It was instant and I took out the rectifier to find it was good. However when it is it tests bad. I did find two tracks had been shorted by some clumsy soldering, went across this with a screwdriver to seperate them but still the issue remains and I still get AC on the DC side of the Rectifier. Perhaps the rectifier is bad? Can they go bad under load? I just tested it with a multimeter.

EDIT: Just tested it again and found 12VAC on the other side of the rectifier, however its all running on a 6.3VAC Winding ......... Its not the valves as I got a higher value when the valves were out. DC was at 5V or so.

HT Stays steady. Now shortly before this issue started occurring this amp was repaired as it wouldn't work at all. Issue was a bad solder joint on a heater connection apparently. Just checked the heater voltage for the valves, on the 12AX7s its 5.8V which is acceptable I think, however on the EL84s its 2.6V! So I unplugged the Heater Winding from the board and read 6.4V directly on the PT itself. Now there are two 100ohm resistors going to ground from either Heater Winding and with the Heater Winding unplugged they are effectively seperated from each other. Measured them and I get 50ohms, odd, measured the ground side and theres 5ohms between them and the chassis. Looks like I'm going to have to take the board out to investigate further.

EDIT: Looking at the schematic the EL84's heaters come straight off the PT Heater Winding and should be at 6.3V, the Pre Amp 12AX7s come off a Bridge Rectifier which is connected to the PT Heater Winding and should be at 12.6V DC. Now when I measured the 12AX7's Heater Pins I found about 3V DC but 5.8V AC. Possibly a rectifier issue?

No but they werent gd, checked the bias, one was at 19Ma and the other at 36Ma! So i swapped them for two that I knew were gd and biased at 20Ma and 24Ma, same issue.

Also I should mention that if you turn the volume back down then volume wont come back until you flip the standby off and on.

Ok I might get my mate in to do that test as its his amp . Its definitely something in the output stage though as when using the D.I. Output which is just before the Output Valves the amp is fine.

Ok no didn't have that. What I will try is connecting a speaker directly onto the transformer to rule out the board tracks, if that doesnt work then i've got a new 50w transformer coming for another project so i'll try that on the amp and see if it makes any difference.

J M: The speaker is fine as I have also tried my own speaker cabinet and get the same issue. I haven't properly checked the solder joints for the secondary winding I must admit as the volume loss occurs at the Anodes and like I said the volume almost cuts out so its like something starts to short when the output voltage goes so high. Wouldn't be a Primary Winding as the Anode Voltage stays constant so I thought it may be a Secondary winding dragging the AC down. Then again would I be right in saying that if the speaker joint was bad then the impedance could change so much and would affect the signal coming out of the Anodes?

Roly: I think I'll try and do that test if I need to as my 18w Output Transformer will be a pain to take out of my 18w amp. First though I need to build myself a current limiter to ensure I don't blow the PT.

Enzo: I checked the Anodes of the EL84s with my scope and when the volume dropped I couldn't see any oscillation. Then again I'm guessing the oscillation will be obvious like a spike in the waveform?

I'll see if I can use the transformer from my 18watt amp.

I should also mention that when the volume drop occurs if you turn the standby switch off and on then sound comes back for a second before dropping again and will keep doing this until you turn the volume down.

Hi Guys

My mate has a Marshall DSL201 which I think may have a shorted Winding in the Output Transformer. Now the issue he gets is when he turns the volume up above a certain level the volume suddenly goes down real low, so low you can only just hear sound coming out of the speaker. Doesn't matter if you have the pre amp volume up and master down or vice versa.

So I've gone through the amp with my oscilloscope and found the volume stays constant all the way up to the EL84's Grid Stoppers, however on the Anodes of the EL84's is where the drop occurs. Soon as you go above a certain level the output from these valves drops significantly. So I believe it could be the Transformer as I would have thought a shorted winding would have the effect of pulling down the Output of the valves. Also when the amp is played at low volume with distortion the distortion doesn't sound as good as I think it should.

Now I've checked the voltages when the volume drop occurs and the B+, Anode and Bias all stay at the same level when the issue occurs. I've also changed the valves which didn't make any difference. My mate did say that the problem started occurring when he lent the amp to a local band who cranked it for a gig, although I don't think this should have killed it as Marshalls are designed to be run at full volume. I should also add that the board inside this amp is actually for a DSL401, looks like a tech changed it a while back, most likely to overcome the Bias Drift issue that these amps are known for (among many other issues!)

Sound like Output Transformer to you guys?

The current draw does follow the valves so its not that particular socket. I've tested all of the capacitors and they're fine so i'm just going to leave it. The valves aren't red plating and the amp sounds great so may as well leave it.

Good point, didn't think of that. That explains why the voltage is changing overall but often i'll find one valve drifts much higher than the other after i've played through thr amp. Then if I turn it off, let it cool, turn it back on and they may then have a similar current draw. Once one of the valves went from 35ma to 40ma whereas the other stayed at 32ma. I'm not too worried as the amp works fine and the bias is still below 70%, just think its odd for only one of them to drift.

Well in that case the Amp is all good. Thanks for your help

Right, Current Draw is 88ma, Voltage is 445V, so that gives us 5562ohms of resistance. 5562 x .0003F = 1.66875seconds so yer thats about right. When I turn the amp off its probably 2/3 seconds by the time I get my meter on it and the highest I've managed to read is 228V. So all good

Hmm I could remove the HT Fuse and put my meter in it's place, suppose it wouldn't hurt as I would only be doing it for 30 seconds. Thanks Roly

Yer thinking about it the PI isn't balanced itself which in most cases sounds better (for the guitarist). The source of interference actually turned out to be one of those LAN Adapters that goes through the power cable. I had the amp plugged into the same four way, just by chance i plugged it into it's own wall socket and realised the amp was silent when I switched on.

Now I think there may still be an issue with a cap somewhere as when I turn the amp off it drains the power pretty quick, yet there is nothing that I can see that would do this? No resistor to ground from the HT line or anything.

Hi guys

I had to buy new EL34s for my 1987xl recently as one went bad and have had a few issues with the new valves. The valves are Svetlana Winged C EL34s from Karltone and get burned in for 30-40 hours. Now when i first received them they werent matched as they were about 4ma out. I didn't think that was toooo bad so biased up to 70% which was 38ma if i remember rightly and then played through it. After I played through it I found one valve had drifted and was now 10ma out, also noticed that the HT voltage was going up and down by 4 volts by itself, with no drive. So sent the valves back, got new ones and these biased up perfectly. However after a bit of playing same thing happened! Altho this time it was only 4ma and the HT Voltage didn't appear to go up and down at first but originally it was 440volts and now its 449volts. Also I found that the plate current had drifted up by 5ma. So rebiased, Swapped the valves and the readings follow the valves. When I first turn the amp on I do hear a very slight hint of motorboating, however it doesnt increase with volume and the valves arent redplating. Played with the amp for another hour, rechecked the bias and its now gone down a bit and both valves are matched again!!!! Any ideas?