Mark IV Volume Loss

[egdon]

Hi

I'm having some trouble with my Mark IV - hoping someone can help point me in the right direction.

A long time back, it crapped out on stage...some smoke came out of the transformer and the sound went to dirt. Had it repaired/retubed at the time, and it worked, but it never quite sounded the way I liked it. Meanwhile the repairman closed up shop and I could not contact him - he just disappeared. I used other amps a lot more over the following years, and eventually stopped using it at all [part of that being that I wasn't gigging anymore]. I always thought the IV cut through well with a band, but at home in an apartment it's a little less useful

Fast forward to recently, I really wanted to get it back into my setup, so I fired it up, and it made almost no sound at all. While I was letting it sit for several minutes, there was a pop sound from the speaker and a slight burning smell. Turned it off [fuse did not blow], put in some older but not dead power tubes I had sitting around, swapped a preamp tube around here and there, and got some sound, although anyone that has a MkIV knows something is wrong, because I was testing at master volume 6-7 in the middle of the night, and didn't wake anyone up. So there is a volume problem. Bought all new tubes just to eliminate that as a suspect...new preamp tubes were even worse, and I tested 4, all the same JJ ECC83s. the new JJs completely kill the volume [and distortion] if I put them in V2 or V3, although they work ok I guess in V1. The funny thing is, the older tubes I had hanging around at least *work* in V2 and V3, even if the volume is still low, so I'm not sure how to troubleshoot the JJs. But something tells me they can't all be bad...maybe it was the smoke that gave me that clue ;-) No smoke or any pops etc at all since then, and it has been somewhat playable with the mix of old tubes, but it's not loud.

So I know something needs to be fixed, but I want to be able to understand the problem, even if just to explain it to the techs. Anyone have a guess at what might be the trouble and what troubleshooting I might be able to do to get closer to the answer?

Really appreciate any pointers anyone can give me.

Thanks

 

[kiff]

I think you might be experiencing two different issues. your main problem sounds like something a tech needs to look at. but the other...

i think m4's must be really super sensitive to preamp tubes or something... I'd replace my m4's preamp tubes a while back and I would lose volume and gain from the preamp when using tweed power. volume from r1 and r2, but the lead channel had almost zero gain, it sounded like r1. swapping back some of the original preamp tubes fixed it.

 

[egdon]

I have definitely had that problem today - putting the JJs in V2 or V3 would reduce the volume, but would cut the distortion too, and with a JJ in V3 my lead channel was clean. Swapping the old ones into those sockets has helped, but the amp still is very low volume.

 

[kiff]

are you using tweed or full power? triode or pentode? simul or class A?

IIRC, tweed power had the most effect and I'm pretty sure using triode and class A added (a little) to the lead gain problem...

Using full power almost totally "fixed" it... but, I swapped tubes until all modes worked...

 

[egdon]

I tried all different modes, tweed/full, classA/simul, triode/pentode...I think I have a bigger problem as you said. Today I contacted the tech and I am going to bring it to them and get it done right. I like to troubleshoot and such, but I don't want to take any more chances with this amp

Thanks for your help Kiff - I'll post what they tell me!

 

[egdon]

So, I brought it in. Turns out there was a blown trace on the board that burned a resistor. They jumpered it, replaced the resistors, and it worked. Also turned out that the guy who had repaired it years ago had put in a bias trimpot, and they removed it and put it back to stock. New power tubes, 5881s in the middle and el34s on the outside.

Brought it home, played it for about two hours, and it crapped out again. Brought it back, and they found a piece of trace hanging from under the board and occasionally flapping against a nearby connection. They fixed it again that same day.

I've had it home for a few days now, and it's been pretty good, and sounding fantastic. But tonight another weird thing started. Funny, but it seems very similar to the problem you described, in that I lost a little power temporarily, but not as bad as it was before, just a slight dip. It happened twice, and both times it was in Tweed. I also noticed that the eq light seemed a little dim at the time. I switched to Full Power and it seemed to be gone, but I'm playing constantly to see if I can make it happen again. Guess I'm going to have to bring it back in again.

I know the EQ light is probably unrelated, and I'm not sure if it had dimmed at other times. I also haven't tried swapping some preamp tubes, which I will do, but I want to wait and see if it happens again. I'm also paranoid by this time, and I'm always listening to hear if there is the slightest dip in volume.

I should also mention that both times it happened I had the Silent Recording switch pulled and was using the recording out. It doesn't sound great that way as it bypasses the power section, but it is a must for me to play quietly late at night

 

[Restless Rocks]

The pull pot switch on your outputvolume/pull silent recording is a known possible problem for volume issues. It is also a known that certain Russian made preamp tubes don't work well in Tweed power setting.

 

[egdon]

Last night, it happened again, and I was just lucky enough to notice that it was the silent record switch - you hit the nail on the head. Thanks Restless Rocks