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

[Konrad00711]

Hi folks!

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

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.

 

[Wally]

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.

 

[Wally]

385 x .039 = 15.015/25 = 60% of MPD.

 

[poor man's burst]

If the plates are turning red, you're looking for trouble.

 

[charliechitlins]

If the plates are turning red, you're looking for trouble.

^^^^

 

[Wally]

If the plates are turning red, you're looking for trouble.

One might think that if the plates are turning red one has found trouble.
The trouble is that the amp will be sounding fantastic….for a little while. Then it will stop sounding
Fantastic.

 

[poor man's burst]

It will probably stop sounding at all. Fantastic or not fantastic.