Back to Basics - Biasing 4-6L6GCs fixed bias

[gui_tarzan]

This Teisco King 100w amp keeps throwing me curves. As it sits with no adjustments yet, I have the following voltages/resistances. I will just post one tube because they are all so close together it's not funny. Because of the lack of schematic for this particular model and that the previous person cobbled things up so badly, I rebuilt the power section with a typical Fender 4-6L6GC output design like my SF Bassman 100 (attached). It's darned close to another Teisco 4x6L6GC though so it shouldn't matter. The Fender schematic shows -57v bias at the balance control, this one is -54v so it's in the ballpark. The grid resistors are currently 10k (original) and I replaced the toasted 150 ohm screen resistors with 1k 5w. There is also the original 20k wire-wound resistor instead of the choke in this amp. That's where the significant drop between OT and screen is happening.

Voltages:

Plate Pin 3: 558v (wall voltage is 125v)
Screen Pin 4: 424v
Grid Pin 5: -54v
Cathode Pin 8: All directly to ground

.5v drop across OT center tap to each plate lead when on.

Resistances:

Plates to OT Center Tap: 38.5 ohm and 38.7 ohm - a half-volt drop makes sense when the resistance is low, no?

I'm just looking for thoughts on these numbers at the moment, they seem odd to me because they are quite different from other readings I've taken on quads over the years.


Edit to add Teisco schematic

[Helmholtz]

That's where the significant drop between OT and screen is happening.

Voltages:

Plate Pin 3: 558v (wall voltage is 125v)
Screen Pin 4: 424v
Grid Pin 5: -54v
Cathode Pin 8: All directly to ground

.5v drop across OT center tap to each plate lead when on.

Resistances:

Plates to OT Center Tap: 38.5 ohm and 38.7 ohm - a half-volt drop makes sense when the resistance is low, no?

0.5V across 38.5R means a plate current of 13mA for 2 tubes or 6.5mA per tube which is close to nothing.
Aim for something like 30mA per tube, resulting in a voltage drop of 2.3V per side
Also the the 20k screen node dropper seems very large.

Also do not measure bias voltage at the grids but before the 68k resistors.

[Jon Snell]

125mA for four seems about right to me.

[Enzo]

So you have the seriously lower screen node voltage plus the much larger screen grid resistors. Isn't that consistent with cooler tube current?

Meanwhile, cold or not, how does it sound as it sits? I saw many 5150/6505 amps idling at 11-12ma as normal.

[Leo_Gnardo]

guitarzan: There is also the original 20k wire-wound resistor instead of the choke in this amp. That's where the significant drop between OT and screen is happening

Helmholtz: Also the the 20k screen node dropper seems very large.

I would suggest reducing that big screen node dropper a notch or two, say 12K, 10K, 8K2, that ought to permit more bias current. AND adjusting bias voltage as well. Doubtless the very high B+ will diminish as you start drawing more current through your 6L6's. I expect you'll land in the low 500's when all is said and done. I'd be looking for a bias current of 20-25 mA per tube, ought to be enough. As it stands, with 6 mA per tube, there must be a horrendous crossover distortion. Unless you want distorted-all-the-time you have to increase that bias current.

Next I would experiment with increasing the value of the next dropping resistor in the power supply string so that the preamp plates are seeing the voltage they were before changing the first R.

[gui_tarzan]

So you have the seriously lower screen node voltage plus the much larger screen grid resistors. Isn't that consistent with cooler tube current?

Meanwhile, cold or not, how does it sound as it sits? I saw many 5150/6505 amps idling at 11-12ma as normal.

Yes, that is my understanding. After I posted I thought about it some more and it really does make sense. If I were doing this full time I would see more variation in designs and would run across more oddities, I'm sure.

It sounds ok, not real warm but not ice cold either. I was just surprised at how low it is. I'm pretty happy with how this is turning out so far given the shape it was in when it came to me. Lots of clean-up left to do, scratchy pots and such but so far so good.

[g1]

You mentioned another 4 x 6L6 Teisco, do you have the schematic for that one? Does it use a 20K dropper to the screen node?
If someone previously cobbled things up, I wouldn't necessarily trust that the 20K is the correct value.

[gui_tarzan]

I do, it's for the 100W PA amp but it shows a 2K/20w resistor, not a 20k. Interesting.

[g1]

I was wondering if maybe it was supposed to be a 2K in there.

[gui_tarzan]

I don't think so in this case, all of the resistors are the same type, all look like a wire-wound resistor from top to bottom with a grey coating and printed values. I can't see the prior mechanic replacing every one of them. There are no typical striped carbon or otherwise.

[g1]

It's still possible it came out of the factory with the wrong value part stuffed. It may have been running cold for it's whole life. I've seen wrong value parts even in amps that were assembled by robots. Some times the wrong value parts get in the bin.

It's just that such a high value dropper for that node is almost unheard of. Do the rest of the preamp supply nodes also run through that resistor? Sounds like that is the case if it is in place of where we would see a choke.
If so, I'd expect low voltages throughout the preamp as well.

[Enzo]

Over my decades on the bench, I can attest to that. Not every day, but many times I have met an amp with a wrong part from the factory. Often wondered how it passed QC.

[gui_tarzan]

You're both probably right. The PA schematic I have shows a 2K there, not a 20K. I will take the early suggestions and see what I come up with.

[gui_tarzan]

It's still possible it came out of the factory with the wrong value part stuffed. It may have been running cold for it's whole life. I've seen wrong value parts even in amps that were assembled by robots. Some times the wrong value parts get in the bin.

It's just that such a high value dropper for that node is almost unheard of. Do the rest of the preamp supply nodes also run through that resistor? Sounds like that is the case if it is in place of where we would see a choke.
If so, I'd expect low voltages throughout the preamp as well.

This is a separate stereo preamp that goes in the head, then the power amp is actually - from what I've been able to gather - located in the speaker cab. There is no way to mount the power amp chassis in the head, though that's where it lives now with a fan added to remove the heat by the previous mechanic.

The preamp has its own power transformers, one for the heaters of five 12a*7 tubes and one for the plates that runs around 160v at the taps. That 20k resistor is in the power amp chassis right off the rectifier diodes. The next resistor in the chain goes to the plates of the PI tube.

[gui_tarzan]

I worked on drawing the power amp schematic as-built with a few value differences based on availability (47k instead of 50k, etc.) and this is almost directly how the Teisco 100w PA power amp compares. I hope it's of some help to anyone that needs it and I added it to my first post. I will be drawing the preamp chassis as well as time allows.

Edit - I just realized I left the decimal point off the .1/600 caps and can't edit the original post now so I'm added the corrected version here.