Hi there,
I am not a frequent poster but I come around here from time to time.
I have a 1979 Mark IIA (60W, non reverb) that I bought attached to a rack mount adapter, nothing else. I bought a Combo shell and a vintage EVM 12L to have a complete package. The main problem with this amp is that it hummed like crazy so, after 5 year of intermittent work on it (I had two kids during this period) I manage to reduce the noise to acceptable levels. This is what I did:
0) Change the power transformer to an export model (I am from Spain, this won't affect to most of you)
1) Replace all carbon comp resistors with metal films. A resistor from the filtering section literally blow up on a rehearsal, decided to change all of them.
2) Replaced all pots with new CTS, with switches when needed. The Volume 1 pot (Input Gain) hummed like crazy at high level setting but stopped humming when reaching 10. This was indicative that the problem was on the pot.
3) Changed the diode that gets the DC voltage for the relay with a diode bridge. The relay made a loud treble hum when activated (clean channel) due to the fact that the DC voltage was not really DC but had a big AC component.
4) Changed the relay with new ones, this helps to reduce the loud pop when changing channels. I still get a pop, I think something can be improved here.
5) Very important, grounding reorganization. Pots, cathode and filter capacitor share the same ground bus on the preamp board. This introduced a loud bass hum.
a) Pots are grounded on a separate part of the chassis.
b) Cathodes are on the ground buss
c) Preamp filter capacitor is now grounded on the same ground of the rest of the filter capacitors. This is the responsible for most of the loud bass hum.
6) Heater voltage on the preamp, replaced it with a twisted pair of cables. The heater voltage runs on two separated bus on each side of the board. This can introduce mayor hum on the cables near by. To prevent this I replaced this bus with a twisted pair where the noise from one cable cancels the hum from the other.
To do this mods I had to cut some of the board bus and this was a major decision for me. But do not regret it, now the hum level is extremely low for a 33 year old high gain combo.
Hope you find this helpful!
I am not a frequent poster but I come around here from time to time.
I have a 1979 Mark IIA (60W, non reverb) that I bought attached to a rack mount adapter, nothing else. I bought a Combo shell and a vintage EVM 12L to have a complete package. The main problem with this amp is that it hummed like crazy so, after 5 year of intermittent work on it (I had two kids during this period) I manage to reduce the noise to acceptable levels. This is what I did:
0) Change the power transformer to an export model (I am from Spain, this won't affect to most of you)
1) Replace all carbon comp resistors with metal films. A resistor from the filtering section literally blow up on a rehearsal, decided to change all of them.
2) Replaced all pots with new CTS, with switches when needed. The Volume 1 pot (Input Gain) hummed like crazy at high level setting but stopped humming when reaching 10. This was indicative that the problem was on the pot.
3) Changed the diode that gets the DC voltage for the relay with a diode bridge. The relay made a loud treble hum when activated (clean channel) due to the fact that the DC voltage was not really DC but had a big AC component.
4) Changed the relay with new ones, this helps to reduce the loud pop when changing channels. I still get a pop, I think something can be improved here.
5) Very important, grounding reorganization. Pots, cathode and filter capacitor share the same ground bus on the preamp board. This introduced a loud bass hum.
a) Pots are grounded on a separate part of the chassis.
b) Cathodes are on the ground buss
c) Preamp filter capacitor is now grounded on the same ground of the rest of the filter capacitors. This is the responsible for most of the loud bass hum.
6) Heater voltage on the preamp, replaced it with a twisted pair of cables. The heater voltage runs on two separated bus on each side of the board. This can introduce mayor hum on the cables near by. To prevent this I replaced this bus with a twisted pair where the noise from one cable cancels the hum from the other.
To do this mods I had to cut some of the board bus and this was a major decision for me. But do not regret it, now the hum level is extremely low for a 33 year old high gain combo.
Hope you find this helpful!
