The Amp Garage

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Helmholtz wrote: ↑Fri Oct 16, 2026 9:15 pm Are plate and screen voltage referenced to the cathode (with cathode bias)?
Or do you use fixed bias?

Plates and screens referenced to ground. Both tubes are individually cathode biased. I checked my math and the amp is putting out about 22.8W, so yeah, less than 25W for the pair.

My power tubes are biased to 24.5W at idle. Close enough for government work. So clean output must be less than 24.5W without exceeding class A. How did you verify that the amp doesn't leave class A at large outpu? See my edited post above. My load line is 8k, plates at 343V, screens at 320V and ca...

That I believe, as I have my own Class A 2xEL34 push-pull amp and it is doing exactly that. So that's why my interest was piqued when I saw the 2xKT88 Vimana which said 54W. Clean class A PP output is always less than the plate dissipation per tube at idle. This follows from the general class A rul...

I don’t think they can do it either. I built a KT88 single ended amp to test the data sheet claim of 19-20 watts. I built my own transformers. The output transformer impedance was something that couldn’t be bought. I don’t remember if it landed on 2.5k, 3k or 3.5k but I couldn’t buy what I figured ...

Personally if your looking for the sweater sounding even order harmonics out of a class A PP stage then your barking up the wrong tree, because you want a SE class A amp in that case. Steve, my interest in Class A p/p was just to compare the difference in sound, if any, to AB. The Class A (push-pul...

It's a guitar amp. Any references to engineering terms, real or imagined, are purely conjecture and ad-speak. "It's class-A" "Wow, is it true class A?" "yup, truly, the finest sounding amp ever, best in class, so that makes it an A!" Could their claim just be marketing hype? Possibly. That ridiculo...

Very nice work!

pdf64 wrote: ↑Thu Oct 15, 2026 2:37 pm So 54W class A output from a pair of 6550 is possible, and cathode bias has no downside for class A, it's just that each 6550 would need to idle at nearly 60W

Hahaha, nice, Pete!

Thanks for posting that, Steve. Interesting design. Yeah, the amp in question can't be class A if it's putting out 54Wrms.

I can, using some dubious theory to help lift a few parameters, get to roughly 33W --- It'll be a tube eater. I'm skeptical there's benefit derived beyond spec'manship. Ultralinear doesn't help juice total measurable power but will smooth off a bit of roughness. .. Thanks. Your results match my own...

Hello Dave — Any chance of actual measurements? They explicitly imply it has some distortion. With high plate voltage and a matching transformer impedance its almost believable that they’d achieve power with wretched looking output pushing a 1kHz square wave through. Best .. Hi Ian, I have not been...

I am thinning out some surplus parts. Payment by PayPal Friends & Family (avoids me getting soaked by the PayPal fee). Shipping will be USPS Ground Advantage by padded envelope or box, depending on what you purchase. I will offer -10% if you take everything (should all fit in a USPS Medium Flat Rate...

jabguit wrote: ↑Wed Oct 14, 2026 10:39 amKT88 SE.png

Yeah, I have spent a while fiddling with various tools and the most I've been able to theoretically squeeze out of a pair of KT88s running class A push-pull is 31W. 27W from g1=0 to saturation is typical.

Thanks, Steve,

I am scratching my head at this one. The =C= Svetlana datasheet shows a KT88 application for SE operation and they list 19W output (with the plate at 400V and screens at 225V). They do not list the Raa however, which is of no help. Other applications list 14W or SE KT88.