5E6 Bassman bias problem

[Luthierwnc]

I'm poking around on a friend's '56 Bassman trying to fix a bias problem. This is a two 5U4 model that says 5E6 on the tube chart but looks more like a 5E6-A. It's bone stock except for a recent filter cap change.

The RCA power tubes are drawing 76ma each (shunt test). I've swapped them and also subbed in a pair of TAD 6L6s which came in at 72 ma. The later schematic calls for -42 at the grid leak junction and this one is -44. The tap itself measures +46 VAC. The old selenium rectifier works fine although I did swap in a UF4007 to be sure. The bias cap is recent but tested good. I pulled the bias tap and checked for shorts in the circuit -- didn't find any. The resistors are off a little but the bias voltage on the grids were -40 and -39 so not the problem. The bleed resistor is 57k on the nose. Plate voltage is 420 and the screens are 410. Those are what the schematic calls for. They should be in the 440 range with modern wall voltage but with the power tubes hogging all the juice, the rest of the rail is just like the picture. Both PI 0.1 caps tested good and wouldn't pass DC.

The temptation is to just sub in a bigger leak resistor (this one already has the lower 3k3 first dropping resistor) but with only 46 volts to work with, I don't see getting the volts up enough to make much difference. Brown and Black Fenders were more in the 60 volt range.

There are some safety issues on the amp -- like a bare choke wire. I did disable the death cap. For laughs, and since the power tubes weren't objecting too much, I ran a 1k sine wave into the amp on very low volume and a dummy load. It looked great all the way through the amp. I didn't want to crank it for fear of red-plating the tubes but there weren't any weird parasitics lurking in the circuit.

So now I'm stumped. Anybody got any ideas? Thanks, Skip

[Luthierwnc]

You know when you assume ...

As I look at the schematic I remember this is a capacitor input power supply. So a shunt from the 5U4s to the plate probably doesn't get me too far. The shunt measurement from the centertap to the plate was about 50ma -- still high but at least in the ballpark.

I'm thinking a slightly smaller first resistor and a larger bleed might get it even closer.

Keep those ideas coming! Thanks, Skip

PS I don't have any 1R resistors.

[Phil_S]

Why are you convinced there is a bias problem? Bias voltage is about where it should be. I'm having some difficulty imagining that 2V colder than the schematic is a problem.

Just exactly what are the symptoms that prompt you to open up this amp? It being so minty inside, it sounds like you are being appropriately cautious.

If bias voltage is really the issue, I hesitate to suggest it, but maybe do a temporary swap of the 56K in the bias circuit for a 25K pot and a 39K resistor. That will give you plenty of range to tweak voltage, just to see if it really is the problem.

[Luthierwnc]

Got it fixed with a 2k2 feed resistor and put a 25k pot discretely (and removeably) soldered to the bleed resistor. Sounds great. One tube is pulling 35ma and the other 39.

This is just a heavier-duty Bandmaster or a one-input-tube Low Power Twin over a 4X10. Absolutely my favorite circuit. You can't make it sound bad at any volume, over any speaker combination with any guitar. My personal version is a Blackface Bassman head modded to a 5E5-A in a combo cab over a 1959 Jensen P15N. All NOS glass with a couple old Tung Sol 5881s driving the bus. On top of that is a stand-alone reverb based on the Ampeg capacitor coupled circuit that just kills the way a Gemini does.

Thanks to all who read and pondered. Skip