Fender Twin Reverb AB763 Stock Bias Range Insufficient

[omalikular]

Pull the PI and check the bias, unless the oscillation is in the power amp.

To confirm: leave in the power tubes, pull V6 phase inverter tube, and check bias voltage or current passing through the cathode?

[omalikular]

I have
.038V across 1R
-35.30V Screen
-36.20V Negative bias at the 220k junction

With PI pulled

[omalikular]

I will say now that I'm in high precision mode I see a .03 fluctuation on these negative bias voltage measurement...is that normal or an indicator of an oscillation?

[SEL49]

It's time to bite the bullet and put a fresh set of 6L6s in the amp.

[tubeswell]

I'd be concerned about repeating the experience with yet another new set of tubes. It seems something fishy is going on with the bias. I'm wondering whether it could be something dodgy to do with the PCB traces - given it's one of those modern reissue jobs, and the amp has obviously been tinkered and meddled with before (with parts being pulled and substituted). I've seen several re-issue fenders with cracked/bad PCB traces that have needing jumpering to get the things to work. Hairline cracks in traces can cause oscillation. Given the seeming insolvability of this, I'm thinking if it were my amp, whether it might be more satisfactory to completely gut the innards and install traditional eyelet board setup etc complete with new pots and jacks and so on. $1,000 bucks worth of parts and work but at least you could exert complete control over the quality.

[omalikular]

I'd be concerned about repeating the experience with yet another new set of tubes. It seems something fishy is going on with the bias. I'm wondering whether it could be something dodgy to do with the PCB traces - given it's one of those modern reissue jobs, and the amp has obviously been tinkered and meddled with before (with parts being pulled and substituted). I've seen several re-issue fenders with cracked/bad PCB traces that have needing jumpering to get the things to work. Hairline cracks in traces can cause oscillation. Given the seeming insolvability of this, I'm thinking if it were my amp, whether it might be more satisfactory to completely gut the innards and install traditional eyelet board setup etc complete with new pots and jacks and so on. $1,000 bucks worth of parts and work but at least you could exert complete control over the quality.

Oh this is not a reissue, this is from '65

[Calboy]

Were the bias capacitor(s) and bias diode updated?
What about setting the bias at 30ma per tube.

[omalikular]

Were the bias capacitor(s) and bias diode updated?
What about setting the bias at 30ma per tube.

All the original capacitors in the amp were replaced with orange drops  and modern electrolytics, though an unknown time ago. At 30ma at each tube it still red plates. I get in to the low 20s before it really stops red plating. Unsure of the diode, I did a test with the multimeter and it drops the expected voltage and drops .58V

[tubeswell]

Oh this is not a reissue, this is from '65

Sorry I misread the '65 Fender Twin Reverb in your first post - my bad.


Is there anything else we don't know about your amp? (Like - has anyone added g-k caps to the output put tube sockets? or other possible modifications that could explain the bias conundrum?) Can you upload some hi-res gut shots?

[omalikular]

Traveling for work for a couple days. I grabbed some old pics from the listing, the only thing different is the caps on the normal channel were replaced and the 1 ohm resistors added.

[omalikular]

More closeups

[tubeswell]

Have to tested the 2  x 0.1uF couplings caps going to the 6L6 grids for DC-leakage?

[Merlin]

Have to tested the 2  x 0.1uF couplings caps going to the 6L6 grids for DC-leakage?

His grid voltage is the same as his bias supply voltage, so the coupling caps are fine. Anyway that wouldn't explain why his tubes are red plating with only ~30mA plate current.

[tubeswell]

Maybe something is wired incorrectly? How else can there be red plating with only 37mA cathode current? How can current get to the plate other than through one of the other electrodes (cathode, screen, suppressor or grid)?

[shooter]

haven't followed close so;


does this amp have flyback diodes on the plate?


has the tubes been verified good in another amp?


The bias was verified at the grid, was it verified at the the tube side of sockets?


guessing this is all being troubleshot at DC level...No AC signal, has the OT primary been ohmed for partial shorts?

[stratomaster]

Appears to be missing the artificial center tap as there is no filament center tap to ground. Confirm this with measuring the resistance of one leg to ground when powered off though. Some of these early 60s PTs have internally grounded filament CTs. Haven't seen it on a Twin Reverb though--but most of my experience with TRs is post '68...aka What my clients can afford.

The bias filter cap looks like it's several decades old.  Its worth investigating. Put your meter in Vac and check if you have significant ripple riding on the bias voltage. You may have a 60Hz AC spike to low voltage that the DC measurements aren't picking up. (Grasping at straws with that one.)

The OT is definitely aftermarket, so that carries the risk of it being improper spec and installed incorrectly.  You can try disconnecting the NFB at the speaker jack and rechecking bias to see if it was oscillating above audible frequencies which is causing the red plating. If this causes the bias to behave and the tubes to stop red plating, then reverse the secondaries and reconnect the NFB.

Finally, I always replace tubes that have a history of red plating.  I would want a fresh set in there after you've fixed the problems, regardless. So perhaps have a set on order.  This isn't as thoughtless a suggestion as it was even 10 years ago with the pricing being what it is now vs then.  But in my experience, once they pop the fun don't stop--so a clean slate is warranted after the servicing is complete.

[tubeswell]

You can try disconnecting the NFB at the speaker jack and rechecking bias to see if it was oscillating above audible frequencies which is causing the red plating.

That would answer my earlier question thanks. UHF oscillation from an incorrectly wired NFB loop can cause thermal runaway in the output tubes