Marshall 1987x issues....

[Dr. Jimmy]

Hi, I'm new here and am desperate for help!!!

It all started at a gig last week in which I popped the mains fuse. Putting the amp on my bench today I noticed one of my power tubes was bad and was the culprit. I put in a new set of Winged C's and biased 'er up.

After getting the bias all set (these tubes are matched really tight, 16.42W and 16.48W) and the amp warmed up, I ran 1k into it and checked the output on my scope. With the signal applied to ch.1 I can barely crack the volume knob before the signal clips horribly, with a high pitched squeal emitted from around the tube area. Putting the signal into ch. 2 gives me a damn good signal on the scope, and I'm able to turn the vol. up to a good level before clipping.

I've replaced V1 in hopes that V1B was suspect, but no dice.

Removed the bright cap tried it with a 390pF cap on the vol. 1 pot, as soon as I turned up the knob just a hair the signal clipped, I heard the oscillation, I measured 59W across the load and the 1k input signal was magically changed to 10k on the output.

Turned off the signal generator, left it plugged into ch. 1, turned up the vol. on the amp and still saw/heard oscliiation, clipping etc...

Plugged in my strat and banged a few chords, could hear the oscillation happening as I hit the strings harder. Vol. on ch. 1 around 9:00. My theory is that this is what caused me to blow the tube at the gig Saturday, I was plugged in and hit a few chords when the amp just died. The oscillation must've redplated the tube and it finally quit.

Removed the cap, and I was able to turn the vol. 1 knob to around 9:00 before it clipped, no oscillation heard, measured approx 55W across the output, output was 1khz.

Ch. 2 showed none of these problems.

I've always run this amp into an attenuator, jumpered channels with both volumes at approx. 1:00. I wonder if I have damaged it by doing that???

Sorry for the long post, any ideas or suggestions?

[Dr. Jimmy]

Just wanted to add that I've tried swapping preamp tubes with known good ones, to no avail. The oscillation is there regardless of what size cap I place across the vol. 1 pot. I've also inserted the signal directly into V2 and noticed no oscillation (doesn't mean it wasn't there, I just didn't see or hear it).

Also I have measured 196V on the cathode of V2b (R10 and R11) does that seem normal?

Thanks again.....

[Chuck H]

If I'm reading your post correctly you have the oscillation in the preamp, correct? If not then scope at dofferent points in the circuit to find the earliest stage where the oscillation begins to show. If it is V1 I would check the bias conditions for that tube. Could be a leaky coupling cap or a bad ground.

Chuck

[Dr. Jimmy]

If I'm reading your post correctly you have the oscillation in the preamp, correct? If not then scope at dofferent points in the circuit to find the earliest stage where the oscillation begins to show. If it is V1 I would check the bias conditions for that tube. Could be a leaky coupling cap or a bad ground.

Chuck

Hi Chuck and thank you for the reply.

It appears to me that the oscillation is beginning in the preamp section, correct. As I've mentioned, the problem is not there on input 2 only on input 1, and only when there is a cap on the pot. It is possible that the oscillation is there without the cap, but I cannot see it or hear it then.

What should I expect to see for bias on that particular tube? And which cap would be the coupling cap to check (there's a few off of V1b, just not sure which one it may be). I've attached a gif of the schematic I am working off of.

One last dumb question: Can this oscillation be the cause of the excessive power (leading to a blown tube) on the output that I am seeing?

Thanks again!
Bill

[Twist]

I've had amps in the shop that had pots with broken traces inside. The amp would sound fine until I turned the pot past the break and then it would either be dead or the volume would be wide open. Since channel two is ok I would concentrate on channel one.

[Dr. Jimmy]

I've had amps in the shop that had pots with broken traces inside. The amp would sound fine until I turned the pot past the break and then it would either be dead or the volume would be wide open. Since channel two is ok I would concentrate on channel one.

I've actually swapped the pots (i.e. put the leads from the ch. 1 pot onto the ch. 2 pot and vice versa) and the problem moved with the leads.

I also tried inserting the signal directly into V2 and the problem appeared to go away, again narrowing it V1 (or V1b, more specifically) I think......

[Twist]

The cathode voltages will be high on the second cathode of V2 because it's cathode follower circuit. The cathode voltages on the first tube should be around 1 to 2 volts. Check your lead dress around the first tube and make sure all grid wires are shielded.

[Dr. Jimmy]

The cathode voltages will be high on the second cathode of V2 because it's cathode follower circuit. The cathode voltages on the first tube should be around 1 to 2 volts. Check your lead dress around the first tube and make sure all grid wires are shielded.

Vk on V1 are both between 1 and 2 volts, as far as the grid wiring is concerned, it's a stock Marshall 50W reissue so I don't know if those wires are shielded or not. Either way, the amp never behaved this way before so I think something went south on me......

Is it possible the tube sockets need to be retensioned? Could that cause it?

[Twist]

It's good to retension the sockets but I don't really think that is causing your problems. Your cathode voltages would be missing or way off if you had a bad socket. Has someone sprayed some sort of cleaner into the socket. I've found all sorts of gook people spray into the socket and ends up creating more problems. Looking at your schematic, there's just not much that can go wrong in that part of the circuit. You've eliminated the tube and pot. Check C4 to see if it's shorted and maybe remove it. That's used as a high frequency pass like the capacitor across the volume pot. Maybe try replacing
C2 and C4. Unless you like the brightness that the small cap across the volume produces I would leave it out. I usually go ahead in older Marshalls and use shielded wire on all the grids.

[clintronics]

Deja vu all over again

I posted on this model 2 weeks ago. After i fixed in ouptut section it had exactly the same problems you discribed. I also replace the gain control because the old one wouldn't go to zero. But it would squeal and have tons of gain. I replaced input jacks because they didnt seem to ground out with nothing pluged in. Then to make matters worse audio stops all together. V2 no longer is lit. replaced the tube socket. amp seems very normal now. only get a squeal when guitar pickup was close to input jacks(less than a foot) when chassis back in cabinet and shielded sounded great never did any of those weird things again. hope my story helps a bit. but this one drove me nuts. BTW what value are your grid resistors 1k or 2k2?

[Dr. Jimmy]

I posted on this model 2 weeks ago. After i fixed in ouptut section it had exactly the same problems you discribed. I also replace the gain control because the old one wouldn't go to zero. But it would squeal and have tons of gain. I replaced input jacks because they didnt seem to ground out with nothing pluged in. Then to make matters worse audio stops all together. V2 no longer is lit. replaced the tube socket. amp seems very normal now. only get a squeal when guitar pickup was close to input jacks(less than a foot) when chassis back in cabinet and shielded sounded great never did any of those weird things again. hope my story helps a bit. but this one drove me nuts. BTW what value are your grid resistors 1k or 2k2?

The grid resistors on the output tubes are 1k in this particular amp.

I don't even have this head plugged into a speaker cab, it's going into a dummy load, the squealing I am hearing is coming from the tubes (or at least that where it sounds like its coming from....).

[Chuck H]

With a cord plugged in (but no guitar), measure the resistance between the grid of v1a and ground. Also, lift the end of c3 that is downstream of the plate for v1a (the end that goes to the volume control). With the amp on and playing (not in standby) check dc voltage on the lifted end.

Chuck

[Enzo]

Jimmy. Clintronics said something you passed over. Once back in the cab it no longer did it.

A lot of signal radiating away in there. In the cab, there is a metal shield that covers the open bottom of the chassis. Set a sheet of steel over the open chassis or slip the chassis back into the cab and se what happens.

[Dr. Jimmy]

Jimmy. Clintronics said something you passed over. Once back in the cab it no longer did it.

A lot of signal radiating away in there. In the cab, there is a metal shield that covers the open bottom of the chassis. Set a sheet of steel over the open chassis or slip the chassis back into the cab and se what happens.

Sorry, I did put it back in the cab, and the problem was still there....

[Enzo]

Oh well, then we are back to scoping.

Look on the B+ rail for the stages affected. Is this signal showing up there? If so, look for a failed decoupling cap or a lost ground to same.