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I found this link where I’m discussing the rebuild of my amp as it’s going along. Amp Garage post I had a moment of clarity and realised that the effects loop across the 2.2M/20pF reverb network has two common cathode triodes due to the phase of the signal. It basically reverb circuit tweaked...

The only was volume 1 would effect the amp when plugged into input 2 is if the volume 1 were wired the wrong way around. Regards Mark

I think the loop will be a cathode follower on the send side and an anode/plate follower on the other side. Valve Wizard version Dumbleator loop Regards Mark

Check out these links. All the calculators you could need. Regards Mark

No problem, if you had the circuit it would have been good to try it out. Having the loop after the master volume seems to be the convention these days. This what I originally had in mind. I can try my luck and ask Mesa, they can only say no at worst. Regards Mark

I dropped a 12AT7 in the P.I. stage, the stage is a bit brighter and there isn’t as much gain as a 12AX7. Regards Mark

The negative feedback reduces gain, that can be good and bad. If you have a high gain amp and you are getting the sound via the preamp and you will want that power amp to stay tight. More feedback is good. No feedback means more gain, but the bass response can be flabby sounding. Getting the...

If you don’t mind I would love to see the circuit of this effects loop as I wish to put a loop into my SOB. I don’t think this is the right way to think of negative feedback as it fails to take into consideration the current of the negative feedback signal. Wouldn’t you think that you have a...

This is the Mark IIC effects loop. Its a strange loop. Have you tried pushing as much signal as possible into the cathode follower stage of the loop. It has this great thickening effect, perhaps you’ve already done it and are use to the sound? I wonder what issues with gain tone were resolved...

Above is the Mark 1 reissue amp and below is the diagram simplified. When nothing is plugged in output of the reverb return stage goes directly to the P. I. stage which is out of circuit. When leads are plugged in the reverb return line going to the P. I. is open circuit. The signal can only...

I didn’t think the SOB’s were as flexible as the Mark 1 amp they didn’t have the pull bright or the presence control. I think the output of the P.I. wasn’t particularly strong as the grid leak/bias resistors were increased to 330K. Converting didn’t require a complete rebuild, it just needed a...

H Fred Thanks so much for tracing out the circuit diagram. The P.I. stage looks the same as my original Mark 1 amp. 1. The 22K is there to decouple the power supply. My SOB amp didn’t have it and when I converted it to a Mark 1 amp, I found the amp went into oscillation with the new P.I...

One thing that puzzles me is all the reports of the early Mark 1 amps being bright sounding. I remember playing through a teachers Mark 1 amp and it was incredibly bright. My Mark 1 wasn’t nearly as bright and with lower gain. Ironically, I’m getting more gain from my SOB which was converted...

Unfortunately, there aren’t circuit diagrams which detail any transitions. That would make it all much easier. Regards Mark

Hi Fred The Mark 1 and Mark 2 amps are very different amps. The Mark 1 is more or less a Fender amp with mods. The Mark 2 is its own thing. Regards Mark

I thought I saw that years ago. I wasn’t that keen on building a Mark II at the time. The irony is the SOB I own has the Mark II transformer set for the build. This is worth mentioning, a long time ago I was rebuilding a Brown Note D’Lite kit for a guy. In order to get the best Dumble tones...

How is the restoration going? Regards Mark

You could do what Mesa did and build in a Princeton chassis. PCB for IV amp

Hi Fred I had a Fetron in my Mark 1 amp. As luck would have it, I lent it to a friends brother who returned it with a blown Fetron. I didn’t get annoyed about it as these things happen, but I was annoyed as he didn’t offer to contribute financially or tell me that it happened. I then bought...

Judging by the photo I posted if your amp, the presence control is similar to the presence control of the reissue amps. The give away is the 0.001uF cap.