Hellllp - diagnose my Rockerverb problem

[morgwn]

Amp sounds great...nice, fat, smooth, clear...for about an hour or so into band practice... Then, at some point, it loses bass, and gets all "gravely" and grainy sounding (high end becomes raspy).

I've unplugged FX loop, etc and plugged the guitar straight in, and it still sounds wrong.

Is this a preamp or power amp tube problem, or something else?

[Andy H.]

<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote">Is this a preamp or power amp tube problem, or something else?
<hr height="1" noshade id="quote"></font id="quote"></blockquote id="quote">Yes, definitely!
If I had to guess, I'd say it's almost certainly one of the first two though...

I'd get some really nice power valves to try out and hear the difference they make. I think your problem will likely disappear too.

If not then start swapping out preamp valves too. It could be either of course, but I'd guess it's probably more likely to be the hotter running power valves.

Andy.

[morgwn]

Ugh...the power tubes aren't that old

Is that typical with power tubes...getting a "falling out"
of volume, and increased top end rasp after they really warm up
if they are about to go?

Also...how does one bias this thing? Should I buy a
Weber Bias thing, and do it myself? I have no good amp
tech in the area.

[fiveightandten]

Tubes can drift over time, especially if they're not tested well and burned in properly (a lost art). When in doubt, always start by replacing tubes.

You can learn to bias yourself. If you want to learn how to do your own amp work, i'd suggest buying a digital multimeter and a bias probe, as opposed to buying a bias unit. They're limiting in that you can only use them for one thing.

I bought my DMM at Radio Shack like 5 years ago and it's been great ever since. It was $50. It does capacitance (a BIG plus), resistance, AD and DC voltage, current draw, checks doides, and has an audible contunity tester (which I use ALL the time). You can get bias probes on eBay or various tube and electronics supply places.

Read this article on biasing:
http://www.duncanamps.com/technical/lvbias.html

The bias probe will alow you to bias without adding 1-ohm resistors, which you can't do in your amp anyways because the power tubes are PCB mounted.

Bump this up with any questions you have about that article.

-Nick