help diagnose jcm 800 type amp. "farting out"

[pmendola]

So I just picked up a bedrock 1200 100 watt head. its essentially a jcm 800 type circuit. forgive my lack of better terms when describing the issue I'm having with it.

I noticed that when I roll of my volume or pick more softly, the amp sort of farts out and the sustain dies it doesn't really clean up at all. I realize im not going to get fender cleans but I had no trouble getting cleans out of my old 50 watt bedrock. The sustain is also noticeably not up to par when I play it balls to the wall, but it doesn't fart out at all, sounds fine when i dig in and stuff.

is this possibly a cap issues? tubes? Not exactly sure where to start. The tubes look like they've never been replaced (amp was built in the late 80s) so I'll definitely start there, I'm just wondering if anyone has experienced a similar phenomenon and it turns out it wasnt the tubes

[soundguruman]

The bias is too cold.

[pmendola]

The bias is too cold.

oh, haha, I didn't think it could be that simple!

[Gtr_tech]

I've seen a similar situation when a coupling cap in the preamp is on the way out. Check them for leakage...

[pmendola]

I've seen a similar situation when a coupling cap in the preamp is on the way out. Check them for leakage...

cool! thanks for the info

[pmendola]

Figured I would post some pics of the guts since I had it open! Bias looks like its turned all the way "down"

[soundguruman]

oh, haha, I didn't think it could be that simple!

If the bias is too cold then it sounds like a fart in a bathtub

[Gtr_tech]

Oh...thats a Bedrock. Brad Whitford of Aerosmith endorsed those back in the 80s.

http://bedrock27.tripod.com/sitebuil...0bradjoead.jpg

[Amp Kat]

Kinda hard to tell where the bias is set by looking at the pot. You really need to get a meter and check the negative gid bias voltage on pin 5 and either use the factory setting or calculate the tube current vs plate voltage and set it to the correct wattage dissipation % you want to use. One mistake lots of end-users make is cranking the bias pot until the farting goes away and then deciding to give it a little more which puts the tubes in a under-biased state and fries them due to thermal runnaway when the tubes give it up. Then they want to know why the amp has died all of the sudden. Don't be that guy and bias it right and be done with it.

[soundguruman]

Well, it's like this really:
A. If the tube is biased hot then it will sound better.
B. If the tube is turning red, it may be biased a little too hot.
C. Ideal may be to bias it hot without making it turn red. Just not "too" hot.
D. The hotter the bias, the shorter the tube life. And so, it's always a compromise.

[J M Fahey]

Avoid "biasing by the plate color" suggestions like the plague they are.
They show total ignorance.
Trust your multimeter and, if available, your scope.
Good luck.