The amp is a combo Marshall JMP 50W Master Volume (two inputs) and on pcb there is "st1 iss.202"
The issue is that the amp when plugged into high input is oscillating. The more the preamp pot is cranked the more becomes noisy
I am checking tubes voltages according to this schematic
Also power tubes are glowing red and preamp tubes too
The preamp pot is actually changing preamp v1, v2 plate and grid voltage values
v1a
plate: 208V
grid: 0V
cathode: 1,8V
v1b
plate: 244V (preamp from 0 to 50% and at 100%), 215V (from 60% to 90%)
grid: 0V (preamp from 0 to 50%), -5V (from 60% to 100%)
cathode: 2,7V (preamp from 0 to 40%), 5V (from 50% to 90%)and 2,0 at 100%
v2a
plate: 155V (preamp from 0 to 40%), 150V (at 50%) and 100V at 100%
grid: 0V (preamp from 0 to 50%), -7V (from 60% to 100%)
cathode: 1V (preamp from 0 to 50%), 1,34V (from 60% to 100%)
v2b
plate: 285V
grid: 155V (preamp from 0 to 40%), 150V (at 50%) and 100V at 100%
cathode: 158V (preamp from 0 to 50%), 100V (from 60% to 100%)
pdf64 wrote: ↑Sun May 15, 2022 7:25 pm
Maybe 1 or more electrolytic caps have failed?
I suggest to bypass each in turn with a known good cap of similar specs.
I have already replaced the bias filter polarized capacitors 10uf, but the issue is still
bmx wrote: ↑Sun May 15, 2022 10:54 pm
Before you do anything, jack that -Vbias up all the way! It's hard to work on something that is melting down.
I've never seen a 12ax7 red plating. The plate resistors should limit current to well below what is needed to red plate. Very peculiar.
Do you mean to make "less negative" the control grid of both v4 and v5?
v1b and v2a have both grids that are reacting weirdly to preamp pot. In fact the more they become negative the more pronunced ecomes the oscillation
I’d wager pdf64 was referring to the dual 50 uF preamp supply can capacitor. If you can’t drive your preamp and maintain preamp plate voltages, it is likely your power supply caps are failing/leaking. V1 and V2 are both powered by the dual 50 uF preamp cap, so I bet that’s the problem.
Yes, need to verify that the electrolytic caps decoupling the HT nodes supplying preamp are working correctly, that excessive internal impedance there isn’t causing the issue.
It’s normal for massive signal levels to change DC conditions. In this case the massive signal is due to oscillation, but the same behaviour happens with a regular audio signal.
RC coupling between stages and grid rectification will pull the average grid voltage negative.
It probably just seems unusual because it’s more usual to check DC voltages at idle (and by definition, an oscillating circuit isn’t at idle).
I mean turn bias as far negative as possible to lower current draw. That may not totally work though as something oscillationg can reach absolute maximums. I'd probably remove the phase inverter tube before testing while powered on.
Good luck. Might be a good idea to consult a tech. This amp is probably worth it
Thank you for all your feedbacks. So i have ordered 3 F+T capacitors 50uf 500vdc rated and i will replace them. I have set the power tube bias as max negative but the oscillation is still there. What i am also noticing is when i lower the volume of single coil guitar the oscillation kicks in, instead with full volume and neck pickup or bridge no. But middle position is the oscillation party.
Could be parasitic noise issue so related to capacitors, again?
Last edited by Turbonigro on Sat May 21, 2022 11:59 am, edited 1 time in total.
The lead dress around the input jacks may be bad.
Is the green wire connectors to the hot terminal of the high gain input?
Try using a chopstick (or other non conductive probe; not a pencil!) to alter the relative positions of the green, pink etc wires.
Is the brown mica cap properly soldered to the input stage’s 100k anode load resistor?
pdf64 wrote: ↑Fri May 20, 2022 6:13 pm
The lead dress around the input jacks may be bad.
Is the green wire connectors to the hot terminal of the high gain input?
Try using a chopstick (or other non conductive probe; not a pencil!) to alter the relative positions of the green, pink etc wires.
Is the brown mica cap properly soldered to the input stage’s 100k anode load resistor?
The green is going from hot terminal of high input to 68k than to v1b grid (first preamp tube on schematic). What is weird is that from v1b grid the wire was soldered under the board, directly to 68k lead because there seems to be a cut trace.
I have used a chopstick and green wires seems to carry the noise. Moving it changes like the frequency. I am checking solder joiints on coupling mica 20nf cap
I have routed away from the rest the grid of v1b, still no luck. So the green wire seems to not be the problem.
The wire under the board from grid to grid stopper resistor is ugly as hell
is it a normal behaviour on v1a (my mistake, second side of first preamp tube) grid? negative voltage? Could be a cathode resistor fault?
Seems a bias issue to me, preamp pot (according to schematic) is forming a voltage divider so when pot is at full the attenuation should be done by 470k resistor, am i right?
Can a faulty filter cap enhance this bias problem?
My guess is that there isn’t a bias problem. As previously mentioned, a negative grid voltage can be caused by a massive signal voltage at the grid, in this case the massive signal may be due to oscillation.
Try turning the gain setting down slightly so as to stop the oscillation, and feeding a big signal into the amp; you should measure something similar. Grid rectification in action.
