The AM Forum

I have a commercial paging transmitter that was used on 35 MHz.....Aside from the other gear in the rack, the PA is a nicely metered assembly complete with HV power supply, blower, etc. It is Class C as it stands now. Of course the tank circuit needs to be modified for 75/160 meters. What approach would be best here ? Building an outboard modulator for this OR re-biasing the amplifier for linear service ?  I guess the modulation transformer would be the big expense with the outboard modulator. I am not looking to run a kilowatt, just some additional power beyond the Ranger output.

Well Steph, you could run it as an amp while you look for a mod tranny and a pair of 813s.

What kind of power does it offer now? What are the voltages? You can modulate that with 811's as well as 813's.

FWIW, I have dismembered several Quintron paging transmitters made for somewhere around 49Mhz. Lots of really good parts to be scrounged! they used a single class C 4-400. They had nice power supplies and metering circuits. However a single 400 is a little lame in leanyour service.

My suggestion is, since you have to rework most of it anyway to get it down to HF,
Rebuild the PA deck for a pair of 400s, and use robust and wide spaced tank components. Run it as a simple GG leanyour for now and as you accumulate parts later change it over to a plate mod rig.

Most of those things were in 6' racks, so you have plenty of room to add more later as you build it.

                                                        The Slab Bacon

The amplifier will currently do about 350 watts out with the following voltages provided...

2600v on the Plate, 400v on the screen and -90 grid bias

Yes, but that is not in linear service. In linear service the efficiency will drop to around 33%. Also the 350w is a max outpoot with no consideration for am headroom. 350w peak output (PEP) will only equal out to 87 1/2w carrier output. Hardly worth the bother of building a linear.

The low plate voltage (2600v) in linear service will require more plate current to make things happen. the sinplest way to do this is by paralleling a pair of them.

                                                              the Slab Bacon

Yes, this is a Quintron !  ;D

Frank, she's using a barefoot Ranger now. Even if this thing gets her a 125w carrier in AB2 it'll be better than what she's got now.

Nice looking Quintron box.
My vote would be to keep it class C and plate modulate.
You have a T-368 type of RF signal. I think legal limit A.M. or close to it.

Fred

Buddly,
          this is true. but a 6' rack amp to put out 100w is one of those "whats wrong with this picture" scenarios. that is the same transmitter that I cut up for parts. there are gazillions of them out there just about free for the taking since low band paging is just about extinct.

the power transformer out of one of those was what I was limping my 4x1 rig along with until I got the mega iron. It is definately good for 1kw PEP or better. It would hold 2600vdc with 400Ma load. I was beating it harder than that, but it was sagging on the voice peaks. So it would easily be good for 250W of carrier in AM linear service. Since she will have to rework most of the PA deck anyway to get it down to 80M, there is plenty of room in that chassis to shoehorn in a second tube. Used 400s are cheap and plentiful. I still see them at festers for $10-20. If you're gonna take the time to convert it over, why not take a few more minutes to get much more "bang for the buck".

Just my take on things.

                                                       the Slab Bacon

Not sure what type of modulation was used in that xmtr could have been FM, but you might already have the means to put it on AM phone.  Many of the old "low band paging xmtrs, such as the Budelman used plate modulation for the tones and voice.  Check out the power supply and see if there is also a modulator on board.  The old Budelman paging xmtrs used a 4-250 or a 4-400 modulated by a pr of 4-125s.  N9HXW is running a Budelman paging xmtr on AM phone and it works great. 

I would also vote for building up a plate modulator, if you don't already have one in the rig.  Another option would be cathode modulation, which could be accomplished cheap and easy.  Ask W8NWF about that.  He is running an 833 cathode modulated by 3 6146s and it sounds great.  No need for a big modulation xfmr.  It would be a shame to not use that 4-400 in class C as it was intended.

73,  Jack, W9GT

If you have a Junque box or even if you want to go out scrounging parts, it could be a nice project to build an external modulator, with a pair of 4-125's or a pair of 810's.
I know that putting an external modulator from scratch can get expensive.
You can get to legal limit and have a nice plate modulated transmitter much cheaper than spending an inflated price of a T-368 which is a 4-400 modulated by a pair of 4-125's.
Or you can save the dollars and the hassle of building and make it a linear. 125 watts is better than Ranger power.
With present conditions on 160M 125 watts and a reasonable antenna, you will have a respectable signal.
80M you get close to being a channel master.

Fred

Jack,
        those Quintrons were FM. Cathode modding it would be a viable alternative. A single 6080 with both sections in parallel would do it just fine. Or (God give me strength) a single power MOSFET would also do the job.

One could also consider scream (screen) modulation. I've been running a single scream modulated 4-1000 for years now.

However, with anything other than plate modulation, the plate efficiency thing comes into play. With 33% efficiency and only 400w of dissapation to play with you come back to the need for multiple tubes.

Ed, KB2NSP used to run a cathode mod rig with 3 (IIRC) 4-400s. It used to pack a whopping signal and sound quite good.

Phred,
         With current band condx, 125w aint gonna make anyone a channel master regardess of what their antenna is. (within reason) But every little bit helps!!

                                                    The Slab Bacon

Nice XMTR!  I vote for hi level plate mod with an intermediate step of screen mod.  There are a couple of ways to screen mod the amp, one being getting a hammond AF transformer and turning it around backward to a good AF amp and using the former primary as a secondary to mod the screen.  If you are not sure about setting up the screen for modulation, ask around but I think you start off at 1/2 the normal voltage.

Later as you collect the proper IRON which might include a choke for modified  Heising you could change it over to plate mod.  Just be careful to get the plate tuning components that will handle the plate voltage plus the voltage the mod deck imposes on the plate circuit on positive peaks (figure for 130 - 140% positive peaks).

Congrats on the acquisition.

Al VTP

Class C is the way to go.  Redo the Plate circuit, Grid circuit along with making sure the RF choke has enough inductance on the lowest desire frequency of operation.  Rotary inductors in the plate and grid circuit would simplify the mechanics a bit.  The Ranger would make an excellent audio driver as well to drive a pair of modulator tubes.  A good screen or cathode modulator would do well in the interum to get it on the air.  This weekend you were putting a very strong signal into S.E. Pa. with your barefoot Ranger. 
Joe W3GMS 

Stephanie,

If these are so plentiful, it would be a good idea to look for another one.  That would give you the extra 4-400, socket, and filament transformer for the RF deck upgrade.  The power supply could use two transformers in parallel, feeding separate filter networks, one choke and cap for the RF deck, and the other for the modulator.   This assumes there is plenty of room in the bottom of the rack for the additional iron, etc. 

That plate choke looks like a Barker and Williamson 800.  Good down to 75M or 80M, but need more inductance for 160M.

A pair of 4-125s, or 4-400s would make great modulators, and in class AB1 they would need very little in the way of a speech amplifier to drive them.  Ranger audio output would certainly be sufficient with the appropriate driver transformer.

If there is not enough space in one cabinet, strap the two cabinets together, put the modulator in one, and the RF stuff in the other.   You could even rack the driver in the RF cabinet, and have room to add professional audio chain components in the modulator rack. 

73,
Rick

Rick,

I think that copper pipe 90 on top of the choke will bring it close to 160? ;D Good luck with the project Stephanie whatever you decide.

Mike

Stephanie,

If the plate voltage were a tad lower, I'd suggest a pair of 811s and some 2:1-ish step-down iron of your choice. That configuration works well up to 2000V. 2600V is cracking the whip on 811s, so maybe a pair of 4-125s or 4-250s, or even a single 4-400, any of which would be triode-connected.

I'm not sure 2600V is a good unmodulated voltage for a 4-400 in AM service, either. Methinks the positive peaks might be a bit excessive. The amp was designed for Class C FM service, and the carrier was already clipped by the time it got to the tube, so they could safely run the voltage up a lot higher than you would want to in AM service.

Maybe I'm wrong, but I would want to see if I could dial the voltage back closer to 2000V B+ and go from there. Perhaps going choke-input would get you close, but there's probably a better way.

This also brings up the screen voltage: if it's derived from a B+ dropping resistor, no problem. If it's a seperate screen supply, particularly a well-regulated one, you'll need some way to introduce modulation to the screen as well as the plate. The easiest way out would probably be to introduce a choke between the screen and the supply. Otherwise, you may need to go the tertiary-winding (or second transformer) route.

FWIW: I run a 4-250 at 1650V, modulated by 811s which are driven directly from my Ranger's mod iron (with the audio gain pot only up to 4). 15W out of the Ranger gives a comfy quarter-kilowatt with gobs of headroom that I can (and do) modulate the piss out of. As Rick said, anything from 4-125s to a 4-400 would work in the same configuration, but I would triode-connect them.

Just a few thoughts off the top of my head.

--Thom
Kilowatt Amplifier One Zero Grid Current

This sounds like a job for a Collins ART-13 mod transformer, already has the screen taps available.
Bob

Gee, Mike, I wondered what that plumbing copper EL was for.  Thanks for setting me straight, I never would have figggered that one out!  Should have been intuitive.  Too many lunch Margaritas, for lack of a better excuse!

Every 1 kW BC rig I've seen with 4-400s ran 2500 + volts unmodulated.  No need to reduce the HV assuming proper components are used.

Yup, I agree.   The higher voltage, and lower current in the final should provide higher efficiency, and less dissipation in the the final toobs.  Just a matter of proper turns ratio in the mod iron.  The higher voltage allows higher power positive peaks before reaching saturation.  Strapescent!

Fair enough... then it's just down to the choice of mod tubes and proper handling of the screen voltage.

 ;D ;D Heee, Heee. the copper elbow just simply directs air through the tubular plate choke at the finned plate cap ;D ;D

That plate choke wont have enough inductance to get down to 80m. the 30Pf vacuuuum variable cap for the plate tuning is way too small for the lower bands, as is the loading cap as well.

the HV transfoma is set up with a FWB rectifier and produces about 3+KV with no load. It loads down to around 26-2700v under 400Ma load. However it does have a center tapped secondary, so you could reconfigger it to a FW center tapped setup and get the lower desired voltage for plate modulation.

However, broadcash transmittahs usually run 400s at well ovah 3Kv on the plates and still plate mod them. (1Kw outpoot 24 / 7/ 365) with lotz ov air ovah them.

                                                          the Slab Bacon

Hi Stephanie,

My first homebrew plate modulated rig ran a single 4-400A, plate modulated by a pair of 833As. I ran the 4-400A at 2750 VDC at around 275 Ma Ip, for about 750 watts plate input on 75M. I got close to 600 watts output at this plate voltage, which represents an efficiency of around 78%; quite good in my opinion, considering the plate voltage I was running. 4-400As like to see a minimum of 3000 VDC for really good efficiency.

In Class C plate modulated service, when you run the 4-400A at around 2700 or 2500 VDC, the Esg must be reduced from the published Eimac specification at the normal level of output loading, or else the Isg will run too high. With 2750 VDC on the plates of the pair 4-400As in my rig, and  at the level of Ip I normally run, I pull about 80 ma of Isg, and the Esg must be held down to 400 VDC at this set of operating conditions. If you run the Eimac spec of 500 VDC Esg, the Isg will come close to exceeding the dissipation rating of the screen.

This is true not only with my rig, but similar type transmitters including the Collins KW-1 and the Johnson Desktop Kilowatt when run at their normal plate input level and an Ep of 2500 VDC. Both rigs ran the 4-250As or 4-400As at an Esg of 400 VDC for the reason I have described above.

My experience with BC rigs employing 4-400As is that the Ep is usually around 3100 VDC; this permits maximum efficiency, and the Esg can be run at the normal Eimac spec of 500 VDC for plate modulated class C operation, without exceeding the screen dissipation rating of the tube.

I personally would modify that Quintron rig for plate modulated service. Most of the sheetmetal work is already done for you, and you will of course have to change the grid and plate tuned circuits for operation on the lower frequency amateur bands, consistent with parts that can adequately withstand the peak voltages developed in plate modulated service.

Hell, if anyone comes across one of those Quintron rigs, please send it my way. I'd modify it for my own personal use,  add a 2nd 4-400A, and put 'er on the 160M band.

Good luck with what will probably be a fun and rewarding project.

73,

Bruce

 :) Wow, thanks so much for the suggestions and really good information !!!!

Very cool ! I guess Ill have to start poking around some hamfests and find the grid and plate circuit components I need.

The Quintron is in good shape....I only thing I am a bit concerned about are the HV caps in the power supply. Would they be the oil filled variety that contain PCB's ? I have not pulled the cover on the power supply section yet to inspect it. Perhaps this would be a good idea.

Stephanie,

If the capacitors are oil-filled, just check the external condition.  If there is no leakage, they are safe, and will probably outlast you and me, whether or not they contain PCBs. 

Most, if not all, capacitors that do not contain PCBs are labelled "NO PCBs".  If oil filled, and no label as such, then they probably do contain PCBs.  But as long as they are not leaking, they really do not present a hazard. 

When inspecting the capacitors, make sure they have been mounted securely and are not chafing or wearing the paint off at the mounting points.  Also look for rust spots.  If there are no weak spots on the casing, they will probably not leak.

Oil caps can withstand a lot more abuse than electrolytics can.  Temperature variations and surges do not usually damage the oil caps, and they often self-heal if partially shorted.  So if they look good, keep em!

73,
Rick

I echo Rick's advice and add to that if they ever leaked, even a small amount, you will see a sticky tan or brown wax like goop. It most likely will be near the porcelain high voltage insulators. Try not to worry if yours are clean. Your safe.

Mike

I've never done it  but, it's been done....  how about using a hi voltage transformer  and hi power stereo amp to modulate the plate?

just curious.. what would a pair of 4-65's do?

modulated by ....say......hmmmm.....

the power supply is a choke input after the FWB rectifier. IIRC they used a rather large choke and a rather small cap. IIRC, the cap is only 4 or 6 Mfd. It is also an oil filled cap. That rig is recent enough that it most likely WONT have PCBs. Mine had the "No PCBs" markings on the cap. Like everyone else says, dont worry about it if it has pcbs, if it's not leaking it is of absolutely no concern. I prefer the ones with the PCBs as this is a very good fire retardant.

And.......er.......furthermore.................PCBs only become a problem if injested, so dont lick the leaking seals or fry your eggs in the oil ;D ;D

                                                                       the Slab Bacon

Oil caps... the other red meat!

I'll echo what Rick said, but also suggest that if you find worn paint or the beginnings of rust, fix those problems now. Don't be afraid to use a wire-wheel brush in a small drill for removing rust blisters, just remember the outer case is sheet metal so you don't want to apply too much inward force, just enough abbrasion to remove the rust. Grab yourself a can of gray enamel at Home Cheapo and spray it down, carefully masking the electrode insulators with a piece of paper (not with tape!), as well as masking the ratings stencils. Go ahead and patch up any worn-off paint while you're at it, as that will just become a rust spot later in life.

When you pull the caps out, you should immediately check for any oil slicks around their mounting rings. If you find any, inspect the bottom seam. Just a drop or two leaking out won't hurt, but you'll need to stop the leak. How to do so depends on the nature of the leak. Oil leaking out means the potential for moisture leaking in.

Don't be too paranoid about coming into contact with PCB oil, just use common sense. If you find an oil slick by touch, just give your hands a good warm-water wash with strong soap and you'll be fine.

PCB-oil caps usually don't explode, so the worst exposure you'll get is film from a leaking one. Non-PCB oil caps, on the other hand, are filled with mineral oil with a fairly low flash point, and no PCB to act as a fire retardant. They can get carried away if they short and the circuit isn't properly fused (or a breaker is defective), but that likely won't be a problem.

WOW!
Steve, KL7OF, has a great idea!!!!!!!
That can be a very cheap way to go.
Power transformers easy to find! Info available on AMFONE for using a power tranny as a modulation transformer.
High powered audio amplifers everywhere!!
And for that nice low-end response a few chokes in series and a 2mf cap for the modified Heising hook-up and you're there.
I'm using the power transformer idea and some chokes in series for the Heising on an Elamc AF67 and those 6550's sound so nice.

GO FOR IT Stephanie........................
Fred

I had my Ranger User Manual out the other day and was poking through it trying to understand it better. AB2EZ mentioned that he used an external modulator. I never realized that the octal plug on the back of the Ranger allowed you to do that.  ;D I have an EICO mono amp that I picked up from an estate that might be very suitable as a building block for this purpose. Ill have to investigate this later in my project.

Question: If one were to use an external high powered audio amp, what would be the level of "high power" in wattage we are talking about to modulate a 4-400 either through plate modulation or screen modulation ?

Re: PCBs......I feel better now  :) Thanks for the input guys.....I always de-ick my equipment with gloves. I have years of experience de-icking equipment. ;-)

Stephanie said:
The Ranger is excellent. You can use it as a driver for the RF section, (should be able to drive a 4-400) and the speech amp/modulator can be used to drive your modulation tubes. I use mine to drive a pair of 813's in the RF and a pair of 810's in the modulation deck.

Plate modulation: 300 watts (+/- depending on the actual plate input power).

Screen modulation: 20-50 watts but you'll only get about 100-150 watts  output compated to 300-400 out with plate modulation.

Stephanie

I agree with Thom's and other's comments on the oil-filled caps.  They will probably outlast us all.  I had a PS using them and the terminals were facing down.  There was a slight greasiness at the site of the caps.  I just kept the area clean with paper towels.  If you are worried about them, just start shopping around at the festers for a replacement.  The caps could then be turned in at a local haz mat outfit.  We have one here in Manchester.  You might be able to identify the properties of the oil by copying off all info on the caps themselves and checking around. 

Come to think about it -- wouldn't it be nice to have a national database for all PCB filled caps etc that we could check against?

Al VTP

A long time ago, I was given 2 or 3 of the same transmittters.
I still have some parts.
Crappy stuff for an AM rig, the power supply was a bridge setup which I dont like, and light duty, the RF parts were a bit wimpy, and for higher frequencies, but I used 2 of the vacuum variables as neutralization caps in the push pull 812a rig, great for that! 3 to 30 pf mine were, and made adjusting the neutralization a snap, I put knobs on them pointing up.

You cant do much with a 10,000 volt 30 pf vacuum cap, but they make very nice (and small) neutralization caps.

I still have a power transformer and choke, and that RF plate choke with the pipe on it.
I gave one transformer and choke away to a local ham to use to power his art-13.

Kind of ugly iron....

Brett
N2DTS



 

I am lighting up this message thread again.

Last year I acquired a 4-400 RF deck. Very nicely constructed.

I finally had a chance to take the bottom cover off the chassis. Well, I found a nice heavy duty filament transformer in there. I also found a full wave 5690 tube in there feeding the screen grids. The 5690 is capacitively coupled from the RF input.  ???  RF drive biases the screen grids ? There appears to be no screen supply line feeding the 4-400s.

could be the notorious G2DAF circuit !   part of the rf drive is rectified and fed to the screens to dynamicaly bias the tube into operation ... seriously, I have heard the circuit work very well but most of the folks here do not agree (many harrumphs...)

A decent write up on the topic here, called "Frinear":

http://pa0fri.home.xs4all.nl/Lineairs/FrinearPG400/fripg400.htm

"The screen grids in this amplifier are neither at a fixed high voltage nor at earth potential but at a voltage which is proportional to the RF drive. To that end, the RF input is transformed up 3 ÷ 1 in T1, rectified in a voltage doubler and applied to the four bypassed screen grids through individual resistors. This method is consistent with good linearity."

Jim
Wd5JKO

 ;D ;D ;D  what Jim said

the advantage of the circuit is that tuned inputs are not needed for each band with as Varney claimed was similar distortion performance to grounded grid circuit... all I know is that Gene W5CBS ran a 4-1000 in this circuit on 40 mtr ssb, 1500W pep out and I listened both above and below Fc and his signal was clean with usual spectrum occupancy

Wowie kazowie ! That is really awesome !  ;D

A very unique design for sure ! This is great because I made a whole set of assumptions about power supplies needed and it appears I dont need ALL that stuff. Coool :)
Thanks guys for the insight !

The "Frinear" is an adaptation of the old G2DAF design.  Depending on what tube was run, how the screen rectification and grid drive was set up, and how the amplifier was operated, a few people swore by them, and many swore at them.  They were not all clean.
Nowadays, with the availability of relatively inexpensive SDR receivers to use as ersatz spectrum analyzers, it would be easy to get a good idea of the IMD products in real time during operation.  There's nothing about the G2DAF circuit that couldn't be "backed out" if the results were not satisfactory and one decided to proceed with a more conventional topology.  I have a pair of 4X500s sitting around that I'd like to try with this circuit, and if it didn't make good IMD I would use the PureSignal correction in HPSDR to clean it up.  It's not first or even second on my priority list however.  I think G2DAF published his design sometime around 1960, using vacuum tube rectifiers.

I say its worth at least the effort to fire the deck up in its current state for an initial evaluation at lower power levels :)

Cool Jim.

An early "Energy Harvester."

Phil - AC0OB