Amp Problem

[JumpMarine]

I was playing tonight through my JCM 601 combo and I noticed a very faint chugging sound coming from the amp. It is audible on both the clean and gain channel. I tried out different preamp tubes to eliminate that, nothing changed at all. I haven't swapped out the power tubes yet to check that. What could it be? And what more could I check?

[Kent S.]

Re: Amp Problem

Define *chugging*? Sounds like you are talking about motorboating as it's called ... if that's the case you might want to hit Aiken's amp site (or others) IIRC it's caused by the filter caps starting to go, and/or by something else which escapes me at the moment ... it's like a low frequency oscillation or thumping-popping that continuously occurs by itself.
Could you be more specific?

[JumpMarine]

Re: Amp Problem

Thanks Kent, motorboating makes more sense. As it occurs, it is a constant popping and not an ocillation. It is very subtle and can only be heard while not playing and the MV is turned up. I noticed it for the first time last night and will monitor if it gets louder or faster in rate. If it is the filter caps, what are my options for repair? Is this a safe self repair job or better to hand it over to an amp tech.

[Curly]

Re: Amp Problem

I'm just taking a guess here, but does it use a tube rectifier? If so, you might try switching it to see if that's the problem.

[jeremy]

Re: Amp Problem

you could change your own caps but there is a lot of voltage in there so it might be better to bring it to a tech

[STRATDELUXER97]

Re: Amp Problem

Is that amp 10 years old or more yet? The filter caps should be just fine...Sounds more like it's preamp related if it does it when you raise the gain up...No?

[Curly]

Re: Amp Problem

one tip when trying to trouble shoot, and isolate which part of the circuit the problem is related to, is to tap (lightly!) on the preamp tubes - if one is microphonic, you'll know it

just use something like a chop stick to tap the tube, and of course, use caution

[Kent S.]

Re: Amp Problem

Jump Marine ... Motorboating tends to happen do to wear, age (sometimes the same ), and sometimes right out of the box ... a steady popping is still an oscillation ... first check to see if that is indeed the problem , again read over Aiken's amp site, and other sites (I think it's Aiken's that has wonderful descriptions of common faults and problems and the respective causes).
I'm pretty sure it's the filter caps, but not 100% sure that's the total problem ...
Curly ... I'm not sure if a diode tube would due it, unless it's a full wave set up, I guess that one plate could not be putting out as much as the other, and somehow agitate the caps enough to allow a small DC pulse ... I hadn't considered that ... maybe that was the other thing ... although tube diodes are generally very reliable ... still worth checking out ....
I'd go with what Jeremy stated as to repair, for one a tech might catch something else along the way and it's always a good idea to check over everything, especially if it's been a while ... just to get a clean bill of health.

[JumpMarine]

Re: Amp Problem

I'm just taking a guess here, but does it use a tube rectifier? If so, you might try switching it to see if that's the problem.

No tube rectifier, SS.

[JumpMarine]

Re: Amp Problem

Is that amp 10 years old or more yet? The filter caps should be just fine...Sounds more like it's preamp related if it does it when you raise the gain up...No?

It's about 8 years old and I'm the second owner, but though it was in mint condition I believe that it was pushed pretty hard. When I changed out the el34's there were distinct color changes to the base of both tubes due to heat. I've had the new tubes in for about 9 months now and they look the same as when new. And I have pushed it hard more than a few times. I get the motorboating on both the clean and gain channels, but I can hear it better when I turn up the master volume.

[Kent S.]

Re: Amp Problem

just use something like a chop stick to tap the tube, and of course, use caution

A good book on troubleshooting amp problems recommended *exactly that* ... wooden chopsticks ... Great little devices, you can also take the rectangle end, and cut a small slot in there for holding wires for soldering ...

[Kent S.]

Re: Amp Problem

It's about 8 years old and I'm the second owner, but though it was in mint condition I believe that it was pushed pretty hard. When I changed out the el34's there were distinct color changes to the base of both tubes due to heat. I've had the new tubes in for about 9 months now and they look the same as when new. And I have pushed it hard more than a few times. I get the motorboating on both the clean and gain channels, but I can hear it better when I turn up the master volume.

Most master volumes come at the end of the preamp, and before the PI, turning the master volume off if the source of the noise is not in the preamp, then you should still hear it, if it you don't it's before, and like John said, in the preamp ... could be the plate supply to the preamp plates is doing the same thing, maybe that plate supply has an extra set of filters ... not unheard of ... I'm not sure what's in your amp. What amp is it? Forgive me if this was covered.

[JumpMarine]

Re: Amp Problem

What amp is it? Forgive me if this was covered.

LOL, it's a Marshall JCM 601 combo. 60w, 2 channel, 2 el34/4 12ax7, MV, reverb, s and p fx loops. It's basically the same as a JTM 60.

[gordon_39422]

Re: Amp Problem

Slightly off-hand, how do you like the amp (other than the current problem)? I am in the market for a small tube amp with large sound, and I drove a TSL601 at a local shop the other day. It wasn't too bad, but you know how things sound at the shop and then how they sound once you actually own them......

[JumpMarine]

Re: Amp Problem

Slightly off-hand, how do you like the amp (other than the current problem)? I am in the market for a small tube amp with large sound, and I drove a TSL601 at a local shop the other day. It wasn't too bad, but you know how things sound at the shop and then how they sound once you actually own them......

Hey Allen,
First off, it's freaking loud. Very good clean channel, reverb is pretty good. I did a major improvement on the reverb by swapping the JJ12ax7 in v3 for an old RCA 12at7. It's much more lush and full sounding. The gain channel is more vintage sounding than modern high gain and it sounds very good. It's a very versital little amp. One weak point I found was the stock Celestion G12 Heritage speaker, didn't like it one bit and just put in Eminence Swamp Thang speaker. Good lord it really sings now. All in all, I'm pretty happy with it. You can find them on ebay for $300 to 500 and it comes in a head, 1x12 and 2x12 models.