misbias issue??

[yunger]

i sure do appreciate all the help i've recieved about this. i'll continue to run through the punch list of things to try...maybe i'll get lucky. if i have to start over no big deal, it's all for fun anyway.


btw, could this beast be of help? it could be a project in itsef to get the thing working but it sure looks cool and has tons of old tubes in it. i know nothing about old tubes but i saw mullard, telefunken, blah, blah...12au7, 12ax7......the guy selling it has no idea what it is and it has no accesories like probes or even power cord. he asks $150 neg but just wants rid of it.

edit: i'm gonna look ito buying a dependable, working scope. i still might see if the seller of this bohehoth will sell me the tubes.

[jmaf]

Years ago a friend of mine took apart a similar scope, it was full of precious parts....precision resistors, pots, nuvistors, ceramic terminal bridges, etc....

[trobbins]

Do you always test with the chassis bolted in the cab with the speaker?

[yunger]

Did you try adding a grid stopper resistor to the "A" section of the 12AT7? As in, each grid has its own stopper, and both of them mounted right at the socket pins. The 12AT7 has gain right up to VHF and could be oscillating.

BINGO!! i soldered a 10k to each grid pin and voila. thank you steve! the oscillating 12at7 must've been polluting the power supply and recirculating weirdness back to the preamp.
now, due to all the grounding and layout adjustments you guys guided me through, i have an immaculately quiet amp...haha. thanks everybody!

[Steve Conner]

Glad to be of assistance!

The scope looks like a Tektronix 535. 35MHz bandwidth I think. It was once Tek's top of the range model selling for thousands. There is a chance it might still work (though I'd use a variac or dim bulb tester to bring it up slowly and reform the electrolytic caps)

If you have the bench space to give it a home, it could be fun. If not, well, there are lots of nice tubes in it.

If it's a 585, that's maybe not so useful, it was a specialised high speed model.