Amplifier help please - cutting out.

[zeegler]

Hopefully Pete or someone else knowledgeable about such things can help me out here.

I have 2 amps here doing pretty much the same thing.

My Randall RGT100 - I play for anywhere from 30 minutes to 2 hours, and it suddenly starts to drop in volume about 50%. It will do this for short periods of time ranging from 1 to 4 seconds at a time. It has brand new power tubes, and 2 of the 4 pre-amp tubes are new. Could this be a transformer problem, or something less serious?

My friend's Laney AOR 50 - He just received this from an Ebay purchase. It has basically the same symptoms. He played for 45 minutes, and the volume suddenly dropped right off, to probably 10%. He brought it over to my place, and it did the same thing after 10 minutes. The tubes are probably ancient, and I recommended to him that he replace ALL the tubes first and foremost.

Anyone have a theory?

[taxistationary]

Re: Amplifier help please - cutting out.

try replacing the other 2 preamps. good luck...

[Whiplash]

Re: Amplifier help please - cutting out.

I dont know, but I have an old solid-state marshall combo that does exactly the same thing unless i run a patch cable through the FX loop (or FX, just something going in and out of the loop). No problems after that. I would like to know why it does this too.

[rps14a]

Re: Amplifier help please - cutting out.

my twin has been doing this even when i by pass the preamp section completely and its got brand new svetlanas in it(two tubes were shot were shot, thought it was the problem). anyone here able to help would be great for all of us it seems.

chris

[zeegler]

Re: Amplifier help please - cutting out.

Well, I found a good website with amp troubleshooting tips on it. It suggest changing the tubes first, because it could be caused by a pre-amp tube going bad. That's what I'm going to start with.

[Twisteramps]

Re: Amplifier help please - cutting out.

Patch cable in the FX loop. Do that first...

Pete

[mrr3000gt]

Re: Amplifier help please - cutting out.

On the Laney (I have an AOR 50 myself, so I know this amp):

1) Do the patch cable thing as Twister says. If this fixes the problem, you got an intermittant connection problem with the jacks.

2) Open it up and inspect for obvious signs of tube burnage (like blackened resistors, PCB tracing).

3) What channel does this happen on? if its on one and not the other, its likely a preamp thing.

4) Most of the time, this problem is connected to power tubes. Even still, a fresh set of tubes may NOT cure the problem, as the board components might be stressed. I have seen retubing temporarily fix a problem that returned because the individual biasing of the tubes was wrong due to bad plate/screen resistors when under load.
If you think the Laney was played untill the tubes failed, replace the 5.6K grid stoppers and the 1K screen resistors.

Also, note that this amp has its power tube cathodes coupled and grounded with the biasing done thru the grid: check that the voltage is -36v (if you can get -36v to the grids, around 400v to the plates, slightly less than the playe voltage to the screens, and the load resistors are good, chances are you have a fixed amp).
Note that bad resistors can and do often measure the correct value, but change under load. When in doubt, replace them. 20 minutes and four bucks in resistors is cheaper than burning a set of tubes up every month.

The Laney AOR 50 is an example of a very well made PCB, hand-wired tube amp economically produced. I would put its workmanship up against any other major player, and the quality surpasses the Marshalls I have worked on.

Also, tell your buddy that a 12AT7 driver works well in that amp: who needs a 12AX7 driver in a high gain amp? And, cheap tubes make that amp in particular sound like crap. I have good luck with ElectroHarmonix 12AX7EHs, a mullard 12AT7 driver, and Sovtek EL34WXTs. The WXTs sound more sterile than the JJ Teslas, but have clarity at all volumes the JJs dont have (in my opinoin).

...Cant help you on the other amp. The problem with it is possibly the same: bad tubes or bad components stessed from some guy trying to get all the life he can out of his $10.00 power tubes, or both.

[mrr3000gt]

Re: Amplifier help please - cutting out.

Also: transformers in 'modern' amps are likely never the problem due to the fact amp manufacturers use components rated REAL low (ex: the JCM900 uses mostely puny, impotent 1/4 resistors). Even a minor current draw more than normal operational ranges will burn these components up just before the B+ fuse blows (typically, these are rated at 1/2 amp).

What that means is the boards burn up long before the transformers even have a clue whats going on.

[siggy14]

Re: Amplifier help please - cutting out.

sounds like a heat issue to me, you might want to try taking a fan and have it blow directly into the back and see if this cures the problem. Alot of times a amp will start to get hot and it will shut itself down or the soder joints will losen up.

[zeegler]

Re: Amplifier help please - cutting out.

Thanks guys. I guess I will try the patch cable in the effects loop first.

This Laney does sound like crap at the moment. It has Sovtek power tubes and some cheapass pre-amp tubes that I have never heard of.

[zeegler]

Re: Amplifier help please - cutting out.

Tried the Laney with the patch. Nailed it! Still gonna replace the tubes, cuz this thing sounds bloody awful.

[Whiplash]

Re: Amplifier help please - cutting out.

Glad we could help! [img]/images/graemlins/band.gif[/img]