Twin RI Bias and output power questions

[nateflanigan]

Hey guys,


Some one dropped off a Twin RI (the 15" speaker model) they said it's breaking up in an ugly way.  There's two different sets of power tubes, plugging in my bias probe one set measured ~18 ma the other ~25 ma.  Plate voltage is 443, the range on the bias pot is -52 to -44.  I couldn't get the bias above 32ma (that's on one of the tubes that was reading higher to begin with) weber bias calculator says I should be at 41 ma.  This seems pretty far off, thoughts?


Secondly, I'd like to look at the output on a scope and make sure I'm getting an appropriate amount of clean power.  What kind of output voltage should I be seeing? 


Thanks as always

[HotBluePlates]

Some one dropped off a Twin RI (the 15" speaker model) they said it's breaking up in an ugly way. 

Silly question: Did you get them to demonstrate/replicate the sound they were talking about when they dropped the amp off? If not, you could wind up chasing your tail...

... I'd like to look at the output on a scope and make sure I'm getting an appropriate amount of clean power.  What kind of output voltage should I be seeing?

Got a 4Ω 100-200w dummy load?

We might guess the Twin can make 80w clean output power (more or less). Since V²/R = Power, then V = √(Power * R) = √(80w * 4Ω) = 17.89v RMS across your 4Ω dummy load.

Or go with the current 65 Twin Reverb Reissue Manual info: 12mV 1kHz sinewave at V1A, all Normal Channel control full clockwise, all Vibrato Channel controls full counter-clockwise, reverb and vibrato off, 9v RMS measured at speaker jack (better have your earplugs in).


Oh... And the schematic indicates bias of -50v; in historic Fender practice this was irrespective of output tube current (because the design assumed typical 6L6 gain & Gm amplifying an input bounded by this grid voltage).

Regardless, I'd try to get the amp to make the bad noise in question, see if it's really out of character for the amp (or just doesn't match the owner's unrealistic expectation for how the amp should sound), and go from there...

[nateflanigan]

Hey HotBluePlates


Regarding the silly question, not so silly.  I dropped the ball on that one, I gotta get better about that stuff.  I'll revisit that. 


I do have a dummy load I got for repairing/testing my SVT.


Thanks for taking the time to type out the math, I really do appreciate it.


Do those bias numbers seem unusual to you too?

[sluckey]

I'd try a fresh set of known good power tubes. Might pick up the power and take care of that ugly breakup too. That twin should not break up until your ears are bleeding.

[HotBluePlates]

Do those bias numbers seem unusual to you too?

They might be cold, but there's no telling really. The "35mA idle current" and "70% idle dissipation" are relatively new inventions, from about the 90's. See where plate current lands at about -50v of bias on the grids.

[nateflanigan]

I'd try a fresh set of known good power tubes. Might pick up the power and take care of that ugly breakup too. That twin should not break up until your ears are bleeding.

Ordered a quad of 6L6's, I was able to recreate the problem, had to turn the amp up to about 5.  It's not exactly break-up as we would normally call it.  It's more like a brittle crackling.  I had a similar problem in a SFDR, the issue was that the two power tubes were WAY off from each other. 

[nateflanigan]

They might be cold, but there's no telling really. The "35mA idle current" and "70% idle dissipation" are relatively new inventions, from about the 90's. See where plate current lands at about -50v of bias on the grids.

At -50v the plate current reads ~21 ma.

[HotBluePlates]

Gotcha. See if you can bias hotter. You'll probably need to modify the bias circuit by dropping the value of a resistor in series with the diode to get more bias range.

Of course, you could start by biasing the tubes as hot as you can get them now (32mA) and see if the distortion issue improves.

[nateflanigan]

Will do, also, that was on one of the sovtek tubes.  There are two sovteks and two groove tubes, the GT's are even colder.

[nateflanigan]

I got a fresh set of JJ's in the amp, they're still running a little cold.  I had the customer come over and we fine tuned the bias by ear, he's happy.  That 15" speaker that comes stock in those amps was also part of the problem, it just has a nasty frequency response in the upper range.