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Marshall 1962 Bluesbreaker: new valves & biasing - is everything ok?


New member
May 24, 2016
Hi folks!

I think this is a question for more "amp tech" members :D

I've just got a 2007 BB Ri, the PCB version. It is fully stock and came with 5881 power valves. I decided to go for KT66s (NOS Gold Lions).
The plate voltage (on pin 3, with KT66s mounted) reads 385V. Kinda small?
I set the KTs at 60%, so 25 / 385 x 0.60 = 0.03896, just 0.039. Kinda high?

So, is it correct? I found the plate voltage with KTs kinda low (it was 407V with 5881s), so the bias is higher. I can't go lower than 37mV with onboard bias pot with KTs.

My main concern is both KTs become physically very hot after like 10 min of working - I didn't check the temperature but I can feel they produce more heat than 5881s pair. Additionally, the plate in both KTs become slightly red. Well, it's nothing like typical, glowing "red plate", but if I watch carefully, the plate become a bit orange/red (didn't saw it with 5881s).

Is it like 66s do that or something's wrong? KT66s in Bluesbreaker is rather popular mod but I feel the voltages are not right here.


Well-known member
Feb 27, 2003
First off…you need to measure your plate voltage after setting the bias because a change in current draw will cause a change in the plate voltage. That said, we will use 385vdc for the Pv.
25 x .7 = 17.5 watts
17.5/385 = .045ma of current draw for a 70% max plate dissipation factor.
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