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Much appreciated Frederik. The voltages in the SOB MK1 are as follows. Mains voltage: 238.5vac O/P TXer node: 460VDC Screen node: 458VDC P.I. Node: 407VDC Preamp 1 node: 361VDC Fetron Pre 2 node: 344VDC The amp was rebuilt, with new caps and resistors, I used the Mallory 150’s as I I had them...

If I look at the early circuit diagrams, the main rail voltage is recorded as 450VDC, 446VDC, 345VDC, 260VDC, if a Fetron was used 225VDC. I know that these voltages didn’t line up with my original Mark 1 amp. Regards Mark

I have a Mesa Boogie SOB which I converted to a Mark 1. I’m think the rail voltages are effecting the tone of the amp. I don’t remember what the voltages were in my original Mark 1 amp. I knew they weren’t the same as the old circuit diagram. I will measure the voltages on the nodes of my...

Hi Joey, What is the post FX loop in your amp? Gut shots? Regards Mark

So what is the circuit in you amp? Regards Mark

The signal from the secondary of the output transformer must surely be out of phase with the input signal to reduce the output of the P.I. stage? I say this as the pot goes to the grid of the other triode in the P. I. stage.

Okay, I understand that the Limit control is a variable resistor, but more importantly the pot is attached to the secondary winding of the output transformer. This surely must be important. Why does adding resistance make the current limiter less relevant? I would think the pot is introducing...

I don’t follow what you are saying. I understand the bit where you say replace the TCR5305 with a 1.5K resistor, but the bit about freeing up the pot doesn’t sound right. Will the AC content be on the cathode? If it were then the grid attached to the pot would have to be grounded. TAG thread...

Again thank you so much for chasing this up for me. I think it would be 47pF and the K would refer to 10% tolerance. I will have to try it out and see what it’s like. Usually 47pF is quite subtle. It might allow more noise through though. Regards Mark

Agreed, the Triac does lower the volume but it comes with some other nasty side effects. SOB circuit diagram There is no negative feedback so the bass isn’t tight, the P.I. Is easily loaded down for some reason. I think that’s the reason for the 330K grid leak resistors. The circuit is a bit...

Thanks for opening the amp and taking the pictures of the first volume control. I couldn’t see in the pictures, but what value did you see written on the cap when you had the amp apart? I still find it hard to believe that an 0.047uF cap would be in that position, I could believe a 470pF (471)...

The are a few factors to consider where using a multimeter. What brand of meter is it and how accurate is it. Did you make a good contact with the meter leads? The other factor which doesn’t effect guitar amps is the frequency range of the meter and the input resistance of the meter. With...

I hear about these speakers and yet there is no repro version of them. It’s really quite strange as they seem relatively popular. Regards Mark

Hi Fred In your circuit diagram, I think there is one error. Vol 1 has an 0.047uF cap across it in red. If this cap were in the circuit the amp would either have no output when the Vol 1 was set to zero or full output when the volume control was turned up. The cap could also shunt signal on the...

I recall back in the day people liked the Sidewinders but blew them up. As your speakers have survived so long you should be pretty free of that worry. Regards Mark

My God, only three posts on such a great player? I guess we can consider ourselves to have the ability to recognise greatness. Regards Mark

Indeed, rest in peace Jeff Beck. I only saw him play once in Sydney and it was a great experience. With the first chord you knew who was playing. The sign of a great player. I too first heard Beck when I was in my teens and it seemed that he had discovered notes on the guitar which other...

I would leave the loop in place. It’s a good design. You may wish to see if it’s transparent by bypassing it with a switch (temporarily without drilling holes in the chassis.) Regards Mark

I received a reply from Mike Bendinelli and the loops he installed were similar to the Dumbleator style loops. There were no loops in place on the reverb section of the amp on amps with no reverb. I hope that is of some assistance. Regards Mark Abbott

To update, I received a reply from Mike Bendinelli and he states: “I have installed a loop on a very few original MK1 Mesa amps. I inserted after the master, at the input to the phase inverter.” In another thread there is talk of effects loops being inserted where the reverb circuit would have...