• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
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    the safety precautions around high voltages.

2C39A as the Output Tube

AbaddonD

Member
2015-08-20 1:49 pm
Hey guys!
I just won a 2C39A in an auction for cheap for RF applications and I was wondering if I could use this as the output tube in a class A amp.
It's pretty powerful with about 100W of max plate dissipation.
Do you guys think I can make an OK sounding class A amp with this and my other tubes?
I have these tubes: 2 x PCL805, 3 x 12AQ5 and 1 x 2C39A (glass version)


I'll ask my simple second question here too.
Can I run 12AQ5s with 320V on the plates? IK it says 250 max in the datasheet, but I get a lot of distortion with 170V with about 3V RMS going in to the control grid.

Thanks guys!
 

PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
...I just won a 2C39A

I think you "lost" that auction.

Best use would be to make a lamp-base out of it. However do NOT!! drill it!!! It is a technology and vintage which may have Toxic Metals. Probably safe to handle (you should wash-up after). Some of those metals are VERY Toxic as dust (such as in drilling). Some major Government programs had to be severely cut-back because workers were sick or dead.

To get any real current and power from that tube you must drive the grid POSITIVE. Then it sucks big current. This is OK in single-frequency tuned circuits. In *audio*, a device that changes impedance from >1Meg to around 100 Ohms means gross distortion.

OK, I found an "audio" condition in the negative grid range. It is only 1 Watt out. It is also about 10W dissipation so a PC fan may be enough blow. The plate structure can be hot enough to boil water but not hot enough to sear a steak (or hand) quickly. Note that the whole plate structure is at +300+V so must not be exposed to touch.

...I'll ask my simple second question here too.
Can I run 12AQ5s with 320V on the plates? IK it says 250 max in the datasheet, but I get a lot of distortion with 170V with about 3V RMS going in to the control grid.....

Something is wrong. Either you are expecting too much of the little thing, or you have mis-biased or mis-loaded it.
 

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AbaddonD

Member
2015-08-20 1:49 pm
Thank you all for your comments!

Providing about 1kV is not a problem for me.
Cooling isn't an issue either. I have serious EDFs to cool the plate.
I have a couple of transformers. If I find a design which is worth it for this tube, I'll have a transformer custom made.
Also, I would NEVER, EVER harm a vacuum tube, so don't worry about that.

I have run it off 170V and it wasn't very pleasing. Will try 500V and 10V RMS from a PCL805's triode section as the pre-amplifier.

Also, thank you for the schematic for 12AQ5s. I will try that as well and let you know. :)

Thanks again!
 

AbaddonD

Member
2015-08-20 1:49 pm
I think you "lost" that auction.

OK, I found an "audio" condition in the negative grid range. It is only 1 Watt out. It is also about 10W dissipation so a PC fan may be enough blow. The plate structure can be hot enough to boil water but not hot enough to sear a steak (or hand) quickly. Note that the whole plate structure is at +300+V so must not be exposed to touch.

Hello again!
I tried the 2C39A with 320V and with a grid leakage resistor of 1MOhm and 100kOhm. It still has a lot of distortion with not much power. Also, it had a cathode resistor of 500Ohm.

I checked and my PCL805's triode isn't the cause of the distortion and it's the 2C39A.
I tried the circuit with two different transformers. One with a 15kOhm to 5Ohm and one with 50Ohm to less than 0.5Ohm. The second one did a bit better.
I know that the transformers are far from perfect audio transformers, but still.
Is there anything that I'm doing horribly wrong?
I expected a lot more power while running off 320V.

Any help is appreciated. :)
Thanks!
 
I may be late to the party, but my opinion is similar: do not attempt to use thes UHF 'grounded-grid' triodes for AF work. For the same reasons. Positive-going bias on the grid, causing very low input impedance (indeed, asymmetric, which is even worse that 'just a load'), near-impossibility of mounting the things securely, while still using forced air head ducting on the anodes.

They're certainly beautiful to look at, to hold. But really not suited for audio-frequency work.

Just Saying,
-= GoatGuy ✓ =-
 

AbaddonD

Member
2015-08-20 1:49 pm
I may be late to the party, but my opinion is similar: do not attempt to use thes UHF 'grounded-grid' triodes for AF work. For the same reasons. Positive-going bias on the grid, causing very low input impedance (indeed, asymmetric, which is even worse that 'just a load'), near-impossibility of mounting the things securely, while still using forced air head ducting on the anodes.

They're certainly beautiful to look at, to hold. But really not suited for audio-frequency work.

Just Saying,
-= GoatGuy ✓ =-
I see.
As you might have known, it's just for fun. Not anything serious.
I am getting into vacuum tubes and trying different tubes to see and learn how they act differently.

Also, this might be the reason that there is only 1 schematic online using a 2C39A. xD
Still, I want to see if I can make something out of it which might sound "OK" and try to boost the power from there.

Update: I have noticed that at low input voltages (less than like 0.3V RMS), it sounds pretty good with my 50Ohm:0.5Ohm transformer. Like, PRETTY GOOD. But if you increase the volume, it gets heavily distorted and when you go too high, it start cutting off and distorting at the same time.
 

H713

Member
2017-11-24 11:21 pm
Madison WI
Better idea:

Go get your Technician (and while you're at it, the General is easy enough) license and use this for a VHF or UHF amplifier like it was designed for.

Some "transmitting" tubes are great for audio usage (let's be honest, the 845 is practically useless for RF anyways), but some tubes like the 3-500Z, 2C39, 4CX series, etc. are not a good choice for audio. In general, if it needs forced-air cooling, it's not a good choice for an audio amp for that reason alone.
 

AbaddonD

Member
2015-08-20 1:49 pm
Better idea:

Go get your Technician (and while you're at it, the General is easy enough) license and use this for a VHF or UHF amplifier like it was designed for.

Some "transmitting" tubes are great for audio usage (let's be honest, the 845 is practically useless for RF anyways), but some tubes like the 3-500Z, 2C39, 4CX series, etc. are not a good choice for audio. In general, if it needs forced-air cooling, it's not a good choice for an audio amp for that reason alone.
I will try my best to make some form of an audio amplifier out of this.
Will keep you all posted on this if I get to make something good which I kinda doubt.
 

H713

Member
2017-11-24 11:21 pm
Madison WI
Quite seriously, every so often people get their hands on an esoteric RF tube and then try (and fail) to build an audio amplifier out of it. Usually they try to build one single-ended, which doesn't help the matter.

Tubes like the 2C39, 4CX250, 15E and some of the lighthouse tubes are better left to their original use. Tubes like the 15E, which were originally designed for extremely high voltages in pulsed RF usage (WWII radar systems), are just a poor choice for audio.

Tubes like the 2C39 and 4CX250 are also a terrible audio tube. Sure you may get it to "work", but the results are going to be sub-par and the effort will be enormous. It will require very high voltages, a custom-designed output transformer (in all likelihood, though I haven't looked at the 2C39 curves in a while), and a ridiculously complicated chassis design to handle the forced-air cooling.

Oh, and one other thing: This is a UHF tube. That means that it will have no trouble oscillating at some ridiculous frequencies. You thought the 807 was bad for parasitics, wait until you see this. The 807 won't go anywhere close to this high.

Just because it's a "big triode" doesn't mean it has any realistic usage in audio whatsoever. Like I said, if you want to build something with this, use it where it shines, and that is in RF service.

Edit: To be clear, there's nothing wrong with RF service. That's another very fun hobby that the folks over on QRZ will be happy to help you with.
 
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AbaddonD

Member
2015-08-20 1:49 pm
Quite seriously, every so often people get their hands on an esoteric RF tube and then try (and fail) to build an audio amplifier out of it. Usually they try to build one single-ended, which doesn't help the matter.

Tubes like the 2C39, 4CX250, 15E and some of the lighthouse tubes are better left to their original use. Tubes like the 15E, which were originally designed for extremely high voltages in pulsed RF usage (WWII radar systems), are just a poor choice for audio.

Tubes like the 2C39 and 4CX250 are also a terrible audio tube. Sure you may get it to "work", but the results are going to be sub-par and the effort will be enormous. It will require very high voltages, a custom-designed output transformer (in all likelihood, though I haven't looked at the 2C39 curves in a while), and a ridiculously complicated chassis design to handle the forced-air cooling.

Oh, and one other thing: This is a UHF tube. That means that it will have no trouble oscillating at some ridiculous frequencies. You thought the 807 was bad for parasitics, wait until you see this. The 807 won't go anywhere close to this high.

Just because it's a "big triode" doesn't mean it has any realistic usage in audio whatsoever. Like I said, if you want to build something with this, use it where it shines, and that is in RF service.

Edit: To be clear, there's nothing wrong with RF service. That's another very fun hobby that the folks over on QRZ will be happy to help you with.

You are absolutely right.
I might terminate my experiments shortly as it has lots of challenges, both experienced and explained.
I will plan to make a 433MHz amplifier for some weird Arduino remote station.

Thank you all for your explanations and guidance.
 

AbaddonD

Member
2015-08-20 1:49 pm
So...
It seems that my previous circuit was the best, with maybe 0.5W of power which is stupid.
I won't be attempting to make an amplifier out of this and I declare defeat.

However, I have made a cool little variable regulated power supply using it as the series pass tube. The comparator amplifier for the 2C39A can't take more than around 400V so that's the max voltage. I used a VR-150 as the reference, if anyone were curious. It goes down to 155V and can have a short-circuit current of about 70mA with an input voltage of 320V.

Again, thanks to all the people who guided me and gave advice on how I couldn't make (or "could" make) one.
 
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AbaddonD

Member
2015-08-20 1:49 pm
So...
It seems that my previous circuit was the best, with maybe 0.5W of power which is stupid.
I won't be attempting to make an amplifier out of this and I declare defeat.

However, I have made a cool little variable regulated power supply using it as the series pass tube. The comparator amplifier for the 2C39A can't take more than around 400V so that's the max voltage. I used a VR-150 as the reference, if anyone were curious. It goes down to 155V and can have a short-circuit current of about 70mA with an input voltage of 320V.

Again, thanks to all the people who guided me and gave advice on how I couldn't make (or "could" make) one.
I just noticed something very weird when shorting out the output of this PSU. The gap between the grid-cathode and the anode of the 2C39A glows purplish-white and disappears when I remove the short-circuit.
Can anyone please explain this?
 

6A3sUMMER

Member
2016-06-07 6:50 am
EiMac built an RF triode that was made for Linear Class A service for NTSC TV transmission.
It needed forced air cooling. The plate dissipation was 1500 Watts.

Well, a very creative and hard working audio person, used a complete 19 inch 6 foot tall rack to house what he built . . . a pair of 200 Watt SE amplifiers using one of those tubes for each channel.

Please remember, that EiMac RF triode was intended for Linear, Class A service.

That is far different than an RF tube that was made for Class C service, with lots of grid current and operation that had much longer off times than on times (not even close to Class B, where there is residual quiescent current).
The same goes for Radar modulator tubes, with 1000:1 or even larger ratios of off times versus on times.
 
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