The AM Forum

I have a rack cabinet that contains, three power supplys, variacs and metering.  I want to build an RF deck for it to use as an SSB and AM linear.

The Screen supply has its own variac, meters and output connector.  0 to 800 volts as tested.
The HV supply uses a 3650-0-3650 transformer rated at about an amp.  It has two big chokes and two big oil caps. There is plate current and plate voltage meters. Tested to 4K volts with variac.
The last supply is a bias supply. I dont know its rating but its adjustable.

This thing is ready for an RF deck.  What tube or tubes would you consider for max output power with these supplys?

It seems that the 4-1000, 3-1000, 1500 ect will not be a good choice for me as the voltage is only 3 to 4K on the HV.

I want to run 20, 40, 80 and 160 with this.  Plan to use a Transiever on a shelf so the entire thing rolls around. Probably a 1000D or FT 101.

I thought of three or four, 3-500s as I have plenty of tubes and sockets. However, I am worried this will produce an incredible amount of heat.  I thought of a couple 4-400s but then I would be dealing with very low Drive requirements.

I have never used the Russian tubes so maybe there is one out there that is perfect for this?

What is a simple, cheap and effective RF deck for this thing?

C

A pair of 4-1000s would be happy as a clam on 4Kv!! Not to mention look pretty kool through a peek-a-boo window!!

Lose the screen supply and biass supplies, run them grounded grid. keeps everything nice and simple and leaves room in the rack for otha stuff!

Just my $.02 worth. I love those tubes and used broadcash pulls are still cheap!

A 4x1 with 3500V will do 1500W out in GG and 100W drive. A 3 holer of 4-400's will do a bit more and give you 200W more of Pd headroom for AM. Either tube can be run GG or swamped grid driven. With the 4-400 you can get away with the ceramic socket with a small fan blowing on the pins and a bigger one on top blowing on the glass. Saves the hassle of pressurizing and chimneys.

Carl

What they said I agree. My old 4-1000A rig did 1500 watts using the pulls that are Frank's spares, (sept for the brown one) Multiple tubes in parallel makes life interesting because layout is much more critical

Hmm.. I am showing 6K to 7K is needed on the 4-1000 for full output.   3 to 4k is fine??? 

C

you need to grow some BA hair before you start messing with 6KV on a 4-1000A sonny. And get a good blower. Above 5KV is a whole new ball game.

Yeah.. I got plenty of help.. = You guys.

Nobody likes the multiple 3-500 idea?  I happen to have a box O tubes and sockets are cheap. 

C

I was amassed on how fast you can blow holes right through 833'A at 3.5 KV. I was going to run them at 4 KV. The arc over's were also frighting at 3.5 KV so i cannot imagine what life must be like above 5 KV.

In my Westinghouse MW-2 I had some arc over's to ground. The rectifier uses three phase and I have a 16 KVA plate transformer on there. It would sound as loud as a 12 gauge shotgun going off which gave me pause to think about what it would be like to get your body across it.

I guess the message here is to BE CAREFUL.

Pair 4-1000 cathode driven with G1 and G2 potentials both used.

According to Chuck/ K1KW, an ex-Varian engineer, keep the HV below 5500V maximum on a 4-1000A in linear. I can't recall exactly what it's called, but something to do with electrons re-radiating off the plate or something similar causing non-linearity. Besides, 5500V max (or less) works FB in my experience and need not be increased.

It's just a matter of getting a good tank impedance match for good efficency at whatever voltage you run.

Cooling will be very important at those voltage/power levels, so use a chimney.... ;)  ;D

T

I ran 5500 one time on 10 meters. The plate color approached the color of the sun. Made some real juice though. 

I used to have a 4-1000 amp build by someone in Cali.  It was very well built with Vac caps and big Dahl supply.  This amp ran 6600 volts on the plate. It ran hot but sure made the power.

C

a 3CX3000?

but if you got the 3-500Z's, use them but put them in chimneys and close together. The blower comment is on target no matter what you do, if you intend to push that plate supply at all.

BA hair? Is that the sticky gray dust bunny things in the bottom of old transmitters?

nothing wrong with 3-500s and 3500 volts should make them sing. Just need to adjust the bias so resting current isn't too high.

I wanted the 3cx3000 tube orginaly.  But was told that it really needs more then 3K volts and alot more current.  I will look into this more. Thanks for the ideas. Keep em comming.. 


So far, I like the 4-1000 idea at G1 G2 ground.

C

FWIW  the 1978 ARRL handbook showed a 1kW input linear amp with 50 Ohm terminated grid driven ... can use triodes ,tetrodes or pentodes .... not having to bandswitch an input circuit is attractive .... the article pictured a 833 in it ...hmmmm

Clark,
        If you can find the odd ball filament transformer (7.5V@21A), and the socket. Grounded grid 4x1s are a very friendly tube. Friendly and non finicky.

I've been running the same one in my big TX for almost 10 years now!

http://amfone.net/Amforum/index.php?topic=18634.0


Clark:


The 4X1 tetrode is an anomaly when placed in hi-mu GG triode service. It is cleaner than most other tetrodes in GG.  That's why the triode-connected 4X1 modulators work so well. Same for 813's.


The above thread contains a rare letter from the Eimac boys spelling out the suitability of a 4X1 in GG. Great info when running the G1/G2 at ground - info not found anywhere else.

The recommended grid current is revealing as well as the suggested output power at various plate voltages. 3KV is the minimum plate voltage due to grid current exceeding tube parameters.


In general, I've found a GG 4X1 in linear service too be reasonably clean compared to other specialized triodes designed for linear service.

T

Interesting. This makes the construction so much easier.  4K on the plates. This thing should do 3K out easy with into the dummy??

C

Well, for a single tube if you run the grid currents within Eimac specs to be clean, it says 4KV gives 1870w pep output, so you wud need a pair as you intended..

When I had Quadzilla going (R.I.P.)  I found about 2KW per tube into a dummy load was the best I cud do at 4KV. These were pulls and I drove the tubes to slightly under saturation.  Driving them harder would make the IMD specs unacceptable, in my opinion.

A pair is not a bad idea in the end...


And as Jay/W1VD says - reduce the full power output by 3db (1/2 power) and the 3rd IMD spec will clean up roughly ~9db better. That's quite significant.

T

Ok sounds good.  Now I need to stumble on two good tubes and sockets and start looking at the network.

C

*3650-0-3650 transformer rated at about an amp*

That will make about 3250 volts with a choke input filter. My vote is an 8877 tube, and use one of those 'medical pulls' available for a reasonable amount. You can adapt an old 4D32 tube socket, and make your own chimney.

I also like the trio of 4-400's...

Jim
WD5JKO

I think it an amp.. Its an electro engineering job. Same one used in Gates or 707 for plate transformer.

I have two 1500 tubes.  I can go that route.. One or two of them.

C

I have to laugh at that Eimac sheet!  If you underload it at 5KV it will take off like a rocket. Load it untill it wont put out more and then continue loading until the power drops about 50-100W. That is the point of best IMD.  If a 4X1 can handle 2500W out in AM Class C then about 2200W in SSB wont bother it since the PD ratings are CCS and quite conservative.  Same with a 4-400 or 4-500.

OTOH a 3-500Z sucks for several reasons. Its a SSB tube and rated as such. At 3500V key down it lights up brilliantly. Use it as an AM linear at that voltage and you are asking for fireworks. Older Eimacs often wont pass a 14KV Hi-Pot and neither will most all Chinkadink versions. Some years back I tried several tubes in an AL-82 as an AM linear before I built a Hi-Pot tester. They all worked fine in my LK-500ZC at 2500V. In the AL-82 (tested 1 tube at a time on 80M) they gave light shows, fried parasitic suppressors, blown fuses which resulted ime losing 2 tubes. Its the last time I tried that dumbass stunt. ::)

The 3CX3000A7 is a fantastic tube and almost impossible to kill, its a favorite with technically challenged CBers ;D and still used in many BC stations. The Chinese version is highly acclaimed.

The 8877 is a great tube as long as you tread lightly and have full protection circuitry built in otherwise it very easily can be a display piece. Its not for AM at any serious voltage unless you have a 3CPX1500A7 version.

My tube of choice would be the YC-156. A 3CX15000B7 internals and a 3CX5000 anode. It will easly blow past 1500W with 100W drive and really wakes up at 300W. Anything from 3500-5500V works just fine. Modest filament requirements but it really wants a 6-8 minute warmup so you turn it on once and then off at bed time. No socket, it bolts right to the chassis; stable as a rock, no parasitic suppressor even needed. Its a medical tube and IMD in the -50dB area is attainable; pulls are readily available around $300 and will be the last amp and tube you will need. You will be Loud and Clean.
http://www.g8wrb.org/yc156/

Carl

So at higher voltages, the gain in GG becomes such that it will go nuts, or, the stress caused by underloading with drive applied will blow it? Interesting that do not mention that but I suppose they expect designers to know it.

I suspect the downrating at 5KV was mostly to keep Charlie off their back.

Underloading any amp is known to create huge RF volatges in the tank circuit and even vacuum caps are not immune. Look at used glass ones, most have pecker tracks on the plates.

They also changed the original 3CX800A7 spec sheet after getting hollered at.

Carl

On the topic of picking tubes, assuming a cathode drive, is there any merit to deleting a cathode tuned circuit and putting a small halfwave rectifier tube across the K-Grid nodes so that it provides loading for the half of the cycle the RF tube's grid does not conduct?

Years ago there was a 3-1000 leenyar got in trade that had this arrangement using a 6AX4 (32V drop at 250mA) instead of a cathode tuned circuit. It worked great but I had no means to check anything on it back then and ended up selling it. The idea was to avoid the need for input tuning.

        

          This "half cycle loading"  system has been used in high powered (50 KW+PEP)
          multiplexed ISB transmitters since the late 1950's. Most of the Airforce Globecom
          point to point Tx sites were packed full of them...        
  
           Eimac's Bill Eitel wrote it up in one of the ham rags back there some time as I recall.

           I user it with a couple band switched 4-400's back in the early 60's, worked fine...

           Ex AF 3034/3016...

No. You will just create more IMD.

Maybe thats why some of those military signals sound like CBers :o

The concept might work better if there was some way to control the hard switch time to something a bit softer. Something like key clicks on CW ::)

Carl

Since we found some 4-1000s and sockets.  Should I use one or two tubes with this Transformer?  Should I do Grounded Grid or Grid driven?

C

   


     No Carl, the military Tx output I'm talking about sounded like a hornets nest on
     the SP600/51j/r390/r390a in 16kc bandwidth at the audio output point (before
     the multiplex demod unit input)...

     They normally transmitted 32 incripted RTTY channels, 16 on each sideband...

Yeah and now Im back to looking for them again since you changed your mind ;D  Anybody got plastic 4x1 sockets and chimneys?

Carl

Clark,  Good news on finding 4-1000s and scokets.

Carl,  had posted this in the e-pay and links section of this site last week,  and appears this guy still has socket(s),  and chinmey(s).   Not at give-away prices but here is the link:

http://swap.qth.com/view_ad.php?counter=839173

Have been following this topic.  IF I had been starting from scratch,  I'd probably have strongly considered a 3CX3000A7.  They are pleantiful,  and a number of the sputtering amps have been floating around,  which are a good starting point for a HB Class 'A' amp,  wink  wink.  The 3X3 does require a bit of drive,  and the Vp max is 5 Kv ...  etc   Had also wondered about a 3CX/(X)2500A, altho not too available.

73  GL  Vic

I am concerned about a couple things here.  Just for learning sake can we talk about this?

I dont think I have enough PS for two tubes.  The transformer says 3650 0 3650 and 770kva on it.  Its for a BC rig doing carrier duty with two 4-400s.

I am also concerned that one 4-1000 will not put out nearly enough power if its Grounded Grid with 3000 to 4000 on the plate. 

My next thought is that if I do a Grid Driven setup with one tube, It might ok but still plate volts seem low for linear service here with SSB and AM amp.

What If I used this same Transformer in Full wave? That would get me more plate volts here.. I looked at the handbook and this is what I have determined.  Good choice?

Another worthwhile thing to be said about the 3cx3000 is that with that enormous 4000W plate dissipation, short qsy's around the same band can be made without having to immediately jump on the knobs and get it retuned.

What tube would I consider???

Anything with handles!

Clark,

Think that your goal is to use this big PS to power a bigish linear amp.   The Kva rating you noted,  770 Kva,  cannot be correct.

If the transformer is from a 4-400 X 4-400 Plate Modulated rig,  then,  the transformer should be about 3.0 Kva.  Would expect it to be designed for a choke input filter,  in Full Wave configuration with the center tap grounded.  This should give you close to 3 KV output.  (I may be wrong on some of this,  as am not an expert).

So you need a tube which can give you good output power with approximately this output voltage.  Some  center-tapped transformers cannot survive being operated with their CT floating (as with FW Bridge config), as the insulation will break down,  and hurt the transformer.

Others more expert than I should chime in about now.

More later GL  Vic

Clark,

That transformer should be fine producing around ~3600V or so in the  full wave center tapped mode using a choke input. You should put a vacuum relay across the choke (short it out) to get a voltage boost near 4KV or so under load. Also, if there are taps on the primary, use a high current relay to add some more boost when desired.  You will be able to stay away from a Variac this way and see better regulation. . The sag will depend on your filter cap size  (at least 40 uf) and the load.  Put a good pair of 4X1's in GG and you will get out about 3500 to 4KW pep into a dummy load. This is a good, conservative set of parameters for all components and the amp will run clean and efficient if you find the correct tank values and use a tuned cathode input.

I'm not a fan of grid driven 4X1's in linear service  due to the added circuit complications and lack of negative feedback - that is free in GG config. (also more stable in GG)

T

Swamped grid driven solves neutralization and a lot of parasitic problems but 4x1's still require a good layout to be stable.

Carl

I will go read it again. Its the sale electro-engineering unit that where in thousand sof 20V's, 707s, gates ect...

I have the PS done. I hooked the old RF deck up to it one night and put a 1kw Carrier in the dummy with it.   I had the variac at 3000 volts.  My brother walked in and like an A-hole said what would happen if I did this.. Then cranked the variac up to the max. It was around 5K volts.  That was the boost of the Variac on my high line voltage.

So 4-1000 in  Grid driven needs move voltage.




C

I've built several amp's with 4-1000's. Both grid driven and GG I have had good results swamping the grid using 100 watts drive. Here is a reprint from 1963 QST.

Here is the rest of the PDF

Thank you! I love good articels like that.