Marshall tsl602 bias drifting

[basher106]

Hi all great forum i hope someone might be able to help me? my tsl602 bias drifts really badly i usually set bias to 80mv recheck an hour or so later and it varies from as low as 60mv or upto over 106mv and tone changes (from a nice full sounding tone to a horrible thin sounding tone) which i notice when playing live, i read a while ago that a bunch resistors in the biasing circuit were prone to drifting (cheapo resistors) when they get hot and when replaced can help to stabilise the problem alot,
any help would be much appreciated i can replace any components no problem but not sure where to be looking ..

[basher106]

Hi all great forum i hope someone might be able to help me? my tsl602 bias drifts really badly i usually set bias to 80mv recheck an hour or so later and it varies from as low as 60mv or upto over 106mv and tone changes (from a nice full sounding tone to a horrible thin sounding tone) which i notice when playing live, i read a while ago that a bunch resistors in the biasing circuit were prone to drifting (cheapo resistors) when they get hot and when replaced can help to stabilise the problem alot,
any help would be much appreciated i can replace any components no problem but not sure where to be looking ..

just bumping my thread

[R.G.]

The first step in maintaining any tube amp is to swap out the tubes with known-good replacements.

Have you done this? If so, does the drift change at all?

If it doesn't, then work on what the bias voltage is doing over time and with increased temperatures.

[888guitars]

Also check the bias circuit. My store's service center has repaired a couple of those type of circuits.

[digthisbigcrux]

i have heard that some of the newer marshalls used resistors in the bias circuit which are temperature sensitive. so the bias drifts while the amp warms up.

the jcm2000 is i guess the most notorious of these.

here is a link to the page i found the info on. lots of other good stuff on the page too. the guy that runs it is super nice and helpful to boot.

The Marshall TSL122 JCM2000 Repair/Mods Page

[basher106]

thanks all for the replies sorry i'm a bit late getting back to you guys i've just revalved this amp again and have the same problem its really annoying as fault usually doesnt occur until a couple of hours normal rock use works great from cool probably wouldnt turn to a fault if i sat in my home playing on it
but when it goes the bias is miles out but not drifiting!! usaully higher 110- 120mv instaed odf 80mv also i think the tsl60 head tsl602 schematics are different to the other tsl100 and dsl50 dsl100 amp schemtic could anyone point me to the resisitors used in bias circuit i have tsl60 602 schematic in pdf format but can't seem to post it?

[Chuckl]

This might not be yer problem, but very interesting. I had to replace 2 valveboards this month. Not the cheapest repair but not too difficult If yo!u take yer time & good notes.
Good luck!

The Marshall TSL122 JCM2000 Repair/Mods Page