6L6GCs w/ST-70

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Hi,
I just got EH 6l6gc quad that I want to use w/ my ST-70 loaded w/ DIYTUBE ST-70 board. The way it is right now, I get around 700 mV drop across 10 ohms, giving me 70 mA per tube. Is this safe idle current? What's optimal for 6L6GCs? I notice on the original Dynaco manual that 6L6s can be used w/ST-70. Thanks. -Min

[erichayes]

Min, I don't know what your plate voltage is, but I suspect you're hammering those 6L6s. I'd drop the cathode current down to around 45 to 50 mA per tube for starters and see how the amp sounds. If you have 400 to 450 volts on the plates, the "maximum" cathode current allowable is in the neighborhood of 95 to 130 mA per tube, operating AB1. That's maximum. Quiescent current should be considerably lower.

I put maximum in quotes because Leo and Ed proved spec's were to be ignored (or sneered at) with guitar amps. Hi Fi is another matter . . .

[Shannon Parks]

Hi Min,

Can't you adjust the bias to 500mV?

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Shannon, 700 mA is the least I can get with the chassis pots turned all the way down. I couldn't get any lower. This is after I had two tubes/side matched. I could get 500 mA with EL34s, so I guess it's not how I wired it.
Eric, the plate voltage is 410 - 415 VDC and I am under 95 mA/tube, so I am ok? I am mostly concerned about hurting the PT and tubes. I guess I could lower the plate voltage, but I'd let it alone if I can help it.
Those 6L6GTs are actually Svetlanas. I briefly turned on the amp and it sounds fine. It does lack a bit of "warmth", too bright for my liking. Probably needs some break in. -Min

[TerrySmith]

The EL34 biases at a lower neg bias voltage than a 6L6GC, 6CA7, 6550, KT88 etc. You will need to shift the bias, ie: replace the 10k resistor on the ground side with a 15k, replace the 10k on the other side with a 4.7k. This will move the voltage range upwards.

[Sal Brisindi]

If you have a 5AR4 installed, replace it with a 5R4GYB, that will drop the B+ a little plus lower your bias too. I did that when I tried my 6L6WXT's in my ST-70.

Rrgards,
Sal Brisindi