Twin Reverb Biasing

[bobloblaws]

I have a 68 Twin Reverb, AB763 circuit. The plate voltage is 460V. The target 70% max dissipation calls for about 45mA current per tube. When I turn up the bias trim pot I can get the current to about 30mA per tube (I'm using the OT shunt method so it's probably more like 32 mA which is what I calculated using the OT resistance method). After that turning up the pot further does not change the reading and I eventually reach the physical limit of the pot. The negative voltage coming from the pot is around -47V and around -45V at the grids. This too does not change once you reach a certain point in the pot adjustment/current draw. Could this possibly have anything to do with bias cap having been changed from 50uF to 100uF? It seems to be a pretty common mod, although I don't know the benefit it is supposed to provide. Could it be one of the 220K resistors between the pot and grids? This seems unlikely as each of the two resistors is specific to one pair of tubes and I'm seeing the same thing on both sides. Or the tubes themselves? All 4 tubes are vintage RCA and seem to be working well otherwise. Any insight appreciated, thanks.

- Bob

[parentheticalfact]

I have a 68 Twin Reverb, AB763 circuit. The plate voltage is 460V. The target 70% max dissipation calls for about 45mA current per tube. When I turn up the bias trim pot I can get the current to about 30mA per tube (I'm using the OT shunt method so it's probably more like 32 mA which is what I calculated using the OT resistance method). After that turning up the pot further does not change the reading and I eventually reach the physical limit of the pot. The negative voltage coming from the pot is around -47V and around -45V at the grids. This too does not change once you reach a certain point in the pot adjustment/current draw. Could this possibly have anything to do with bias cap having been changed from 50uF to 100uF? It seems to be a pretty common mod, although I don't know the benefit it is supposed to provide. Could it be one of the 220K resistors between the pot and grids? This seems unlikely as each of the two resistors is specific to one pair of tubes and I'm seeing the same thing on both sides. Or the tubes themselves? All 4 tubes are vintage RCA and seem to be working well otherwise. Any insight appreciated, thanks.

- Bob

I don't believe your bias cap being a 100uf is causing what you're seeing as a lack of good adjustment range. 100uf takes a little longer to charge but should be more effective to filtering any ripple or noise in the bias circuit. On a twin this longer charge time shouldn't be an issue in my experience.

It's always a good practice to check your 220k resistors for accuracy, though I don't think that's what you're experiencing.

Since you're using the transformer shunt method the problem is you don't actually know what your mA draw is per tube, only across each side of the transformer. Keep in mind this will be showing you current for 2 tubes at a time, as opposed to the same method on a 2 output tube amplifier like a bassman.

Have you tested the tubes in a good tester? If you can read the plate current (octal bias probe is good here) on each tube the amplifier can serve as somewhat of a good tube tester as it will run them at operational voltage. You can see if one is drawing more or less current than the others this way.

I suggest the transformer shunt method only if it's a known good matched set of tubes. The transformer shunt method lacks certain data that can clue you in on if you have an amp problem or a tube problem.

[The Dude]

Firstly, Fenders are usually biased on the cold side. IMO, 30mA is fine. No need to bias hotter, unless it's causing the amp to sound bad- which I doubt.
Secondly, are you sure it's a bias pot? I'm thinking you have a pot for output tubes matching and the bias is fixed. There are a few different Twin Reverb schematics. Which is yours and can you upload a schematic or tell us the model number that's on the tube chart?

[bobloblaws]

It is the AB763, it's the blackface circuit. It's definately a bias trim pot. It works as expected until reaching a certain point in the wiper travel. Maybe it's just a defective pot. I suppose it wouldn't be hard to swap in an equivalent pot temporarily to find out. I hear what your saying about biasing on the cold side. The schematic calls for -52V at the pot which would make for even less current draw than the ~30mA I'm currently at.

[g1]

Hey! I'm a big fan of your law blog! I take it you're a canuck.
Agree with The Dude that 70% was never a target used by Fender, especially in that era. Possibly the increased modern line voltage has upped the bias voltage from the stock value. If you really want, you could knock it down some with a resistor change. But the real vintage sound will be with fairly cold bias.

[The Dude]

Yep, the AB763 has a bias pot. Just wanted to make sure. Some have output tubes matching instead of bias. No need to swap the pot. From your description, the wiper is at one end. Just measure wiper to that end and see if it's at or near zero resistance. You may have to modify the bias circuit a bit to get where you want to be, but again, I wouldn't worry about it.

[bobloblaws]

Hey! I'm a big fan of your law blog! I take it you're a canuck.
Agree with The Dude that 70% was never a target used by Fender, especially in that era. Possibly the increased modern line voltage has upped the bias voltage from the stock value. If you really want, you could knock it down some with a resistor change. But the real vintage sound will be with fairly cold bias.

Canuck indeed. Based on yours and other comments and the -52V indicated on the schematic, 30mA (or even lower) might be actually be the sweet spot for this amp. I'm still curious why I can't go above that though. I guess I'll start by troubleshooting that pot. Thanks all.

[g1]

I'm also surprised that's where you are (30mA) with -45V on the grids.
Are the screen resistors stock 470 ohm type and do you have proper voltage on the screens?

[Mick Bailey]

I've had plenty of Twin Reverbs that are biased cold. Some not much more that 20mA, and they sound just fine. Take care not to troubleshoot something that actually isn't a problem. I sometimes get old Fenders that still have their factory tubes. I call them time warp amps. The only reason a tube can last that long is if it's biased cold.

70% is like the advice you get for setting the idle mixture on a carb. Your manual may say something like "Turn fully clockwise and then adjust 2 1/2 turns out" Your car may run like this, but it may not be the best setting. Bias is the same - adjust it to what the amp needs and sounds good.

[bob p]

Time Warp Amps. I know exactly what you mean. It's amazing how many amps like that are actually still out there. I see them all the time.

There's something to be said for using a cold bias setting with high powered amps. In an amp like a Twin Reverb, it's not all that likely that the average user is going to bias it hot enough that most of the amp's tone would come from the output stage clipping hard. Twin Reverbs are so loud that most people can't get away with that. As a result most of the people I know who use Twins tend to use them as signal amplifiers rather than distortion generators. They typically obtain most of their signal processing through pedals on the front end. At least that's what I have to do with my Twins, because they just so damned loud that I can't stand the volume that it takes to crank a quad or a sextet of 6L6 into the sweet spot.

That sort of thing also explains why Mesa deliberately biases their output stages cold in their big amps. Mesa likes to generate a lot of their tone through cascaded preamp gain rather than doing everything with output tube distortion. In that context it makes sense from a reliability standpoint to bias an amp on the cold side so that the output stage is used as a volume control for the tone-shaping preamp. that makes the amp more reliable and the tubes tend to have longer life.

Of course, all of this goes cold bias theory goes right out the window if you're one of those guys who has a BF amp and wants to drive it into the sweet spot as your ears bleed...