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To measure the taper of the pot you put the wiper of the pot in middle and measure the resistance. If you have a 250K with the wiper in the middle and measure 25K then the pot has a 10% taper. Regards Mark

The presence pot is 10K, you won’t get much out of a 100K pot, it would be more like a switch. The brand of the pots isn’t that important, what is important is the taper of the pots. I really like a 30% taper on the 250K treble pot. If the treble pot has a 10% taper I find the response is...

The bias circuit is on the power supply board of the SOB. The bias circuit is on this 1976 vintage power supply board too. The SOB has a bit more realestate for the pot. The board might be made of Garolite, it’s very popular with boutique amp builders.

One thing I remember about my Mark 1 amp. The valves in the amp were top shelf valves. There was the Fetron in position 1, and the rest of the valves were bugle boy 12AX7’s and Sylvania STR 6L6’s. Unfortunately, we don’t get valves of this quality now. Regards Mark

This is how my SOB is set up to deal with biasing the output valves. As shown in the first shot, one resistor is replaced with a ten turn pot and it’s stuck in place with avionic grade silicon. The second shot shows one ohm resistors going to ground which allows me to measure the current going...

The early amps are all class AB. The bias control puts a negative voltage on the grids of the output valves. I can open the amp later and show you where I put the bias pot. Regards Mark

In Australia a stock Mark 1 amp 60 watt with an Eminence speaker, reverb, in a black cab were $1200.00. There was only one seller at the time so they were charging what the market could tolerate. The speakers available at the time were JBL and Altec. I think JBL speakers were in the other...

I ditched the factory select resistors and put in a ten turn pot to set the bias. This worked for me. Regards Mark

I suggested the 418 as they were in my Mark 1. The Dumble cloners use 6PS as a substitute for 418’s etc. The 715/716 poly have a reputation for being brighter than polyester caps. I haven’t A/B the caps in an amp, but I thought I heard a difference in the output stage 0.1uF caps. Your preamp...

I wouldn’t use carbon composite for screen resistors, I personally prefer metal oxide as they are very durable. Randall Aiken on resistors I had carbon composite resistors in my Mark 1 amp and the resistor values drifted up as high as 2K, it’s amazing the CC resistors in your amp were so...

In this picture of your amp you can see the 470 ohm cathode resistor. Two 1M ohm grid leak resistors, the 15K resistor, 56K feedback resistor, the 10K tail resistor which has a 10K presence pot across it. The is a picture of the 470 ohm cathode resistor, two 330K grid leak resistors or a 330K...

Okay, SP-3 is the number on the pcb. The brown drops might have been Mallory caps, I have seen them in Orange and Blue. I have seen pictures of a Mark 1 with brown Mallory caps. In my late 70’s Mark 1 amp it had Sprague 418 caps. These are a polyester cap. 6PS/418 caps The thing that has me...

Ha, I only have more questions, what is a SP-3 and a SP-8B? I would appreciate the explanation if you have time. Regards Mark

Did you find out what you needed to know from your other post? Regards Mark

I don’t know anything about the early Mesa amps and how they evolved into the Mark 1 series. I’m aware there are different variations of Mark 1 amps. There are two different P.I. stages, there are differences in the triode gain stage on the front of the amp. Regards Mark

Hi Fred You are very good to us and it’s appreciated. I would have to rewire the co-ax in my amp to implement that. I did a search for a photo where the resistor is more easily seen. There is also an example of where the wire wasn’t implemented into the amp...

Thanks for clearing that up Fred. I didn’t think the wire would be around the co-ax cables as it would effectively be around the earth on the cables as opposed the actual grid wires themselves. I can’t see the 220K resistor in the photos, but I don’t doubt that it’s there. Regards Mark

Good spotting Thomas, there are photos of Frederick’s amp on the thread titled “ Help with my 1978 Mark 1”. Mind you I had a quick look at the pictures and couldn’t find wire wrapped around the grids of stages 1, 2, & 3. The first and second stages use co-ax cable and I couldn’t get a good look...

Thanks Fred for your assistance, I appreciate your help. I can delve inside of my amp now. The voltage to the output transformer is about the same which is promising. Regards Mark

The other SOB and more common I suspect circuit. I have included the circuit as it’s useful and links disappear from the internet. Regards Mark