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[gui_tarzan]

I finally got to power this up today to check voltages and unloaded they don't look too bad. I am a little concerned about the bias cap voltages though, they are different

C1 - 514v - 32mfd (can)
C2 - 522v - 32mfd (can)
C3 - 479v - 32mfd (can)
C4 - 498v - 16mfd
C5 - -48v - 8mfd (bias)
C6 - -61v - 8mfd (bias)

Why would the bias caps be different voltages? All the resistors checked out within 10% tolerance of their values.

I think I just answered my own question, there is a 15K resistor between them. Coming off the 220K from the power tubes there is -48v so I think I'm good there.

[martin manning]

No problem, the second bias cap (the one that the bias voltage is taken from) is connected to the first through a voltage divider so it should be working at a lower voltage.

[gui_tarzan]

I've got 458v on the KT66s pins 3 and 453v on pins 4, Tung Sol's data sheet says not to go over 450v on the grid. The bias is at -48v on pins 5, which seems high to me. Aren't they supposed to be in the high 30s at 450v on the plates?

[gui_tarzan]

Original Mullard pre-amp tubes installed. Holy cow does this thing sound great!

[martin manning]

These are new (New Sensor) Tung-Sol KT66? You're within a percent or so of the stated maximum screen grid voltage so that is ok. Re the expected bias voltage, there is no screen voltage specified for the anode curves in the New Sensor data sheet, so they are pretty useless. In any case the bias voltage is set to get the idle current where it needs to be, somewhere around 38-46mA for 458V Va-k (KT66 was originally rated 25W, New Sensor data sheet says 30W). It doesn't look like you have a bias trimmer installed, so how are you setting that?

[gui_tarzan]

It's only adjustable by changing the 56K resistor [edited for accuracy], there is no adjustment pot on this. There's no impedance switch either, I'm trying to figure out what it's set at without lifting the circuit board. It just covers the transformer leads so I'll have to try to find a small mirror to see if it's 4, 8 or 16.

[Firestorm]

It's only adjustable by changing the 220K resistors, there is no adjustment pot on this. There's no impedance switch either, I'm trying to figure out what it's set at without lifting the circuit board. It just covers the transformer leads so I'll have to try to find a small mirror to see if it's 4, 8 or 16.

Not the 220Ks, the 15K or the 56K.

[martin manning]

Changing the 220k's will have virtually no effect on the bias. It's the resistor that is in parallel with the second bias cap that you would need to change, or better yet install a trimmer.

[Firestorm]

What Martin said. In the next iteration of this amp, Marshall used a 25K trim pot in series with a 47K in place of the 56K that's probably there now (under the second bias cap).

[gui_tarzan]

Yes, you're right of course. I was reading another thread about an amp like this and that's what the guy said he did to lower the bias.

[martin manning]

Adding a trimmer would go something like this:

[gui_tarzan]

Took the amp back to the shop that hired me to fix it. They're ecstatic and very happy with the results. As am I. The guy that played an LP through it said "listen to those harmonics - they're sparkly!" lol!



This one makes me feel good, it's not just another Fender but a timeless classic. I'll probably never see another one like it but who knows? The word is starting to get out that I can actually repair these. Of course some of the credit goes to you guys for steering me in the right direction on some of the theory end and I greatly appreciate it.