73 Twin Reverb

[g1]

My book is version 3, it's on pg.491. This appears to be the same:
http://www.el34world.com/charts/Sche...mv_noboost.pdf
Hopefully it's the right one!

[Euthymia]

V5 is the oscillator for the trem circuit. Since it is not directly in the signal path it is less likely that it is causing problems, but those voltages indicate something wrong there. With the amp off and discharged, pull the tube and measure the resistance from pins 3 and 8 to chassis. Have you tried substituting a known good tube at this position?

Have you tried substituting a known functional set of power tubes?

What are the voltages at each pin of the power tubes?

[andyjazz]

The V's at the power tubes are exactly what the schem says a 6L should run at. I have a tube checker - All the preamp tubes check out OK. As I posted before this ground-like hum continues with all preamp tubes removed. I just put the new bias caps in and the problem persists. I'm going to do a little more poking and hoping with the soldering iron. Maybe I can find a bad ground or something-idunno. I'm ready to give up. I've got to move on to something that i know how to fix so I can make some money-time are tight. Maybe somebody with more experience or luck can fihure this goddamned thing out!

[pdf64]

Does adjusting the bias balance control help with the hum?

[g1]

If the speaker jack is relying on the chassis connection for ground (no ground wire) you might want to have a look at it for corrosion etc.

[andyjazz]

Thanks everyone for your ideas. @g-one - yes I've seen this problem before. Not the problem here. I'm thinking it has to be something simple - just haven't found that simple thing yet.

When you turn up the volume the 60-cycle-ground-fault-like hum gets louder and when you twist the treble, middle, and bass knobs around- play around with them a little - you can get the screeching and howling to oscillate like an old 60's moog synth envelope filter only really godawful ugly sounding. Like a B horror movie "here comes the monster" sound. It reminds me of when I wired an output transformer the wrong way one time- sort of.

Anyway I've tested the OT for continuity and it's ok. I pull all the preamp tubes - still makes the hum. The howler/oscillation phenomena is not present (Although the hum is) without preamp tubes so I checked the reverb transformer and it checks ok - no short betweem coil(s) and ground anyway. I went all through the power supply to make sure everything was good. All the tubes operate fine in other devices. What I'm left with is the OT or the main PT. I've had it with this fucking thing - I'm giving back to the owner - There is a big repair shop I know of that has the facillities to test these transformers-I don't so....I'm sending it on down the road - I'm sure that there's smarter and more experienced techs than me out there that will figure it out. thanks again everyone!