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Old 14th May 2013, 10:48 PM   #1
cancon is offline cancon  Canada
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Default 72 Twin reverb

Hi again,

I've got a '72 twin reverb on the bench and I'm looking for advice. I've never worked with a twin before. This model is silverface, pots and serial date it to 1972. It has a master volume but no push-pull switch. It has a 3 prong accessory jack and thermal protector (currently out of circuit). This leads me to believe this is the approprate schematic

The electrolytic capacitors have previously been replaced, but all resistors are original. There is one non-original resistor; a worn out, high-wattage (15K? 2-3W?) resistor between the high-voltage "A" supply and the reverb transformer's red primary. The schematic shows no resistor in this location.

I'll start by removing the ground-switch circuitry and replace the 470 ohm screen resistors, they're looking a bit worn. If anyone has other recommendations...
Also, at the plate voltage of 455vdc, the 6L6 current draw is only 11mA per tube (screen+cathode), why is it so low? I know twin amps are notoriously clean, but is this the reason? Worth adding a variable bias?

Thanks!
adam
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Old 14th May 2013, 11:34 PM   #2
Enzo is offline Enzo  United States
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Location: Lansing, Michigan
The linked schematic has the pull switch. See if this one matches your amp better:
http://www.webphix.com/schematic%20h..._nomstrvol.pdf

And for a lot of amps:
Schematic Heaven. Where All Good Amp & Effect Designs Await Resurrection...

Why biased "so low"? Why does it need to be hotter? If the amp sounds OK< and you are not going for power tube overdrive, I'd say leave it alone. Your bias control started out life as a power tube baqlance control. I will say many people convert it over to bias loevel and just wire the two sides to it. But I don't do that unless ther is a reason. Gain monster amps like the Peavey 5150 have similarly low idle current through the 6L6s.

SOmeone may have added that resistor to the reverb drive to tame the tube current some or prevent overdriving the reverb, or short tube life. I would see if the thing sounds good on reverb as is. If you don;t like it, then revert to stock, otherwise, let the reverb drive run cooler.

Other than standard maintenance stuff like making sure hardware it tight and cleaning jacks and controls, I'd probably assess its performance before doing much else. I believe in loosening nuts on pots and jacks and turning the star washer a little so the points dig into fresh metal as I retighten the nut. Just to insure good grounding.

Some old film caps work great forever, others get leaky. SO you might go through the amp looking for unwanted DC showing up on the wrong end of the coupling caps.

CLIp a wire to ground the trem pedal jack to turn on the trem. Does the trem work? Start right up? Work throughout the speed range. If the speed range is too high - usually is for my taste - increasing the 0.01uf feedback caps in the trem oscillator to 0.02uf helps a lot. I hate it when the slowest speed is still too slow. I like it to dial down to a slow sexy throb, and not be too fast to resolve into pulse by the time the speed is on 5.
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Old 15th May 2013, 12:01 AM   #3
cancon is offline cancon  Canada
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The balance control still exists in this amp - it has not been modded for bias (yet? hah..).
I'm not a fanatic about adding bias mods...but I was just asking about this amp in particular as it seems odd to have the tubes working at only 5W dissipation.

Thanks for that schematic, looking better already. I'll print it out and see if it matches.

I've tested the amp before breaking it down, that's when I got those plate and current values. All the features work (broken treble pot), vibrato is good (yes, far too fast even by 5), and the reverb works (although the RCA plug needs to be replaced). The reverb isn't noisy either, which is a nice change. I'll keep the resistor in place with the reverb, only problem is that its hard to read off the bands after so much wear. Does 15K seem ballpark?

I like your grounding trick with the star washers, very nice!
Thanks very much for the tips.
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Old 15th May 2013, 01:54 AM   #4
Enzo is offline Enzo  United States
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Location: Lansing, Michigan
The resistor doesn;t appear to be stock, so it is whatever someone thought appropriate. I have no reason to like any value over another.
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Old 19th May 2013, 08:38 AM   #5
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As long as those resistors are good keep them, they are adding to that vintage mojo
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Old 19th May 2013, 08:40 AM   #6
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11ma is waayy too cold
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Old 19th May 2013, 10:24 AM   #7
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Agreed!

The Twin Reverb is famous for it's super clean sound. If the finals are biased too low, you'll get increased crossover distortion especially at low volume levels. This ain't what you want?!?

High gain monsters ain't that critical in this aspect, as high gain aligned guys rather seldomly play clean.

30-35 mA cathode bias current per tube is more practical for the Twin Reverb.

Maybe that nowaday's 6L6's in general need lower grid bias voltages than the originals, maybe that the specimen in your amp simply are worn out.

Best regards and have phun!

Last edited by Kay Pirinha; 19th May 2013 at 10:26 AM. Reason: Grammars
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