White Amplification Little Rock

[Robert M. Martinelli]

I undrstand all that, but the fact that the scorched remnant of the original resistor measured 1K seemed like a good starting point. (I know that its not unknown for resistors to change value under extreme thermal stress)

I tried 560 ohms and although it was stable at idle a couple of the valves went into thermal runaway within 30 seconds when I ran it at full power.

Out of interest, I tried 150 ohms and the EL84s anodes were glowing within a minute at idle...

Talking about the cathode resistor, if you choose to stick to 1K, a 3W resistor is not enough ( 60V*0.06A=3.6W! ), I'd use a 7W or even a 10W one for peace of mind.

The schematic kindly posted by Alex uses a 470 Ohm cathode resistor, but with an HT around 350 V; the 560 Ohm resistor you tried is way too low IMHO, with this value the valves already are at their limit when idling IMHO, 150 Ohm simply is overkill!

I' d give an 820 Ohm ( 5 or 7W ) one a try, and I wouldn't probably go any lower.

Cheers

Bob

[Ted]

My comment about the 560R causing redplating was wrong - that was with the old valves.

Looking with a scope, there was severe crossover distortion with the 1K. I gradually dropped it one value at a time...

With a 240R cathode resistor the HT drops to around 470V. Combined screen grid and anode currents are 25mA per valve. (Thats about 11W)

I tried an ECC82 in the PI position and suddenly everything came right - distortion under 10% up to 30W - increasing slowly up to around 30% at 40W where my courage ran out.

Tried a guitar. Oh my word. Nice amp.

[Robert M. Martinelli]

My comment about the 560R causing redplating was wrong - that was with the old valves.

Looking with a scope, there was severe crossover distortion with the 1K. I gradually dropped it one value at a time...

With a 240R cathode resistor the HT drops to around 470V. Combined screen grid and anode currents are 25mA per valve. (Thats about 11W)

I tried an ECC82 in the PI position and suddenly everything came right - distortion under 10% up to 30W - increasing slowly up to around 30% at 40W where my courage ran out.

Tried a guitar. Oh my word. Nice amp.

Oh, I see....
Well, maybe 240 Ohm is still too hot, 11 watts are very close to the tube's limit ( 12W ), I'd try a 270 or a 330 Ohm one, just to keep the amp from becoming a tube-eating machine, much like an AC30.

40W out of four EL84s? That's really pushing them hard, as I said, you might want to try 7189s or 6P14Ps, as they would better tolerate the extra stress.

Congrats on your now nice sounding amp!

Cheers

Bob

[bluesfreak]

My own AC30 ('65) runs well with 68 Ohm cathode resistor (but with lower B+ as the beast here) but when using JJ EL84 he's still in the ballpark what these tubes can handle. However, since Orange uses 150/pair he should be safe using a 330k.
Ted:
Splitting the cathode resistor into 2 has the benefit that the load on the r is less and therefore the emitted heat drops. As you might have noticed at full power its getting warm a bit down there...