though the vacuum cap on the loading side of the pi-net works "backwards" (clockwise rotation of the shaft decreases the capacitance). Since it has a ceramic body I can't see its plates.

Also I should have known better, that adjusting the loading cap for optimum output with the unmodulated carrier is NOT the correct loading at full output  :-X

Furthermore, I may have been planning to use this amp with the Heathkit single-banders I used to own - there is a T-pad that I tucked under the chassis and forgot about, before the bifilar step-up unun (4:1 Z, 2:1 voltage) with a 200 ohm swamping resistor.

I jumpered around the pad and eureka! 280V p-p carrier into 50 ohms (which is 200 watts).  Maximum on voice peaks is just about double (540V p-p) which is 730 watts PEP  8) Not sure yet if the ARC-5 or the amp itself is the limiting factor.

AM Noon Forum on 7290 was just ending, naturally (and being interfered with by on- or nearby frequency SSB activity as usual). There was a good round-table going on 7160, mostly 5-land, but I only tried a couple of times to break in barefoot and they didn't hear me. I'll see if I can try the noon forum again in the next day or two.

Don, what's a MKII? Pics?

I was on 40 today till about 12:30, 7295.
If you hear me on, call me.
It would be fun to work screen mod to screen mod rigs.

What did the 4-125 plates look like at 200 watts out?
I would think you would want to run them at about 150 watts out and 600 watts pep..

Didn't hear you but it was just about that time or later when I got on the air... will listen to 7295 and 7160 tomorrow or Mon. morning if I get a chance.

Plates were moderately orange which I believe is normal behavior for these tetrodes. Even at no-signal idle, 85 ma at 3250 V = 280W, or 70W per tube, there is still a dull orange glow on the plates.

I have a good sized muffin fan blowing across the bases and envelopes whenever the filaments are on, and large finned plate caps, which should keep the seals out of trouble. The RCA and Eimac data sheets for 4-125A/4D21 claim that convection cooling alone (sockets mounted in the clear to allow circulation) is sufficient at the full 125W dissipation if below 30 MHz, and that no more than 2 cfm of air is plenty at higher frequency max ratings. With four tubes mounted close together I wanted a bit more cooling than that - these tubes are cheap but not that cheap.

I have not seen the meter show more than 200 ma at 3200V = 640W DC input, and that's with 200 W coming out, leaving the remainder safely under the 500W total max dissipation  8)

It'd probably be a good idea to turn the wick down just a bit, though, in the interest of extended tube life. 1 or 2 dB shouldn't make much difference. But going from 10W to 150W (12 dB) sure will!

Normal operation is a nice bright glow, no problem doing that.
What you want to do is (somewhat) match the carrier power to the peak power.
Things can be adjusted to give about 4 times carrier power on voice peaks.
200 watts carrier would be 800 watts pep.
If you can adjust things to do that with the tubes below white hot you are golden!

If things do a maximum of 600 watts pep you should run the carrier at about 150 watts.
You do not want to flat top, or run 100 watts carrier and 800 watts pep, the least distortion will be around 4 times carrier power.

Its going to be a mess adjusting things, the screen modulator (output voltage, audio level), the transmitter grid drive, loading, the amplifier.

Heard some guys on 7295 this evening and joined in the roundtable, then continued a good QSO with Tim KC9QJE. He told me my signal sounds good once I turned the audio gain down a bit, which is good news. Still S9 barefoot, but nearly S9+20 with the amp so it'll be really helpful in marginal conditions... that's what it's for, after all.  :)

However, he reported that on his Softrock SDR display, my signal is about 3 KHz wide, with or without the amp, but that nearly all the energy is in the upper sideband, hardly any lower sideband.

I'm trying to figure out how that's even possible. When screen modulating a MOPA transmitter, I would expect both sidebands to be equal and 1/2 the amplitude of the carrier at 100% mod. Could there be a tuned circuit in the ARC-5 that is tracking slightly high - but it would have to have an impossibly high Q to reject the LSB less than 6 KHz away at 7.295 MHz. What's going on??  ???

I have seen that on some transmitters.
I have been told its phase modulation, audio getting into the vfo.

3 KHz is very narrow, what would limit the audio?
What do you have for screen bypass caps on the 1625's?
Should be about 500 pf I would think.
 

3 KHz is not that narrow if it's only one sideband though - would be 6 KHz with both sidebands, right?

I'm using an old handheld Shure "CB-10E" mic. Very heavy cast aluminum case and heavy coil cord. I'll hook it up to my HP 3580A audio spectrum analyzer and see where the high-freq rolloff is with my speech patterns. I suspect it's 2 to 3 KHz by design. SSSSSibilance. Sibilance.  ;D

There is a small LC filter I added to the input, to keep RF out of the audio. It's 30 uh series and .0022 to ground. Now that might be rolling off the incoming audio - don't know the Z of the mic. Really I'm not looking for hi-fi audio anyway - why run highs up to 5 KHz and use even more bandwidth?

No other low-pass elements between there and the PA (except the screen bypass caps). Those caps are original to the ARC-5 transmitter (.002 uf with a series 620 ohm resistor).  A 0.002 uf cap at 5 KHz has a reactance of 16k ohms. Shouldn't be doing too much shunting with the cathode follower driving the screens...

Phase modulation from audio into the VFO? OK, but how's it getting in there... The VFO is now regulated by two OB2's at 216 volts nominal, and the only other load on the 280-300V unreg supply that feeds the VR tubes is the first stage of the speech amp, which pulls less than 1 ma and is RC decoupled anyway.

Or is the oscillator pulling with the modulation on the PA? Several KHz though? I'm still not clear on how that would eliminate the lower sideband anyway...

3 KHz is ok if its only one sideband.
I do not know how it does it, but if the vfo gets pulled with modulation, its in one direction.
Most times I have seen it its the lower sideband that is thin or missing.

That is also what Tim 'QJE said... he saw the spectrum of another ARC-5 (couldn't recall whose, though) that was also missing most of the lower sideband. It would still takes a lot of incidental FM (or PM) to suppress it near-completely though. Maybe they do "all do that"!  ::)

This article was interesting reading:
http://ece.wpi.edu/analog/resources/hp-am-fm.pdf

(in particular, page 46 on asymmetrical sidebands).

Anyway I don't think I'm going to worry too much about it, since the signal sounds decent if I'm careful not to overmodulate (no ALC in this system), and I'm not about to add a buffer stage to an ARC-5 transmitter.

Congratulations. I knew it was something minor. 
Mk19 AKA No 19 Wireless Set. (2 to 8mc low power transceiver with also a 235mc transceiver inside. Jeeps Tanks AFV and occasionally even the odd aircraft. 1940s to 1960s.)They were supposed to arrive yesterday by bus but have disappeared . Hopefully it is the weekend thing since they are bought and paid for. Will picture them when they arrive. I totally ignore SDR reports.  This mode is going to kill AM and all the fun. I just say thank you and move on.
don

SDR is a mode? Who knew?

Come on 'HX, you knew what he meant... "mode" as in what equipment he's operating, not the more specific definition of the type of RF emissions  ;D

I actually am interested to know what the output of my transmitter looks like in the frequency domain as well as the time domain. I don't have a spectrum analyzer with sufficiently narrow resolution to get a good look (min RBW on my HP 8557A is 1 KHz and there's a fair amount of displayed jitter at that setting).

Fortunately it's "minor" in the sense that it does not seem to affect intelligibility and is not splattering onto anyone else in the crowded 40m band. If anything I'm using less bandwidth!

 ;D Yea, hyperbole used to illustrate. I think the "mode" is not giving a damn what sort of signal is put on the air. That's a mode I cannot endorse.

Clearly, you learned that there was FM/PM going on in your rig from the dreaded SDR report. OH NOZ! Was the fact that your ARC-5 had FM/PM the end of the world? Not really. Was it useful that someone used their SDR (OH NOZ!) to give you a report? Yes.

Not all signal reports are equal, but they should not be universally ignored. That's just plain stupid.

(http://amfone.net/misc/ohnoes.gif)

A less than successful morning...  heard KC9QJE just signing off, then W9BHI(?) calling CQ on 7295. I came back to him, he got my call, then disappeared! I called him again and got a testy "This frequency is in use" from somebody... Oh well. Shortly thereafter the added rx gain control pot arced audibly and the rx went dead.  :o

I suspect that the pot's carbon track failed from electrolytic bypass caps charging up (to a measured 122V), with the cathodes disconnected via the muting relay I'd added. When the PTT is released, it dumps that charge through the pot, which is usually close to the end of rotation... it may be too much current on a small area of the track, not just the surge, but it'd been working fine until I actually started keying the relay. I think I'll replace the pot, and just use the relay to short the secondary of the audio transformer instead! There is a neon bulb at the input to the RF stage for overload protection anyhow...

Anyway I got to work on my 3-4 Mc. transmitter (BC-696A). I desoldered and removed the original connector on the rear (to which I'd previously soldered external wires), and replaced it with an Amphenol octal plug after carefully enlarging the hole slightly with a rat-tail file. The plug wiring is of course identical to my 7-9.1 Mc unit so they can be interchanged easily.

I hadn't had good results with this transmitter when I hooked it up ten years earlier, having taken the "Conversion Manual" shortcut of connecting the output of the roller inductor directly to an SO-239, then to a 50 ohm load. Now that I'm more familiar with the design of the ARC-5, and also screen modulators, I can see why that is the wrong way to connect one! (Recall that they are designed for a very short antenna that appears capacitive and only about 5-12 ohms impedance. Not at all the load that our typical dipole presents). On the scope, although there is decent carrier and a warm dummy load, there was no upward modulation at all at any setting of the inductor and coupling. Largest carrier at no inductor and 100% variometer.

I tacked in a small air variable set to about 68 pf between the inductor and the output (68 pf Russian surplus small doorknob cap on the way). Now, just as with the 40m tx, there is decent upward modulation once the roller inductor is set to about 60% tuning out the reactance, but even with the variometer at 100% coupling, the loading is still insufficient for good 2:1 (100% mod) or better peaks. Looks like this one will need two turns added to the output link - the 40m tx needed one additional turn.

That's enough fun for one morning. No contact, accidentally PO'ed someone, receiver blew up. Typical day in the shack  ::)

Charles,

   Sounds like a fine day. Maybe you will feel better if I told you my 8877 based amplifier croaked today. I drive the filament and cathode with RF equally through a trifiliar RF choke, but one side of the filament transformer is still grounded. Looks like I got a hard filament to cathode short. The amplifier keys at 300 watts carrier (was 200w before with an 8V zener in series with the cathode), and then when unkeyed the cathode current stays pretty high. A heater-cathode short will bypass the 50K cathode bias resistor when unkeyed, and will bypass the 8 volt zener when keyed..

   As far as your ARC-5 with incidental PM causing dissimilar amplitude sidebands, you are in good company. A high percentage of prized BA transmitters such as the EFJ Ranger also do this. The issue is the VFO runs at 40m when the transmitter is on 40M. My Gonset G-76 on 15m does this so well that there is a 30 db difference between sidebands. Switch to a crystal and they are equal.

   W8JI touches on some of this with the Valiant:

http://www.w8ji.com/johnson_vfo_chirp_jump.htm

Jim
Wd5JKO
 

Thanks for the link. Might be interesting to try pulling the 1626 and injecting a signal from a crystal osc... don't have a 7295 handy though - and then how do I know if the sidebands are equal anyway ;)

I've been outside cutting up the remains of a tree I had taken down 3 years ago, because I'm getting low on firewood just in time for a significant (for southern MO) snowfall and temps in the single digits (also low for here). Some of the pieces must weigh 4-500 lbs. I'm getting too old and fat for this lumberjack stuff! Maybe fun in the shack is a better way to spend my time.

Ouch on the 8877. I hope you can rewire your filament and cathode circuitry so you don't have to buy a new vacuum bottle...

Amazingly today there was a Scripture commentary that rang a bell on this subject from Psalm 135:16,17 regarding "eyes but they do not see, ears but they do not hear" etc. Being subjected to this mind set and attitude over and over on the air, made me think I was in some bizzaro universe where left was right, up was down 5% modulation was good and usable etc etc. ;D

Don't be an "expert" or a contrarian.

Al VE3AJM

It'll be a few days (probably more, with the snow/ice everywhere) before I can get a new pot for the receiver. Fortunately only one end was burned, so I just turned it around temporarily, and the gain control knob works "backwards" for now.
Also rewired the PTT relay inside the rx to leave the gain control line connected to the pot, and instead disconnects the audio from the front panel jack and shunts it to a 1K 1 watt resistor to ground instead.

Meanwhile I added two turns of Teflon-insulated wire to the 80m transmitter's tank coil in series with the variometer. Naturally I guessed wrong in the direction of the turns (with one turn on the 40m tx it didn't matter) and had to redo it. Why is a 50/50 chance wrong 90% of the time?  ::)
Edit: Now that I think about it, actually it does matter even with one turn, I must have guessed right the first time.

Anyhow, now the 80m tx also has a nice 2:1 carrier/PEP (or better), although the screen bias pot needed to be readjusted for a different pair of 1625's. At the setting for the 40m tx, the 80m won't go below about 20% carrier.
Nothing on 40m except broadcasters and what sounds like some kind of contest (is the K1N DXpedition still on the air?)

Now I'd like to have a 3-6 Mc. receiver to go with it, but may have to wait a long time for a bargain like the 40m rx/tx I found at the flea market here. The only one on the auction place has a starting bid of $25, missing all the tubes, dynamotor and both covers, has lots of "hammy" wiring hanging out underneath, and he wants $20 to ship it 500 miles. I don't think so.  :P May just have to drag my Hallicrafters SX-110 out from under the bench!

Edit: there's another 3-6 Mc. rx starting at $49 and $20 shipping, and it looks like it was run over by a deuce and a half... also there's a new listing for the low-freq (190-550 KHz) buy-it-now for $26 plus $19.90 shipping, looks complete except for the top cover. If you want to make a "Q-5er" this might be worth buying, since it has a very sharp 85 KHz IF.

6=8

(http://amfone.net/misc/ohnoes.gif)

there should be one of these with "KEEP CALM" and "ALL IS WELL" over the figures. maybe with flames in the background.

Somebody bought the 190-550 Kc receiver within hours. $26 even with $20 for postage is not bad for one of those in decent shape. Oh well.

Anyhow there was a good roundtable going on 7295 this evening, which I joined while making some RG-58 cables (attaching PL-259's is something I haven't done in a long time, but it came back to me quickly... and I didn't leave any outer shells off before soldering)  :D

Just wondering, is there much AM on 80/75m these days, or is it mostly 7160 and 7295? Not many posts on the 80m activity board here.

Attachment for Opcom:

Just consider it part of the charm of a  classic - - or a challenge to "fix" it.   ;D

Al

Lots of AM on 80 meters in the mornings and mostly at night, starting around 5 pm till whenever, 3870 to 3885 and some lower down in the extra section.
What equipment do you have?
No general coverage receiver?
If you have a computer, you can download the sdr-radio software and use an on line receiver.

For AM I also have the BC-696A 3-4 Mc. transmitter which is now wired up to match the 40m unit (with an Amphenol male plug in the back) so I can swap it in a matter of seconds. The amplifier will work on 80m also. I did mention in passing that I have a Hallicrafters SX-110 so could use that for reception. And my Heath SB-102 transceiver but I'd be twiddling the knob constantly, trying to zero-beat everyone's carrier...

Problem with 80m is the range is so short in daylight, and too many hold-the-frequency-but-never-talk types in the evenings. Will see if the SX-110 is working and can take a listen tomorrow on 80.

80 meters at night takes some power most times.
Around here, there are the usual suspects on 3873, and some groups on 3880 and 3885 out west or down south, they get taken out mostly by the 20+KHz wide class E guys on 3873.

Daytime 80 is mostly very local, at 5pm you can have a nice qso before the zoo starts sometimes.
40 is often good all day and night as long as the broadcast is not on.
That comes and goes, some stations only transmit an hour or two.

Hmm... I may stick to 40m then (besides, I am waiting for a reasonably priced 3-6 Mc. ARC-5 receiver to use with the transmitter).

On 7160 just now I heard a VERY loud signal from W5WN in Plano TX talking to W0ERE who I could copy only some of the time. QRZ says he's (too) close to me in MO and he said he's only running 13W carrier. Fading is particularly bad today on 40, too. Of course the guy I could hear was just signing off...

Anyhow I was considering building a 3" modulation monitor scope directly into the mostly blank lower rack panel of the amp. I have a 3" CRT, bezel and neck shield, and a HV board with some caps, high-value divider resistors and socket/harness from an old Tek scope with the same CRT base. Don't even need a vertical amplifier, and a simple sweep generator is trivial. The hardest part may be cutting a neat 3-1/8" hole in the 1/4" thick aluminum rack panel...

For  opcom 6 does =8. slant six against 427 Hilborn injected and racing heads up.(I crashed on this run , broke my spine.)
No 19 wireless next. 8 watts of AM
Anytime you wish to try I will listen. Early morning here and early evenings is best for 80M. Just give me a heads up and a freq
don

Any reason you do not use the sx110?
Or get a viking 2 or something?

I did start this project with the aim of using ARC-5's, which has been a reasonable success on 40m. Of course, I could use other equipment to get on AM be it 40 or 80 meters...

But there is definitely no room in the shack for more equipment (and anyway there are few hamfests within a reasonable drive of BFE, Missouri).  8)

I remembered that I had a Tek RM561A (rackmount) scope just gathering dust in the other room, so I pulled it out, dusted it off and mounted it in the rack underneath the amp. It's only good to 15 MHz but that's fine for 80 and 40!

With a small cable running from the vertical plug-in to a simple divider tapped to the amp's output, now I've got modulation monitoring in real time. And the added advantage of having a scope right there at the rig, should I need to measure anything else.

Unfortunately the scope's muffin fan is extremely noisy. It sounds like a vacuum cleaner, or a prop plane running up its engine in the room, and dwarfs the (new) fan I use to cool the 4-125A's. They are fairly cheap, and it's on the back of the scope so I won't have to disassemble anything to get to it. I may even have another quiet fan in the parts bin.

I picked up a BC454 at a hamfest. The guy gave it to me and I use it with the 80M rig. It was badly beat up but electrically perfect. You could do a crystal converter and tuned front end in front of you 40M rx which would be easy . I use a monitor scope , 2 inch tube I built from an old ARRL book although I only hook it up when necessary. Uses a 220 volt AC transformer for sweep.
 I have seen BC 454s on ebay recently even. Anyway sounds like you have it all in hand.
don