Trouble biasing HRD, help please

[ShortBuSX]

Lemme start by saying Ive biased my Deville a handful of times, Im familiar with the procedure...I attempted to help my buddy last night bias his HRD, hed replaced the tubes bout 2 months ago, everything sounds fine, just curious if the bias drifted and I wanted him to hear the effects of bias on his tone hotter or colder...well the bias read 61, I wanted to bump it up to 70....but then it jumped to 94, I tried to fine tune the adjustment more but then it went down to 61 again, still aiming for 70s it jumped back to 90s....I found this odd, so I set it 64 and buttoned it back up. He played for abut 5 - 10 mins and then his sustain was gone.
I took his amp home, tried it again this morning samething...BUT I knock on the cab and it appeared one of the preamp tubes went microphonic, I assume this cause everytime Id knock on the cab itd howl, I pin pointed it down to the invert tube, swapped it and the howling stopped...but the bias still reads the same, what gives? That tube sure seemed fine before I started(and Ive been careful the entire time, no slips, no bumps, no accidents

HELP!?
(someday I will realize that no good deed goes unpunished)
I posted this over a Tdpri, but Im getting no responses...I could really use some help or advice.

 

[duhvoodooman]

Hmm, I've only got a limited amount of amp biasing experience, but that behavior seems pretty odd. Are you saying that the tiniest tweak jumped the bias reading from low 60's into the 90's? Are you sure everything was full warmed up and the bias reading was completely stable before you started adjusting it?

I can't understand how a microphonic preamp tube could affect the power tube bias. That would seem to be just a coincidental occurrence. BTW, when you replaced the bad tube, did the amp's sustain return?

BTW, there are some great instructions for biasing the Fender Hot Rod amps HERE....

 

[ShortBuSX]

Yeah I think the microphonic tube was just coincidence and having nothing to do with the bias, but more of a reflection of the can of worms I opened.

Fully warmed up and the standby switch has been switched off.

The tiny tweaks never leave 60-64ish, then a scant bit more goes to 90's, tiny turn down still stays in the 90s then another lil turn back to 60s...no 70s or 80s to be seen as of yet.

It was weighing heavy on my mind all last night and began again as soon as I woke, so I checked the bias and found that microphonic tube this morning before work...I was hurried so I wasnt able to plug a guitar into it...Im just hoping the microphonic tube was responsible for the lack of sustain, and I'll just leave the bias in the low 60s, thats where it was and is also factory spec...just lame about the lack of adjustment on his amp(mine works great).

Im no amp tech, but I figure worst case scenerio its a cap or resistor responsible for the lack of bias...or could even be the bias pot itself...I may need to take this to an amp tech.:thwap:

Also, thanks for the link, Im totally familiar with Justin's site...its a constant source of reference! Anybody with any Hotrod amp should be familiar with that site, bookmark it!

Thanks for the reply DVM, Ive been racking my brain and scowering the internets...I got nothin

 

[duhvoodooman]

Hmmm, maybe a bad bias pot. That "all or nothing" behavior sure isn't right....

 

[ShortBuSX]

Now that Im home and getting to spend some time on it..it kinda appears to be a slooooow lazy bias pot. If I turn it up and wait it slowly creaps, like 5 mins creaps...if you turn it too much it jumps rapidly.

 

[ShortBuSX]

Appearantly the longer its on the higher the bias climbs...Im going to try a different set of power tubes this afternoon, if that dont remedy the situation(or get better advice) Im gonna take it in to a tech.

 

[duhvoodooman]

Appearantly the longer its on the higher the bias climbs...Im going to try a different set of power tubes this afternoon, if that dont remedy the situation(or get better advice) Im gonna take it in to a tech.

Yeah, sounds like a pro should look at it. Bias "creep" like that can't be a good thing....

 

[ShortBuSX]

Nother set of tubes did nothing...its at the techs.

 

[ShortBuSX]

Just got the amp back from the tech...2 lil tiny resistors, one week and $45 its good to go!

Such a cool ol dude, great service and awesome labor rate...Im kinda anxious to take my Deville to him to mod a little, probably do that in the next couple of weeks.