73 Twin Reverb

[andyjazz]

This is a 73 Twin Reverb. SS rectification. Master volume. Turn it on you get massive hum like a ground fault. The amp is running way too hot, The bias pot measures -58V where it should be like -37V. The output tubes have decent readings. on V6 however I got 310V on pins 1 and 6 (Plates 1 & 2) where I should get 245. Worse on V5! I get 435V on pin 1 where I should be getting 245V.

The filter caps have been replaced and look good. I checked the OT for continuity and it checks OK. I've replaced a few R's and C's in the preamp circuit just cause they looked old (They metered OK) I've retouched solder connections. I'm wondering where to go from here. Any help would be welcome!

[g1]

When you say the amp is running hot are you referring to bias? If the grid voltage is -58 instead of -37V then it is biased quite cold. Where you have the high DC readings what do you measure on AC range? A bad filter cap would put excess ripple on the supply which your meter might measure as DC.

[Chuck H]

There is only one plate on a 6l6. I think you mean plate and screen grid. The socket with the high reading is not drawing current due to a bad tube, wiring fault or bad pin connection. High -bias voltage makes the bias cooler, not hotter.

[andyjazz]

Thanks for the replies. I guess I didn't describe things very good. The high V reading is on the pre-amp tubes - not the 6L6's. And yes a higher neg. v on this grid/screen (I think it is refered to as the control grid) of the pentode means it is under biased. When I said "running hot" I was refering to the V's I'm getting on the preamp tubes. (And the fact that they're glowing way too red)

On a Twin Reverb V7 - V10 are the quad 6L6's. The V5 and V6 I was refering to are the 12AT7 and the 12AX7 on the pre-amp side. My first thought was the filter caps but when I popped the plate I saw that they were not the originals, they had been replaced - of course they may have been replaced 10 years ago and are now bad - I dunno. I don't have a Cap checker for 100mF caps - mine only goes to 20. I guess I was just wondering if there is something I could be checking before I lay out the $70+ for new filter caps, and then find out that wasn't the problem anyway.

[Jazz P Bass]

Use your voltmeter to see if there is any Vac riding on the Vdc of the preamp tube plates.

[andyjazz]

Jazz P Bass - Yes I got some AC riding on these pins - a small amount, like 2 - 4 volts. Also when the amp starts warming up it's starts making the "Old short wave radio oscillation between stations" type noises. Does this small amount of AC mean these filter caps are bad? Like I said earlier that was my first thought when I checked this amp out but when I saw that they were not the original ones (Like I was expecting) I thought, "well now what?"

[g1]

Those are small amounts of AC on your filter caps and should not be a problem. The hum and radio type noises may be due to a bad ground.
Can you post all the voltages for V5 and V6 and also the schematic you are working from would be helpful.

[andyjazz]

g-one,
Thanks for your help. The V's I get are:
V6 pin 1. 308V
2. 116V
3. 154
4,5. htr
6. 311V
7. 119V
8. 154V
9. htr
V5 pin 1. 431V
2. -40V
3. 0V
4,5. htr
6. 378
7. -40V
8. 0V
9. htr

The schematic I was working from was the AA769. But it could be the AA270. I don't know how you're supposed to tell the difference. Neither one has the master volume on it. And there is other stuff on this amp that does not conform to any Twin schemo I've been able to find. The ground issue you describe was one of my first thoughts when I heard this amp, along with bad filter caps - trying to track down a "loose" ground connection is proving to be difficult and the caps have been replaced by someone else.

You know it kind of makes it hard for us people who don't have the experience. I know a few things about electronics-even went to VoTech for electronics. But VoTech didn't cover this stuff. I remember asking one of my teachers about tube amps and he looked at me like I was asking him how to shoe a horse! Tubes? What the hell do you want to know about tubes for?

[g1]

Does yours have a push-pull switch on the master? If so: http://www.webphix.com/schematic%20h..._100_schem.pdf
Otherwise I am not having luck finding the master without push-pull switch schem. right now.
Either way I don't think your voltages are too far off. V5 voltages are read with vib. on and speed and intensity set to max.
Try removing V1 & V2. If hum is gone the problem is before (or at) V1 & v2. Still hums? Put them back in and remove V4. Still hums? Put V4 back in and remove V6.
Post your results.

[pops]

Try replacing your bias cap, a lot of times filter caps are replaced and the bias cap is not. It will cause hum and is a cheap cap if it is not the problem.

[andyjazz]

g-one, this model does not have the push-pull switch. By the serial # I was able to determine it is a 1973 vintage. (If my serial # matrix chart is right). I have pulled all of the preamp tubes in various configurations and even have them all out and the amp is still making the 60-cycle type-ground-short obnoxious hum. So I'm back to the power tube section, or maybe a transformer, idunno. Thats where I was before when I was checkng the output transformer. The OT checks OK with just a basic continuity check.

Pops, This may sound like a stupid question but which one is the bias cap? Are you talking about the cap(s) feeding the bias pot? I checked with a cap checker and they checked out OK. But mayber there is something happening when the amp powers up that the cap checker isn't able to detect?

[g1]

Yes the bias caps are feeding the pot. They are shown as 80uf75V in the drawing linked to above. There are 2 of them, and fairly often they are mistakenly connected backwards (as you can see from the drawing they should be positive end to ground). You can measure for AC voltage on them, should be very little. Easiest just to replace them rather than using cap tester.

[pdf64]

Try to identify which of the twin reverb schematics on this page
Fender Amp Heaven Schematics - Tweed Blackface Brownface Silverface Bassman Champ Deluxe Princeton Super Vibroverb Reissue
best match your amp.
The circuit around the bias pot is the best giveaway (unless it has been modded), along with the MV arrangement.
If it has a bias balance control, does adjusting that help with the hum? Pete.

[g1]

Pete, they don't have the master vol. without pull boost at schem.heaven link. I know there is one in pittmans book but not sure where it can be found online.

Andy, if you do replace the bias filter caps, upgrade to 100Volt caps to better withstand modern line voltages.

[andyjazz]

I just ordered some 47mF 100v caps from triode elec. today. The ones on the amp are 50mF 70V.
BTW I have Pittmans book and it doesn't have the Twin schemo with the master volume. Maybe a newer version does - mine is about 10 years old. I couldn't get the schemos at the link to upload anyway-go figure