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    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

6CB5 in triode PP?

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I have 8 of these, and two 250V power transformers.

So, assuming ~325-350VDC, can the 6CB5 run in triode at this voltage? What would the ideal plate load be for a PPP amp using 4 of these.

Has anyone tested output power of a PP triode strapped 6CB5?



Due to the limited screen voltage rating, these make lousy pseudotriodes. Why would you want to? Run 'em as pents, with screen regulation, and you'll get more output from one pair than from four as pseudotriodes. These TV HD finals also have some excellent sonic performance as pents. Since these were made for pulling bigamps through horizontal deflection coils, you don't need much more than ~350Vdc anyway, the load resistance is a good deal lower than for most audio finals, making for easier to design and build OPTs with good high and low frequency performance.

Anyway, that's how I'd use 'em. I did a project using 6BQ6s instead since I had all the iron and a good deal of the other parts in the junk box already, so that kept the cost way down. A PP pair proved capable of doing 37W, and was more than enough to saturate this OPT that was rated for 30W (probably for PP Class A 6L6s -- it also included what look to be screen tertiary windings for UL operation, unused since 6BQ6s don't need any extra help from lNFB) and a 4K4 (P-2-P) primary.

If you insist on pseudotriode, you might want to do a search on "Tubelab". I think he's done some investigation on these as pseudotriodes. Depending on the type and condition, sometimes you can get away with busting the screen voltage spec. YMMV.
Thanks Miles!

Your posts are always insightful. Pentode is fine really. Just a toy to play with while I buy parts for my bigger amps.

Maybe 3.3-4K / 60W Edcors for a pair in pentode?

Forch, I saved all the original notes. Loadline (attached) Do keep in mind that this particular project never got built, so it's very much a preliminary design.

With an Rl= 635R/phase, you'll need ~2.54K (P-2-P) A 3K3, 60W Edcor should be just fine here, and the somewhat lighter plate load will drop the output a bit, and maybe the distortion as well (though it doesn't look like you're gonna get very much of that anyway).

Even though these H3 estimates are rough, and high-side errors, they do correlate somewhat to sonic performance. However, in the final analysis, you still have to spend some time listening to the open loop performance to see how that distortion sounds, and what's likely to be necessary to correct for those sonic defects. If the 6CB5 is like the 6BQ6, all it'll need is enough gNFB to take the "edge" off. There shouldn't be any of that "pentode nastiness".


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So, assuming ~325-350VDC, can the 6CB5 run in triode at this voltage? What would the ideal plate load be for a PPP amp using 4 of these.

Yes you can use them this way. I have built plenty 6CB5A amps like that and even at higher voltages. There are many articles about this tube, amps and circuits on my blog:

VinylSavor: 6CB5A

I mostly use them SE but I have built Class A PP amps with the 6CB5A too.
With for of them triode strapped in Class A you could get up to 25W. Much more in Class AB

Best regards

Hi Thomas,

I've looked over your blog several times. Nice builds!

I do not see the PP class A amp you are referring to.i would probably go AB1 anyway. What plate load did you use for your PP amp?

I'd like to get 45-60W out of a pair.

Miles, I'm good with Pentode. Even screen drive at lower voltage than Tubelab uses. I have some old Fisher KX-100 (or at least access to) output iron to use as a Phase inverter for a screen drive type setup.


Hi Thomas,

My speakers are fairly inefficient. I use a 185W Emotiva most of the time to drive them. I have a ST-70 Dynaco, but it doesn't cut it at all during transients.

I'm not trying to go "Tubelab" here, but agood amount of power would be nice. I think I found your old schematic somewhere. Uses an interstate transformer for phase inverter duty? In a post I found, it said you used a 7K output transformer and the amp was good for ~15W class A. I cannot figure out how to get the image off my phone.

So, AB Pentode should have a realistic output of at least 40W I would think.

I think I am going to order some edcor output iron for this project. Too much going on this summer to swing the $400 for an octet of KT-120s for my other project.

So, is ~2.4K the proper load for a PP pentode pair also?


Maybe this transformer? I've also read that the 6CB5 and other horizontal sweeps are relatively easy to drive. Could I use a simple, single tube driver using something like the 6GF7A as a driver?



I am testing power transformers for using 4X 6CB5 triode connected; I intend to use Choke input and set the voltage using AC caps; I intend to use 400 volts.
Luckily the power transformer has a 10 amp at 6.3 volts and 6.3 at 6 amps. If I don't like the amp I can always use 6550's at a higher voltage either UL or Triode.
I intend to use 2.2k A---A.{also siuts 4 X 6550's} and cathode drive using 6BL7's and 12BH7's or Russian 6N6 as drivers.
Blair - Not sure if you've seen the 6CB5A thread linked below . . . I posted some PP schematics that I had breadboarded (that project is still together but dormant again - too many distractions!) There's also a pdf of Thomas Mayer's PP design that uses an interstage transformer and a few posts by Bandersnatch about his design that uses 6BQ5s as drivers. Maybe he'll see this and post a schematic and give us an update on how it all turned out, if it did. Anyway, check it out . . .


. . . Charlie
For Pentode, I would probably just use a separate small transformer with a choke input supply for the screens.

There is also a 1.7K @ 100W Edcor output transformer that I could use for PPP (4) 6CB5As.

The $40 difference in price is not too bad. I suspect around 40W per pair at 350V B+. Or will is be less?


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