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Okay, I was talking with reference to my original Mark 1 amp. Given the position of the switch on your amp, I would agree that it is the 60/100 watt switch. This works by open circuiting the cathode to ground connection of two of the output valves. Regards Mark

The boost lifts the earth off the mid pot or in this case the mid resistor in the tone stack which removes the load presented by the tone stack and changes the tone of the amp.

No I wouldn’t worry about it at all. Some amps had it and some amps didn’t. Regards Mark

I find it hard to work out how the circuit progressed over time. I thought your amp would have had the tweed Bassman P.I. stage, but as far as I can tell it’s more like a Blackface circuit. Who did you talk to at Boogie? As far as I can tell there are no accurate records of these early amps...

Thanks, it will be interesting to hear what they say. Regards Mark

Thanks so much for posting the pictures up. I appreciate the effort you went to on my behalf. Regards Mark

It’s most unusual. You won’t know what is going on without looking inside. The amps the tweed P.I. typically have a stack of gain, much more that the Blackface style P.I. stage. In the case of the amp pictured there could be many reasons for low gain. The Fetron could be shot etc. Regards Mark

The P.I. stage seems to vary in Mark 1 amps. It’s either the tweed Bassman style or the blackface Fender style P.I. stage. I wonder why there was such variation. The tweed Bassman circuit does have more gain than the blackface style circuit. It would be good to hear a recording of both amp...

I would appreciate that as there isn’t a lot of information about on the different circuits used on the Mark 1 Boogies. Regards Mark

I would love to see close up gut shots, these amps varied a bit circuit wise, typically in the phase inverter. Regards Mark

I would love to see gut shots of it. If that’s not too much to ask. I don’t think the handle is original. Regards Mark

I assume the sound getting fuzzy and then cutting out could be the rails to part of the power supply failing. Hard to know without opening the amp up. Regards Mark

I wouldn’t get rid of the pot. A bias pot is a good thing as it’s adjustable. Fixed resistors have only one advantage over pots and that is resistors can have a higher wattage than the pot. Other than that the pot is the way to fly. Regards Mark

When you say the worse the worse the amp sounds. Under what conditions does the amp sound worse? I can’t imagine the amp sounding worse when clean. Do you mean when the power stage is overdriven? Regards Mark

As far as the bias circuit goes, it very simple and the same circuit as used by Tweed Fender amps. So it’s likely to be correct in its topology. Power supply circuits tend to be pretty basic. Regards Mark

I don’t understand how pedal in the effects loop affects or could be affected by the bias of the output stage. 40mA isn’t a bad bias point. I assume the valves are 6L6’s, but what brand are they? In my SOB amp (encountered a decade ago) I found Sovtek valves were lasting 18 months and would...

The bias control should be mounted where the factory select resistors are. The pot should be a ten turn pot where possible as this allows finer adjustment. Regards Mark

Has your issue been resolved? Regards Mark

I think the first thing you need to know is the bias of the output valves. Typically, 6L6’s require a larger negative bias voltage, this would mean the EL-34’s aren’t running in class A. The screen is tied to the plate so this will result in less wattage from the valves. It doesn’t account for...

Thanks for the link. Regards Mark