super twin reverb - biasing power tubes

[wagdog]

In my never ending messing around with the super twin reverb, I installed 1ohm resistors between the 6L6GC cathodes and ground to measure how much current is going through the cathode.

My readings are pretty weird though, much colder then expected.

I'm getting 509 volts on the plates.

Looking at the amp from the front, left to right, I'm seeing 20, 20, 18, 18, 19 and 19 ma of current through the cathodes at idle. That would be a hair over 10 watts of static dissipation per tube (509 * .020ma) - seems like it's running very cold.

The bias is fixed using a 2.7K resistor on the bias balance pot.

Is the amp setup this way due to the high voltage on the plates and screen? I'm hesitant to adjust the bias as it might be this way by design. Anyone have any info on this?

FWIW, I'm getting some great tone out of it with some of the mods and tweaks I've done recently - there is a twin reverb in the super twin, but you have to work at it a bit to find it!

[Jazz P Bass]

What bias voltage do you have on pin #5?
The schematic calls for -61Vdc.
By the way , that is one tight matched sextet.
My personal opinion is, if the amp sounds good at a reasonable bias (ie: no crossover distortion) let it alone.
If it really was set "too cold" you would have crossover distortion & the amp would not sound good at all.
The tubes will certainly last longer than if you Internet bias the amp way hot.

[Pedro Vecino]

I use 28mA per tube measured across the plates. With the tubes I use I get it adding a 15K/1W parallel resistor to 2K7 (this is variable with different tubes/grade).
Good symmetry in driver and buffer (basically the value of resistors and parity between triodes) is also important. Normally some of these resistors move away from its original value.
Regards

[wagdog]

What bias voltage do you have on pin #5?
The schematic calls for -61Vdc.
By the way , that is one tight matched sextet.
My personal opinion is, if the amp sounds good at a reasonable bias (ie: no crossover distortion) let it alone.
If it really was set "too cold" you would have crossover distortion & the amp would not sound good at all.
The tubes will certainly last longer than if you Internet bias the amp way hot.

I'm getting -62v on pin 5.

It does sound good, some I'm reluctant to mess with it. I was surprised to see it biased so cold though.

[Enzo]

If you had never gone on the internet to find out that it was "wrong" you never would have known you had this situation, would you?

The bias is set this "cold" by design. The amp is LOUD, freaking loud, and strong. It doesn't have to have the bias turned up to where all those tubes will bake the poor thing into oblivion. Would anyone REALLY drive those output tubes to distortion in this amp? This amp will drown out jet aircraft.


If it isn;t broke, don;t fix it.

[wagdog]

If you had never gone on the internet to find out that it was "wrong" you never would have known you had this situation, would you?

Me? I didn't say it was wrong, was just wondering why it was so low. I've worked on several amps, built one (so I pretty much know enough to be dangerous!) - just seemed weird, that's all. Just wondering.

The bias is set this "cold" by design. The amp is LOUD, freaking loud, and strong.

Yeah it is, really loud! I bought it to play and work on - couldn't afford a real twin, so I picked this one up...

If it isn;t broke, don;t fix it.

Just wondering...

Like I said, I bought it to mess around with. Like the stock tones, but it's got some things about it that bothered me. Undertook some more mods tonight...

There are all the makings of a '65 twin reverb pre-amp in this amp, but it's really muddled up with the extra EQ section and the "distortion" control. Tonight I completely re-arranged things to match the pre-amp of a '65 TR (that is if a '65 TR had a master volume ) - BIG tonal difference. That overbearing mid-range honk is gone and it works even better w/a tubescreamer. Prior to this, it seemed almost as if the amp was fighting the pedal - there was the overdriven sound, and a clean sound on top of it. Hard to describe, but if you've played w/one of these amps and a dirt pedal you'll know what I'm talking about.

It's sort of a mess right now on my bench, but I'm going to clean it up tomorrow. Have to figure out what I want to do w/the presence pot too as it was part of the EQ section. Also now have another triode to play with in addition to the "distortion" control.

Last night I messed around w/removing the cathode follower after the PI. Sounded OK, but not really that different. Put it back.

Fun amp to mess around with.

[Pedro Vecino]

I have also tried to 18/20 mA. but sounds thinner and less round/energic. The bias adjustment also depends on the type (manufacture) of tubes. Original american STR 6L6GC are able to sound good with lower bias settings (more relaxed) that 6L6GC I use.
Regards

[The Kirbster]

I bought a brand new Super Twin in the late 70's. Took it home, blew it up within 30 minutes. Back to the music store, they had another one in stock, brought it home, blew it up in 45 minutes.

It had to go to the shop, took over a week, they said the caps had blown up. I finally got it back, managed to get a couple of weeks out of it before it did it again! I HEARD it go-sounded like big firecrackers inside it. I actually pulled it open to look myself that time-looked & kinda smelled like a dozen firecrackers had went off.

After that, I determined that IF it was gonna blow up, I was gonna do my damndest to make it do it while it was still under warranty. I'd get my guitar to feed back, lean it up against it, & leave it for 30-45 minutes howling & screaming. Went thru 2 more sets of caps, 2 full sets of tubes, 1 speaker, & a window (cracked from the vibration). Got it so hot, the paint on the back of the amp smoked & scortched. They finally got it to hold together-for awhile. Sounded GREAT thru the clean channel, the distortion channel was a waste of wire. Most places we worked, I had to point it at the wall & put a temporary solid back on the cab to get it loud enough to sound good though. When it finally blew again about 7 years later, I traded it off for a SS Peavey Renown. I'm back to tube amps again, Fender Bassman Ten. I still miss the Super Twin sometimes, but it really wasn't practical. Neat hot rod for a kid, though!