Randall RG50TC low low low output....

[nsbluesman]

Hi All:

I have a RG50TC opened up on my bench right now and it is puzzling me, all tubes have been testted and swapped out for known good ones, heater voltages check out, VDC on tubes 1, 2 VAC on 3,4,5,6. Plate voltages appear normal for a 6L6 AB amp. The gain channel has volume, not sure if I would say normal, but definitely way louder than the clean channel. with the volume all the way up on the clean channel it's really really quiet. I have a copy of the schematic, but there aren't too many voltage refs listed, complicated little sucker and I would love to be able to try it out at full power.

Any thoughts or suggestions of where to start looking, I did see some posts about the bridge rectifier in the heater circuit for valves 1 and 2, but the voltages look good..

Puzzled in Nova Scotia..

Attached is a schematic:

[Jazz P Bass]

Randall 50T

What test equipment do you have available?
Signal generator, scope, DMM?
My first gestimate would be check Q16 FET.
If it is always on, that would squash the clean signal.
The tube circuits do not appear all that alien.
Measuring static plate & cathode voltages will tell if the tube is conducting.
The schematic indicates EL34 output tubes, just so you are aware that 6L6 tubes will require a colder bias point. (not that this is the problem)

[nsbluesman]

Only have a DMM available right now, and it will take 6L6's or EL34's when properly biased, I have tried both with bias ranges set accordingly, What should I be looking for at Q16? btw thw 6L6's are biased at around 35ma each.

Thanks for your help!!

[Jazz P Bass]

Are the channel switching LEDs working when you change channels?
Q16 clamps the normal channel when OD is selected.
It may be that Q16 is always pulling the normal signal down.
By checking circuit resistance to ground at the bottom of R74, you can tell if this is the issue.
Hook up your DMM, Red to R74, Black to chassis.
Static test. No signal needs to be applied.
By toggling between Normal & OD you should see a very high resistance on Normal.
OD selected should be a straight shot (more or less) to ground.
Look at the schematic & make sure you are on the bottom of R74.
That should connect directly to Q16.

[nsbluesman]

75K when clean selected, 150ohm when Drive selected..

All lights work indicating channels selected.

Cheers

[Gtr_tech]

These are actually decent sounding amps. Iffy build quality tho.

Have you verified the 15v rails are both there? There is a 78/7915 reg for those.....you can check the voltage either right at the regs or at a 4558 op amp, which is always a handy test point.

The connectors that litter this chassis are a source of problems too. I've had a couple in and it was problems with these little guys that brought them down. Verify *all* of them.

Checked the loop jacks? Patch a cable from the send back into the return and see if that makes a difference. Could also be a failed op amp in the loop as well.

Have fun t-shooting it.....

[nsbluesman]

Both 15v are good... tried the loop patch, no go.. I am starting to think this is really quiet overall.. maybe OT fading?

[Gtr_tech]

Naw....these things are pretty damn loud. You have an issue somewhere. Connectors, op amps, plate load resistors, all possible culprits. The output xfmr, maybe. Do this: Got another amp with a loop? Use that to play into and patch that amp's fx send into the Randall's fx return (a small amp with a headphone out will work also). Is it normal volume now? At least that will localize the problem to either the front end or the output section.

[nsbluesman]

Well that sorts it out, plugged into the return of another amp, and tons of volume... so I have an issue with the output stage. I need to revisit the schematic and see what the scoop is. Any thoughts on the output? I could pop some EL34's back in and rebias, but going out of the preamp into another amp has crap loads of volume..

Colin

[UnluckyFett]

Same problem

I have the same problem did it ever get solved?