The AM Forum

I have decided to use a Pullen Mixer in an  am  receiver that I am building.

I will use a 6C4 local oscillator. I have seen some comments that say a cathode follower buffer might be necessary and some say that the Pullen only lightly loads the LO. Do I need a CF?

I am in the process of punching holes and trying to get this thing layed out.

Pat
N4LTA

The second section of the Pullen is in effect, a high Z input, so it takes care of the loading situation.

Mike WU2D

Anyone have any suggestions regarding the terminating impedance for a Pullen mixer?  I assume one wants to avoid the high off resonance impedances when driving a xtal filter; so what would be a good starting point for the grid resistor on a cathode follower? 15K or so?

Pete K1ZJH

If you research the subject you will find that Keats Pullen and follow up published info suggest a buffer.

This may not be required with a crystal oscillator.

Carl
KM1H

Yep - That is why I asked the question - Pullen said a buffer might be necessary - yet the input impedance is very high and others have not used one and said it is not necessary.

So - has anyone built one and used or not used a buffer?

Pat
N4LTA

Impedance of the LO or Output port?  I'd think you'd want the output feeding a fairly well defined, broadbanded termination, lest the voltage gain soar when the crystal sees signals that are off resonance and the filter goes to a very high impedance.

Pete

There will be no crystal filter. I am asking about the LO port.

The problem with the buffer stage is deterioration of the performance of the mixer.  It inevitably has some nonlinearity and internal noise.  You want as few active stages as possible between the antenna and main selectivity filter.  This includes rf amplifiers, mixers, i.f. amplifiers and buffer stages of any kind.  This is the reason the dynamic range of multi-conversion receivers is inferior to single conversion ones.

Ideally, a receiver would have no tuned rf stage at all, but instead a passive tuned network between the antenna and mixer, and the mixer would feed directly into the crystal, mechanical, L-C or whatever selectivity filter is used.

You need a mixer with enough gain that the signal coming in directly from the antenna is able to overcome internal noise in the mixer and deliver enough signal through the filter that the weakest signal received will overcome the noise floor of the i.f. amplifier and stages that follow.

One of the better mixer circuits uses a beam deflection tube, such as the 7360 or similar.

Diode mixers are extremely linear, but have insertion loss instead of gain.  An extremely good receiving antenna that collects plenty of signal would be needed in order to use one of these without an rf stage to directly feed the selectivity filter, and still be able to pick up the weakest signals all the way down to the atmospheric noise floor.

It would be interesting to compare the performance of the Pullen vs beam deflection mixer circuits.

Ive built quite a few Pullens Pat while using a S-40B as a test bed. Tubes tested have been the 6SL7, 6SN7, 7F8, 12AT7, 6ES8 and 6J6. In that radio pulling was noticed on the 2 higher bands.

Its also been apparent on the highest HF band of a NC-183, SX-28, a pair of HQ-140X's, and HRO-5. However since my primary use for vintage radios is AM with a bit of low band CW its not a real issue.

As a product detector driven by an unbuffered BFO Ive experienced no pulling in any radio and Ive used up a lot of my NOS surplus 6J6's that way.

On a project that will be used for SSB/CW Id suggest buffering a free running first LO that doesnt have a high degree of built in isolation. Also be sure there is sufficient LO voltage.

I havent modified a xtal controlled LO set yet but a R-388 and 75A2 are on the project list where I doubt there will be any pulling.

A cathode follower at the IF output of a Pullen would be a good idea for isolation and feeding any low impedance circuit or filter.  Any noise contribution can be simply dismissed as inconsequential as the front end has already established the system noise figure which should be below band noise even on 10M. I use a HP-8970A NF meter to verify performance. In a multi conversion design I wouldnt use circuits that would degrade the benefits established by the Pullen. This means dont use a 6BE6/6SA7!

I have not spent much time on the W2DXH Pullen revision. The single attempt using it as a first mixer led to instability. Maybe revisit when I have time but for now the basic Pullen is fine for my needs.


Carl
KM1H

Carl,

This is a receiver that I plan to use on 80 and possibly 40 for AM reception. I have left some room for a BFO - But initially it won't have one.

My plan was to use it to test some ideas and make a very decent receiver for monitoring the local AM net. It will be a table model radio as such. It will have no RF amp (not needed at these frequencies) will have a 6C4 LO that will be as stable as I can make it, a 6ES8 Pullen Mixer  - The 6ES8 is probably overkill but I have some, 2 stages of  455 Khz IF, a detector that I am still pondering - (6AL5 biased diode or an infinite Z detector and I probably will have a space for Steve's (QIX's) solid state detector/AVC circuit. I made a PC board  for it and have not built it)

I thought seriously about a 7360 or the TV tube you suggested (I have both of these) but the problems of the input port not liking complex impedances made me a little gun-shy. I don't need or want a RF amp ahead of the mixer.

It is more or less a receiver experiment for fun but I do plan to use it. So far  - I have the power supply working and the 6BQ5 output tube working fine.



I got around to wiring the 6C4 LO tonite. Using a hand wound inductor on an old 3/8" ceramic form (tapped for feedback) I got it working at around 3.9 mhz first time with a single 270 pf NPO cap. That's in the ballpark of what I was looking for (4300 Khz)so that is working OK. I'll figure out what size variable I need and add 2 or 3 NPO's and maybe a polystyrene to get the LO as stable as I can.

 I get about 7-8 volts Pk to PK on the feedback tap.

Pullen says that for some tubes a LO injection voltage of 2-5 volts Pk to Pk works fine. Has anyone used a 6ES8 and remember what the injection level was?

Thanks for any additional info. I hope to get the mixer wired tomarrow night.

I answered your PM already with the injection info Pat.

Let me know how the 6ES8 handles with no RF stage. Thats something I dont believe in even for 160 for any radio.

Carl
KM1H

What would you suggest as a RF amp ahead of the mixer for 80 and 40?

Pat
N4LTA

I am with you Don!
I use a very high Q tuned circuit with a link coupled antenna input right into the mixer, then out of the mixer is the filter.

You need no rf amp before the mixer below 14 MHz if you use the transmitting antenna.
Many radios were designed for use with little short antenna's and even with the RF amp work poorly.

My homebrews have more gain than is needed, and I run the IF gain at 3/4.

When does mixer noise become important?
I just use a crappy setup, LO in one grid, ANT in the other, and with the antenna input shorted its dead quiet.

With the antenna port hooked up to the station control, but no antenna selected, or a different receiver selected, I can hear stations, that is with no connection to the antenna, a 1/4 gap on the rotory switch.....

When I had the R390a, and the 756 pro3, the homebrew was always better copy of very weak signals.
On the R390a and others, the signal was always down in the hiss...

Brett
 

I think Don misunderstood  what I was talking about in using a buffer.

I had not and wouldn't put a buffer in the RF path.

The buffer that I was asking about was in response to Keats Pullen's comment that the Pullen mixer showed a tendency to "pull" the LO at higher frequencies. He suggested  putting a cathode follower to buffer the LO. Carls tests showed the same LO pulling at higher frequencies. Since this receiver is not going to be used above 40 meters, I am not going to install it.

Conventional wisdom is that you have as little signal altering ahead of the mixer as practical. That is why I am not putting a RF stage ahead of it at the low frequencies that this receiver is designed for.

Carl disagrees and I would like to hear his opinion.

The 7360 mixer is supposed to be a very good performer but has been reported to have bad tendencies if the RF port is not matched to the design (usually 50 ohms). The usual way to correct this is to add a RF amp stage with a known constant output impedance. That somewhat defeats the purpose - in that the 7360 requires an active stage ahead of the mixer - or is rumored to require one.

Wire antennas rarely supply a constant impedance to a receiver.

This receiver is somewhat of a test bed and I want to try to observe some of the state of the art  tube technology. I have already build many simple and not so simple solid state receivers and a few superhet tube receivers. The ability to hear weak signals is not my goal - that can easily be done with a simple tube regen receiver - I find them to be about as sensitive as you can get - and also nearly unusable for communication.

Dynamic Range - IMD and Blocking are much more important to me than MDS

The best receiver that I have ever used is my K3 - hands down - I have not used a R390 but have heard most everything else including a 74A4 Howard Mills receiver I owned a few years ago..

The K3 Handles itself with close in blasting signals and is a joy to listen to. I have an ICOM 718 that I bough to play with a few years ago and after listening to the K3 - I can't stand to listen to the 718 with the grating background noise.

That's my opinion only.

Any AGC controlable tube will work Pat, even a 2.5V 58 setup for unity gain. ;D  The RF stage is used to present a reasonably constant load and match to the mixer and a single LC stage to the antenna doesnt cut it. A 7360 directly to an antenna is a known disaster with some antennas and Ive no experience running a Pullen that way. Reading thru Pullens notes and analyzing the circuit leads me to believe it isnt a good idea. An RF stage with gain wont faze it and you have some place else to use AGC control for an all around better performer. I doubt that even a double tuned passive preselector will provide sufficient isolation so why take a chance?

A 6L7 or 6K8/6SA7/6BE6 noise generator will be covered by a decent antennas noise on 160-40M and several basic radios do well that way, and if all you are looking for is absolute basics get a S-38, SW-54, or NC-60 and be done with it. But what you are going for is signal handling with the Pullen which is the whole reason for this exercise.

Once you get it all working it might be interesting to see what happens with the RF tube replaced with a 100pf cap.

Carl

I have room for the RF amp - I am building this thing piecemeal - so I will try it with and without.

The article that got me thinking about this was in November 2008 Electric Radio for a high performance front end using an Amperex frame grid tube (I don't remember the number) with a double tuned LC preselector in front of it  and a single tuned LC circuit after it all on a 3 gang variable cap.

Would you recommend a tube for the front end - reasonably high performance - in case I won't to upgrade this animal later.

Thanks,

pat
N4LTA

I use the 6GM6 almost exclusively for swaps. Lowest noise figure of the 7 pin tubes on 10M, and darn near overload proof. For 9 pin the 6EH7 and 6EJ7 are hard to beat.  The 7788 is hot as a pistol with obscene Gm but short lived.

The 6DC6 beloved by Collins can be a good overload performer but there had been some supplier problems resulting in inconsistency. Also RCA had been caught rebranding 6CB6's. Ive replaced the 6DC6 in my 75A4, SX-101A and SX-115 with noticable improvement. I havent touched the R-390A.

Carl
KM1H

I ordered several this afternoon (6GM6) after some searches showed some of you older posts.

I suppose a tuned circuit before it ( I like to use toroids) with a  link coupling to the antenna and a tuned circuit after it using a dual gang cap would be suitable?

Thanks again for your help.

Pat
N4LTA

Im a big dual winding toroid fan Pat. Ive installed several in boatanchors for a substantial SNR performance without tube mods which are then the next step. Having a HP Q Meter helps evaluate and select the best performers.

I get more enjoyment squeezing high performance out of vintage radios than I do using them every day for hours on end!  Kinda sick, huh ::)

Carl
KM1H

I'm more of a builder than operator myself. I am trying to do more operating.

Don't have a Q meter - Would like to pick one up - I do have some decent test eq. - HP SA - HP RF Gen  400 Mhz scope - HP Pwr meter and some decent HP power supplies.

I have had some pretty good results using toroids in tube transmitters - I have a 40 watt CW transmitter using an 807 with a T130-6 in the pi-net. About 28 watts out and the toroid runs cool.

I had a 160 meter 4-1000A linear with a T400A-2 as the pi net inductor. Easy 2000 watts out and never had the slightest problem with the toroid.

As far as Q goes, what do you think has a higher Q, a toroid or some big B+W coil stock?

I dont have a Q meter, but had much better results with the B+W coil stock and an air variable cap than with any sort of cores.

Brett

I'd guess the B and W. It would depend on the type of core material. The ferrite material Qs are lower.

The Q on a piece of wire with an air core is going to be about as high as you can get. There is also a toroid core with a permeability the same as air - to allow the self shielding of the toroid form and high Q of an air core inductor.

I'd never use any type of core material on an VFO inductor toroid or otherwise - only air.  Some say the type 6 and 7 material is OK but it all has variable permeability with temperature.


The problems that I have with the B and W stuff for low level inductors is that it takes up space and is not self shielding.


BTW - Thanks for the info you sent me. It has been very helpful.

pat
N4LTA

What I did was mount the B+W coil for the input on an 8 pin octal plug, then installed a socket for it on the chassis. That way I could experiment with different setups.

I tried tunable ceramic forms, untuned ceramic forms, and the B+W coil stock.
I also tried various multi coil and cap setups, but what worked best was one B+W coil with link antenna input, and a 365pf cap.
If you get the coil right, it can cover 160 to 40 meters and is has very sharp tuning.
The Q must be high enough that I don't seem to have any image problems at all.

The first receiver used slug tuned forms for the LO, and along with the octal tubes drifts a lot on startup.
Its good enough for AM, plus the frequency readout is digital so shows the actual frequency (no calibration needed), I just have to retune for the first 2 minutes or so.

The 2nd receiver LO used the B+W coil stock in a small metal box, and it drifts very little, at startup or after.

I read all sorts of stuff in old QST's about guys building uber stable VFO's using huge coils in even bigger boxes, like transmitting coils!

Brett

A mix 6 or 7 powdered iron toroid makes for an extremely stable VFO for a FET. Id suspect that it would be more than adequate in a Vackar 12AT7 type circuit also but I havent gone that route yet. Never use ferrite cores in a VFO. Even slug tuning requires playing forever with TC circuits.

A lot of my HP and other high end test equipment came from various internal company auctions where I worked or publicized belly up ones. The HP 4342A Q meter shows up at times at ham prices. A real sleeper is the military version TS-617D/U. The TS-617C/U is a superb late 70's tube Q Meter that doesnt have the inherent problems of the Boonton which are also available under earlier TS-617 letter designations.

Ive had no problem using various size 2 Mix powdered iron cores to add 160M to commercial amps up to the 1500W level. Alpha and others have used toroids since the 70's at QRO levels yet hams generally seem to shun them.

In vintage receivers the coils used are pretty lossy on the higher frequencies with Q's measured in the 100-125 range. Some 30's sets with less developed insulator technology have soaked up enough moisture and pollution that Q's are much lower. It makes me wonder what will happen to the uber cheap Heathkit and other basically paper cores as they age? A 2 or 6 mix of at least a T80 size will measure around 225-250 and offer a 5-15 dB improvement in images just as the antenna coil. Tracking is straightforward by tweaking turns spacing for the low end and the original trimmer at the high end. I havent used them in the following stages.

My HB project is using all toroids with relay switching. Nothing destroys the Q of an airwound coil faster than placing it inside the chassis with other coils, parts, and wiring. Its not much better with slug tuned coils and all the associated switching and haywire associated in the average boatanchor. The last time I used plug in coils in a receiver project was around 1956 ;D.  However they do the job in the FB7-XA and various HRO's in use here.

Carl
KM1H

It's *always* best to use a buffer stage between the osc and next stage - be it mixer, mulitplier, or whatever.

Instead of using a 6C4, why not use a 6J6.  Takes the same room.  Cathode couple (ie don't completely bypass the common cathode lead).  Grid leak of 100K or so.  Take the signal off the plate of the second section.   That's the way I did it in my HB and I didn't notice any pulling at all.

A buffer can't hurt and a 6J6 will fit into the same hole - It is eating up my heater current budget though. I ill probably install it instead of the 6C4.

I have built quite a few stable SS VFOs with the 6 material in a better temperature situation than a tube VFO -  No core should be better than a type 6 core - I have some of the zero material and might try it in this LO.

Thanks

Pat
N4LTA

A bit OT but Ive been waiting to have a 6C4 problem in a Hammarlund and give the 6J6 a shot with sections in parallel.

Carl
KM1H

This is for AM right?
I just used the 6C4 LO right into the mixer, no pulling...

What would it be pulled by?
I can do stuff (tune the antenna) without any pulling.
Strong signals dont change the frequency, so I assume I have no pulling.

It sort of sounds like you guys are worrying about problems that wont be problems?

In a VFO where the load changes, I can see using buffers, but in a receiver the LO just looks into a grid of the mixer, which is fixed output (filter or IF trans).

Am I missing something here?

Brett

Right now my oscillator has a tapped coil with the tap feeding the cathode and the plate bypassed to ground.

If I read you right - you are saying that I can add a  100 K to ground on the second grid and a 47K or so from the second plate to B+ and capacitively couple off the second plate?

LO pulling is pretty common at higher frequencies - especially with high dynamic range mixers.

Dr. Pullen suggested that a buffer be used - it probably may not be necessary up to 20 meters but it is never a bad idea - especially with a free running oscillator. That was what my original question was about - I had decided not to use one for this receiver - then W3JN came in and messed things up  - and I fully agree with him - it can never hurt - I probably will do it if my heater budget allows - I have to come up with another .3 amps.

OK guys  - I need you to make me doubt my design again - this is AM only and I want to have good Hi Fi performance - What detector is best   - a true diode 6AL5 like HH Scott used or an infinite Z detector?

Pat
N4LTA

I tried a number of different types, the only one I could get to work well with a wide range of signal strengths and agc was the one I am using.

For a simple tube circuit, distortion is very low.

Maybe try few different designs...after all, you may end up with whatever you can get to work well.

My entire receiver is a bunch of circuits I could get to work, I tried different LO designs, different mixer designs, different IF amp designs, different antenna input designs, and what I wound up with was all the designs I could get to work well or at all!

Once I got the basic design down, I built another receiver with all the improvements and less extra holes....

The experimentation was the fun part, all the different LO designs and parts value changes, watching the signal on the spec-an for amplitude over the freq range, and stability.
I was lucky to have a real good spec-an that allowed me to watch the drift, frequency range, output level, etc.
I did all sorts of tests, even running the LO plate voltage down to something like 10 volts (it still worked!).

I think you might be disapointed if you think you will have a final design, then build it and have it work well.

Brett


 


Brett

Most vintage commercial radios with free running LO's pulled on strong signals or when the RF gain control was run up to copy a weak signal and a strong station fired off nearby.  Some did it as low as 40M. Ive stuffed several 6C4's on brackets under the chassis to fix.

Yes, Ive mentioned modular construction on here and elsewhere before. Its something Ive been doing since the early 60's and it makes changes and trouble shooting a lot easier.

The best diode AM detector Ive run across so far is in the HRO-60. Jay, W1VD, had good things to say about its performance recently also in his review series. Im still up in the air what to use in the HB project. ER also had some mention of the subject but with the sometimes questionable articles they run Ive put any serious analysis aside for now. I havent reached that stage yet.

I dont know if any of the real high end consumer radios going back to the Scotts, McMurdo Silvers, Zenith 1000Z, and later ever really addressed the issue. More reading I guess.

Carl
KM1H

Here is a HFO that I originally put into my Super Pro 400 using a 6SN7 but I modified the schematic for a 12AT7.

Mike WU2D

LOL - sorry for the thread hijack - didn't mean to wreck the consensus  ;D

Build the osc as you were going to using the first section o f the 6J6.  If you're using a design with the tank coil tapped to the cathode, this will provide enough coupling to the second stage.  You don't need to feed the grid of the second stage, you can ground it or just put a grid leak resistor to ground.  Then feed the second plate thru a 5K resistor or so (experiment here) and capacitively tap off that.  Or you can go with a wideband toroid transformer off the plate as well.  I wound a toroid with a split secondary to feed the deflector plates of a 7360 180 degrees out of phase.

There is nothing new about the cathode follower oscillator either, CQ for July 1947 had a 6J6 version that claimed 50 cps drift on 80M in an hour after a short warmup. Im pretty sure they were used in the late 30's also. Cant get any simpler than a 6J6 for sure.

Carl
KM1H

I like the idea of the 6J6 with my existing oscillator. I'll try to get it built in the next few days.

The 12AT7 would do the same and be less of a strain of the heater supply though.

I am not interested in building a simple S38 or equal - did that a long time ago.

Pat
N4LTA

The 12AT7 would give you the advantage of having separate cathodes, and thus you could plate-grid couple the buffer rather than cathode couple it.  I thought maybe you had a 7-pin hole punched already for that 6C4.

I do already have it punched - and the 6C4 working. That's why I will probably go with your solution.

I am going to try your method this afternoon - Your saying put a 47K or so on the second grid to ground.

Then put a 5 k or so plate resistor (second plate) and tap the RF off the second plate with a 100pf cap or similar.

Or maybe a T50-43 with 30 or so bifilar turns in the plate circuit - high z toward the mixer input?


What's your opinion on a good decent am detector?

Thanks,

pat
N4LTA

Yeah, maybe try it breadboard style first on a scrap chassis if you have the 6C4up and working to your satisfaction already.  I'm overseas and can't look at the project to see how I did it, it was almost 10 years ago.

All of this is from memory so you may have to experiment.  You could try putting the primary of a transformer in series with the plate of the 2nd tube and you can wind the secondary for the appropriate voltage.

Best AM detector is the infinite impedance detector in the ARRL handbook.  You need a separate AVC detector for this though, preferably a whole nother 6BA6 IF stage to drive the AVC detector separately.  But this might blow your fil current budget.  If not, just copy the HRO50 or 60 detector, it does very well.

Sounds like I need to add a 6.3 volt filament transformer. The HRO 60 detector is very similar to the Scott Hi Fi am detector using a 6al5




Pat
N4LTA

Guys:

I have both an early and late HRO-60. The early '60 has lotsa detector distortion. The later one is pretty clean. Dunno the circuit differences.

I built an infinite impedance detector using an MPF102 for a SS rx. It was one of the cleanest detectors I have used.

Dan

The HRO-60 did a respectable job of low distortion am detection...too bad the i-f bandwidth is so restrictive. Good for battle condx but not so good for hifi listening.

http://www.w1vd.com/NationalHRO-60.html (http://www.w1vd.com/NationalHRO-60.html)

JN touched on an important point...separate AM and AVC detectors. This is one of the biggest problems with the BA receivers and causes increased distortion at the low audio frequencies. Some receivers are terrible in this regard...others are okay. This is one area where the SDR stuff really shines - essentially complete separation of recovered audio and agc.

I'd even consider giving each detector it's own last i-f amplifier toob...

My last HB I did a phase splitter as the last IF stage and used 1 phase for AGC and the other for detection.

How did you split the phase - with a wideband transforerm center tapped? I might give this a try - I have seen seperate AGC with a small cap to a diode but worry this will load the stage.

pat
STF

How well are untuned buffer stages going to work say much above a few MHz?  I'd think phase splitters would even be more problematic?  JN, how did that untuned toroid work for you?

Pete

I have used bifillar toroid at 50 Mhz with a type 61 core and had no problems. Impedances were less than 1000 ohms though. A tuned splitter with type 3 material would work with a tapped secondary I guess. I have some T130-3 - 75 turns gives 200uh - would take about 600pf or so to resonate it at 455 Khz. An ARCO mica would work fine. An untuned one would be easier. Or find a 455 Khz transformer with a center tapped secondary.

Pat
STF

I used a 2N4416 AC couple into the gate with a 10 K to ground off the gate. 1 K 0n source to ground , Drain connect 1 K to 12 volts.  AC couple Source and a second cap on the Drain for the two phases. Very easy to do with a tube but might need higher resistor values.

The untuned toroid worked OK.  As I recall about 10 turns primary and 20 turns each secondary, bifilar wound, each driving a 7360 deflector plate.  A real PITA to wind.  But this was for a single-band RX.

Check out the skizmatic for the Squires SS-1-R.  It uses a 1N34 for the audio and a 6AV6 in a triode amplified diode detector for the AVC.  Never seen one like this before.  http://bama.edebris.com/download/squires/ss1r/SS1Rsch5.gif

That's an interesting circuit. Do you have any idea how well that radio works on AM?

Looks like they are forward biasing the diode and grid at 1.7 volts or so at the anode?

Pat
N4LTA

Gives me a headache to look at it.... :)

What are the series # of those HRO-60's?

Carl
KM1H

I put together a 6F6 oscillator on a small chassis to test. I have a conventional tapped coil inductor with the tap tied to the cathode and a 47K grid resistor to ground and a 100 pf NPO from the grid to the "top" side of the inductor. The low side is connected to ground. Th e plate is bypassed to ground and has 165 volts on it. I get a good 10 volts or better PK to PK at the grid.


The second triode has a 5.1 K resistor from +165 to the second plate with a .001uF coupling cap to the output. I have a 100K from grid to ground.

I get about a volt output non sine wave - almost looks like it is doubling or trying to.

Doesn't seem like I am coupling via the common cathode to the second stage.

Removed the 5.1 K resistor and replaced it with a wideband transformer wound on a FT37-43 core. Output still bad and low.

Changed the 100K grid resistor to a 47K - Waveform looked a little better. Put a 22pf cap from grid #1 to grid #2 - 20 volts Pk to Peak and a much better waveform but still not a clean sinewave - rounded at the top .
Maybe not enough inductnce in the coil?

Pat
N4LTA

Yeah, there may not be enough inductance in the feedback tap of the inductor to get good coupling between the 2 sections.  THat's OK though.  Less feedback is better - use as little as you can for reliable osc operation.

Looking back I don't think I used a L with a FB tap - I used the Colpiss (capacitive divider in the grid)... I may have used a RF choke in the cathode circuit.  Crap, I can't remember now.  But it did work FB.

Where you're at now, use as little coupling between stages as you possibly can.   You're probably overdriving that second toob a bit.  Try a 5 pF or so, and try a 10K resistor to ground?   Dunno.  Wish I had my stuff here, I could breadboard something up and try and replicate what I did  :-\

I'll cut back on the drive and use a 10K - That is the direction that seems to look better. When I get this thing working - I need to publish it on the net - as I have searched high and low for a schematic. it is a nice tube and circuit and as easy as a single 6C4 tube but with much better isolation.

Thanks for the help.

Pat
N4LTA

Cut the coupling cap from 22pF to 4.7 pf and dropped the gid resistor to 15K.

Much better - almost good enough.

Nice looking sine wave at first look  -  25 V Pk to Pk

(http://i620.photobucket.com/albums/tt286/n4lta/6f6osc.jpg)

[But the tops and bottoms were a little fuzzy with a thick trace. So I put my signal generator on the scope at the same frequency and got a nice clean sine wave with no trace thickening on the corners?

Not good - it's almost like a little FM - in my earlier days I would have said looks good enough. Put in on the spec analyzer and found the 2nd only 12 db down and the 4 th down about 22 db. Everything else was 45 db or more down.

I read that a problem with a Hartly or Colpitts is that they are not clean - yet I see them used as LOs all the time.

Any comments? Is it good enough - Do I need to filter it like I would in a SS rig - I sure don't like the second only 12 db down.

(Maybe I'd be better off without all this test equipment - not really)

Pat
N4LTA

I would experiment with values (operating points) some more.
After playing around with my LO, I was able to get a clean sine wave, with a really flat output level across a wide frequency range, a very low harmonic content, and stable operation with plate voltages as low as 20 volts, or maybe less, cant remember but it was stupid low!

Maybe isolate the sections, the osc, and the buffer and see where the fuzzy peaks are originating?

Dont settle for any trash on the LO....

Brett



 

I have a 47K grid resistor on the first stage of the oscillator. I may try to reduce it and see what happens.

Otherwise I may have to use a 12AT7 where I can separate the cathodes.

Pat
N4LTA

Less coupling.  Put a 5-20 or so variable cap in there and adjust, see what happens.  You could also mess with the plate resistor value.   Mine was very clean, yours will be too.

Right now I don't have a plate resistor - I replaced it with a wideband transformer. A resistor would be easier to use.

 When looking at the inductor tap with a scope - It has a hitch in the waveform - it is not evident when looking at the grid of the oscillator. I am using a 47K grid resistor. Maybe I should decrease the value.

Well, I finally gave up on the 6J6 LO. I tried changing the coupling with a trimmer and changing the bias resistor on both grid but nothing would clear up the am fuzzyness. Maybe I had a bad tube - I had a new one though -

This afternoon I punched a hole for a 9 pin socket and installed a 12AT7 using the same LC parts. I got a nice clean sine wave on the first attempt. I'll wire the other side as a cathode follower and couple it to the mixer which is built and seems to be biased correctly with 155 volts and 51 volts on the two plates. I don't have enough drive voltage on the inductor tap so I can't couple the Cathode Follower there. Plenty available at the inductor "hot" end - so i'll hook up the CF input there via a small NPO cap.

I still have to build two IF stages but all the sockets and IF transformers are mounted and ready to wire.

I still wish I could get the 6J6 oscillator working and documented.

Pat
N4LTA

I have a couple of old time 6J6 VFO/LO circuits that I'll scan and post here maybe tomorrow. Been kinda busy so havent replied earlier.

Carl
KM1H

I think maybe the tapped coil has caused the problems. Love to see the circuits.

Pat
N4LTA

Here are some from 1947-48 CQ's.

I built a few of the PTO version to go with the CE 10A, 10B, 20A and Phasemaster Jr I used in the 60's to drive various VHF/UHF converters. Didnt take too much TC work to get good SSB stability. Having use of a machine shop at work took care of the mechanical stuff ;D

Carl
KM1H

Thanks Carl,

I had searched high and low for something like this.

I think the hartley circuit does not do well with the common cathode. The untapped coil seems to be th eway to do this properly.

Pat
N4LTA

I also had no luck with feedback coils, or tapped coils for feedback.
I know they are common circuits but I could not get any of them to work in casual experiments.

Brett

I have always used the tapped coil Hartley. I usually tap about 20% above ground even though I have seen some old references stating a 50 % tap is correct.

Never had many problems in getting them to oscillate and find them more stable than the Colpitts. The extra caps in the Colpitts add to things that can be thermally unstable.

The problem I was having was  using the 6F6 second triode as a buffer. The common cathode made it difficult.
For some reason, I was getting some small amount of AM or something that looked like AM - a slight thickening of the trace at the corners of the sine wave. Using the same tank components with a 12AT7  - and it is not there.

I switched to a 12AT7 an the output is very clean. The second was down 58 db measured on the coil tap.

I then go through a conventional cathode follower with the second triode. I haven't got the biasing on the second triode perfect yet.  Hope to get that working tonite and get the IF strip wired.

I am cramped for room in the preselector. The dual gang 365 pf cap is bigger than I left room for. Wish I had a couple of 100pf V caps that have a rear shaft so I could separate them and put a test RF amp in the middle.


Pat
N4LTA

Get creative :o

Cut the back off a 2 x 365, strip the back plates and use a coupler and shaft to reach the other cap. Pull plates on the good sections to reach 100pf.

Carl
KM1H

Pat, did you check for VHF parasitic oscillations? Sounds
like the osc. may be breaking into parasitics over part of
the operating cycle.

Pete k1zjh

I looked on my Spectrum Analyzer out to about the 10 harmonic.  That was about 70 Mhz - I didn't look any further and I should have. I got a 2nd only 12 db down and a 4 th down only 22 db or so. Everything else out to the 10th or so was way down.

That tube would be prone to a parasitic as it is a VHF tube. I still have a test breadboard so I'll check it out.


Thanks ,

Pat
N4LTA

The 6J6 made a nice modulated oscillator transmitter at 420 mc back when it was still legal. However I suspect you have AC hum modulation and not a parasitic. Maybe some heater-cathode leakage.

Here is an interesting tapped coil plus cathode follower LO. Its from the Drake 1A. Its the best I can do with the crappy BAMA original PDF, printing to the HP All-In-One, then scanning back to Photoshop. The values are discernible if you squint a lot!

Carl
KM1H

The good old 12AT7 will be hard to beat as an HFO.

The 6J6 is a tough one to tame. It is certainly used even at audio frequencies, but it is really VHF tube. It is hard to tame as a regenerative receiver as well as you can read in the lengthy thread on the regen Yahoo group. One guy started with a 1G6 regen with perfect results and utterly lost it with the same exact circuit using a 6J6.

I assume that you have a regulated plate voltage of 100 -150 volts or so for the buffer.
 
For the follower try 680 Ohm on the cathode, a .01 uf on the plate with a plate feed resistor of say 1K. A 220K will do on the grid. Feed in with a very low value cap like 3.3 pF and out with .001 uF or so.

Mike Wu2D

Yes,

I am running th e buffer and oscillator on 108 volts regulated with a 0B2.

I decided to clean up the shop this afternoon - that is a major undertaking in my shop and just finished getting it about half cleaned up. I am feeding with a 68pf so I was probably overdriving it. My feed resistor is a 22K

I'll try your values as a start tomarrow. When I get it cleaned up - I'll wire the IF and see what the front end looks like.

I actually found a NOS set of two 140pF Hammarlund MC140S variables (don't ask what I paid for them) for the preselector and should have them mid next week.

Thanks

Pat
N4LTA