Marshall 1959SLP Problems - Please Help

[Peter Nerves]

Thanks Ray, that helps.

This is what has just happened - I could not get a sound out of the amp, not even intermittent, so I put the new power tubes in. Still no sound, until I plug into input 1 high, and turned the vol up to about 5, back comes the sound. I plugged into input 2 high and turned the vol to about 2 and backed off the high gain vol. All is good, I am giving little thuds on the srings to see if the amp is going to cut out but it never. Then a very loud bang bang popping noise coming out of the cab and when I looked at the valves the 4th power valve (V7) had huge arcs and sparks inside it. I have switched off the mains to the amp now.

Any ideas on what has just happened? Will my brand new output tube be fried now?

Cheers,

Pete.

[Ray Ivers]

Pete,

As Max mentioned, it may be time to take your amp to the shop.

In the hands of a seasoned tech with years of troubleshooting experience, a DMM can be a consummate revealer of problems both big and small; I've seen this with my own eyes, more than once. OTOH, it can be very misleading; for example, the 0V reading at the V1 cathodes - which seems to come and go (does that sound like a problem to you? - it sure does to me!); is that caused by an open cathode resistor - which I mentioned earlier, but was evidently never checked as "the meter reading was OK" that time - or a shorted cap? An oscilloscope would tell you in a heartbeat, but a meter... well, I surely don't have to tell you, obviously you know the shortcomings of DC-only troubleshooting as well as anyone right about now.

Is V7 toast? I don't know... but the real question IMO is, "did you damage your output transformer - or something else - during the 'big bang'?"

If you're intent on continuing using just DC meter readings, I would pull the power tubes - no sense in repeating a high-power failure event. FWIW, I still believe the problem is somewhere in the cathode circuits of the first 2 or 3 gain stages, and the output tube arcing could have been caused by a huge preamp-generated transient (or a number of other things).

Ray

[Peter Nerves]

Thanks Ray,
I think the time has come to get this into a repair shop as you have said, I would have loved to have solved this, but I dont have the experience or equipment of a seasoned tech. I will get it in tomorrow, and ask the tech to give me the full lowdown on what the problem is and post their findings here.

The mains fuse blew when V7 started arching, I replaced the fuse put all of the old tubes back in, and it has gone back to the way it was when I started, just cutting in and out when it feels like it (I can make it come back by turning the vol to above 5, and it stays in when I turn down to 1, it stays in while I play softly, but whack a chord and it cuts out, repeat the above to get sound back). I wish I had hold of some Progold, as the problem was not anywhere near as bad as this before I pulled the original tubes, so it screams out to me that disturbing the tube seats has made the problem worse. Tapping the tubes does not cause the sound to cut in or out, and the heaters are lit all of the time.

Thanks for you help Ray and everybody else who has given me advice.

Its is going into a shop before I cause some major damage

Cheers,

Pete.