parameters for push pull class A output?

[swt]

Hi. I'd like to make a class A push pull, pair of EL34s tubes. What are the suggested values for plate voltage, current dissipation, and plate impedance?. Also...do you know any dimensions for an OT transformer of this specs?. thanks a lot for you info.

[Ray Ivers]

swt,

When I saw your post, my first thought was. "oh, c'mon, this data must be all over the Web!" Well, guess what - I looked through every resource I know of, and I couldn't find any data whatsoever for two EL34's running in Class A push-pull pentode operation (which I assumed is what you wanted). Perhaps someone else here can help you out with a link or datasheet.

Anyway, here's a link that shows practically every other audio-related operating condition:

http://tdsl.duncanamps.com/show.php?des=EL34

I'd say just use the P-P triode operating conditions at the very bottom of the chart, possibly with a lower B+ and/or higher load impedance if desired.

Ray

[Matt T.]

When I saw your post, my first thought was. "oh, c'mon, this data must be all over the Web!" Well, guess what - I looked through every resource I know of, and I couldn't find any data whatsoever for two EL34's running in Class A push-pull pentode operation (which I assumed is what you wanted).

I went through this a couple of years ago. I posted here (well...the old crib that is) and elsewhere and didn't get many answers.

I ended up using a Heyboer PT that is fairly low-ish voltage; 390vdc B+ when fully loaded. I used a GZ34 recifier and a JTM45 type OT (with the 43% UL taps) and used the 6K6 primary and wired the secondaries (3 separate windings) for 16 ohm operation. First I tried it UL, then 'regular' (no NFB at all in either case). I used separate 390R cathode resistors with 100uF caps, I have around 30vdc on the cathodes. I prefered 'regular' over UL but either way I get some strange, nasty high end fizz that kicks in at high volumes. Sounds like something is crapping out. It's not just 'fizzy distortion'; it's way more over-the-top than that. EL34s sound way better than the KT66s I tried, more highs and lows and more punch with the EL34s.

I've since asked around for help on choosing/specifying an OT but nobody ever has an answer. Maybe I should try a stout AC30 OT? FWIW, there are some Matchless amps that run 2xEL34s in hot cathode bias (I'm trying to steer clear of saying 'Class A' ) but I know nothing about the OTs they used.

[Ray Ivers]

Matt,

Did you try the 'Ruby Zener mod' on the output stage grids? I wouldn't be surprised if it made that treble crap just vanish.

Also, 6,600 ohms is really high for EL34's at low plate voltages - did you try mismatching it down to 3,300 ohms?

Ray

[Arthur B.]

Hi. I'd like to make a class A push pull, pair of EL34s tubes. What are the suggested values for plate voltage, current dissipation, and plate impedance?. Also...do you know any dimensions for an OT transformer of this specs?. thanks a lot for you info.

The good thing about class A is that is that the operating point and load line for each tube in PP is the same as SE. So use the SE bias point double the SE load impedance to get the correct anode to anode impedance.

[Matt T.]

Ray, no I didn't try that...yet. It sounded different (and way more prominent) than just fizzy distortion (which is what I thought that Ruby-Zener mod was for). I don't recall measuring the grids for an offset voltage when cranking the amp though. This amp was a project that I got so frustrated with (it also has a very slight intermittent hum that I can't find) that I just put it on a back burner.

I don't know what you mean by 'mis-matching down to 3300 ohms'. I did briefly compare it plugged into the 4 ohm and 16 ohm jacks in one of my Marshall cabs and it was louder using the 16 ohm jack.

[Alex/Tubewonder]

Triode mode (screen grid connected to anode), cathode bias PP class A:

B+ = 400V
Cathode resistor, common to both tubes = 220 ohm
Ia+Isg at idle 65mA
Ia+Isg at full power 70mA (5mA increase due to Ig2 current)
Raa = 5kohm
Max signal amplitude on control grid = 22V
Output power ca 16W
Bypassing cathode resistor will increase output power a little and throw the amp slightly out of class A.

Have fun

[Ray Ivers]

Matt,

It might also be that the amp is oscillating ultrasonically on high-power signal peaks, due to coupling through the power supply, ground, the OT primary leads, or any number of other reasons. Even so, IMO the Zener mod would still be a good idea just to try out.

By 'mismatching down to 3,300 ohms', I mean connecting the 16 ohm tap to an 8 ohm load, 8 ohm tap to a 4 ohm load, etc.

Ray

[Matt T.]

By 'mismatching down to 3,300 ohms', I mean connecting the 16 ohm tap to an 8 ohm load, 8 ohm tap to a 4 ohm load, etc.

That was the way I was going to run it, 16 ohm 'setting' into an 8 ohm load. This OT has the three separate secondaries (like a JTM45 OT) and I just connected them for '16 ohm' operation. Time to order a few zeners, all I have are 10V and 15V (both 5W). Any suggestions on the voltage drop?

Maybe I'll give triode operation a spin.

swt, sorry for the hijack...but hey...it's still good info right?

[swt]

we're all in the same boat...so no hijack intended. i guess we have to learn together the hard way...by trial and error. i've also looked at the matchless and bad cat amps, but don't know the impedance, but they run the voltages too high in my opinion and that's why those amps are really tube eaters. A single ended EL34 likes about 390 volts and load between 6 to 8k, so i guess that a target load of 4k to 5k will be ok. I'll try to keep on investigating a little more, and report back. Any help from you guys will be appreciated. Thanks a lot!

[swt]

Hey arthur!! should i double or halved the impedance for two tubes??.
Alex!! do you know if triode operation requires higher load? because in triode you suggest 5k, maybe we can get an aprox for pentode...
thanks again guys...

[Matt T.]

AC30 OTs are ~4200 ohm, correct? And they are 30W so it should be stout enough, correct?

[Ray Ivers]

Matt,

Maybe you could start out by series-connecting different combinations of your 10V and 15V Zeners to get a rough idea of the voltage 'ballpark' you want to be in, then order every value of Zener in that range (500mW or 1W should be fine) for fine-tuning.

Ray

[Alex/Tubewonder]

AC30 uses 4xEL84 and is class AB1 even though it's cathode biased. EL34 is a different animal.
Raa ~4kohm in that application is correct.

[Matt T.]

Alex, yes I realize that but the AC30 OT is within the primary impedance range given, is about the same power, and when he said 'Class A' I didn't take him 100% literally (he could have meant "AC30 Class A" which we know really isn't Class A but...well you know). Two EL84s have about the same power as one EL34 so I don't see where the AC30 OT isn't suitable. What's the main reason it's not suitable?