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Motorboating Issue

Baba

Silver Supporting Member
Messages
7,522
Need some techie help folks. I have a Carvin V3 that I bought used a couple of months back. Sounded good, and I was lazy, so I never changed the tubes. 3-4 gigs in, after playing it for about 6 songs, it starts motorboating. The next day at home, I played it loud and long, it took a while then, but finally started doing it again. It looked like one tube was doing it more than the others. They were GT EL-34's. Amp was biased kind of cold, around 32 IIRC, which I think the factory does (I could be wrong).

I had a spare matched set of JJ E34L's from a previous amp with low hours, and put them in there to see if it would fix the problem, and it did, for about a month now. So, then I ordered some JJ KT-77's after talking to Bob at Eurotubes. I get them in, and bias the amp at 38 as suggested by him and on his website. Played the amp at a good clip in the house for a day or two. Didn't notice too much of a sonic difference BTW.

Took it to the first gig with the 77's, was on standby for a few minutes, flipped the switch, motorboated instantly.

So I talked to Bob yesterday, told him what was happening, he told me to run them in pairs to try to figure out which one or two were causing the problem, so after a couple of hours of doing that last night, and playing it this morning, I couldn't get it to motorboat. He also said it could be the sockets needing to be cleaned, but I've yanked and installed the tubes a million times by now, even before the problem a few days ago.

Any other ideas? I could just throw the trusted 34's back in there, but i have quad of new 77's that I can't seem to find the problem with but that I don't trust.

I've done a search on here about it, and found some answers about caps and things, but I'm wondering why the amp has functioned flawlessly for more than a month, 2 gigs a weekend and rehearsals, with the E34L's with no issues whatsoever?
 

Baba

Silver Supporting Member
Messages
7,522
Please describe "motorboating" as it applies to a guitar amp.
Motorboating is an oscillating sound that gets its name from the "putt putt putt" sound of a motorboat. The exact cause is sometimes very hard to find b/c it is related to some type of positive feedback happening in the amp that will not be obvious. This positive feedback could be related to improper lead dress, improper grounding, bad filtering (inadequate de-coupling), or a conductive component board.
 

Ray Gianelli

Member
Messages
1,343
Baba's right. And motorboating is the term that tech's use to describe it.

He's also right that it can be a bitch to troubleshoot!

Carvin will mail you a schematic at no cost, from my experience. Maybe email it by now.
 

danel59

Member
Messages
980
Definately check the preamp tubes. I had my Fuchs do the same thing but it was after I had immediately subbed in some GT 12AX7 M's that I heard were good to try. I subbed the old preamp tubes back in and voila everything OK. As mentioned above start with V1 but you may want to check the Phase Inverter tube as well

Dan
 




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