Help with schematic

[axer57]

I have owned my first Marshall for a couple of years now. I have a JTM60 212, I know they are not popular but mine works fine. I decided to try a few of the mods I had seen here so I got the schematic and opened her up. I ran into 2 very strange things. R121 on the schematic is a 100k resistor. In my amp it's a 470k resistor. Also R124 on the schematic is a 22k resistor but in my amp it was a 1.5k . Does anyone know if the schematic is right? I would imagine if I change them to match the schematic , my boost channel is gonna lose alot of gain since they both are used in biasing V2.
Neither one looks like it's ever been changed , they both look like factory solder .

 

[VanR]

Why change them if you like the way it sounds? Amp makers often change the value of components mid-production without notice.

 

[axer57]

I wasn't planning on changing them. Just wanted to know if they were right.

 

[mickeydg5]

Did you purchase it new?
It seems as if someone modified the preamp stages.

 

[axer57]

No , I bought it off a guy who was using it for a practice amp. He was having the usuall overheating issues and just wanted to get rid of it. I installed a top side vent and a cooling fan , slapped in a set of RCA 6CA7's and that's it. The solder on the 2 resistors in question looks original though. If it has been modded then I would think there would have to be more to it. If you put these back to the schematic values I would think it would drastically change the gain on the boost channel , since each would reduce gain on V2.

 

[VanR]

Well you could parallel resistors across those you have in there to get the schematic values and see if you like the results. If not remove them.

 

[axer57]

Well curiosity got the best of me. I changed both resistors back to the schematic values and as I expected the gain of the boost channel was dropped dramatically. Something I didn't expect happened also. I didn't think v2 had anything to do with the clean channel , but now the clean channel seems to have more bottom and a more Fender sounding tone. Now if I had a way to put back the gain on the boost channel , the amp would be a really good mix of clean and hot.

 

[mickeydg5]

With the amplifier back to stock you can raise gain on V102A & B stages by

1. paralleling a 22k resistor or somewhat lower resistance at R124 22k (or change it to a lower value)

2. paralleling a bypass capacitor of about 1uF or somewhat lower value at R123 1.5k
3. paralleling a bypass capacitor of about 1uF or somewhat lower value at R124 22k
4. The same can be done at V101B at R117 1.5k

Find a happy median for all channels.

 

[axer57]

I appreciate the suggestions. I would like to find a way to increase the gain on the boost channel without changing the tube bias though. I have tried a cathode bypass cap before and it rolls off too much bass. This amp has a perfect mix of tones on the clean channel but the boost channel has always been thinner sounding. If I could get the same tone as the clean channel with more gain , on the boost channel , it would be great.

 

[mickeydg5]

Well, altering the voltages at V2's plates and cathodes is one way. A lot of people alter the cathode circuit to suit first.

But another is to alter signal attenuation and bandwidth. Try increasing or paralleling coupling capacitors at C118 4.7nF and C119 10nF just a bit. Parallel or decrease resistance at R4 470k. If possible increase resistance of potentiometer VR5 or place a lower value resistance between it and ground to get a signal level and gain range you like.

 

[axer57]

Thanks to everyone for the input. I finally decided just to go old school and I changed R117 and R124 both to 820ohms. worked great. both channels sound great and I have more than enough gain on the boost channel. I also lifted the leg of C105 and took the diodes out of play...It sounds great. I think my old Princeton will get alot less play time now...

 

[mickeydg5]

great!