BeePre voltage issue?

[joepiecuch]

I corrected a filament board mis-wire as pointed out by Chris65 and then got a good set of resistance readings. Proceeding to voltage readings, everything looks according to spec with the exception of the terminal 17/B2 reading, which is showing 72VDC. I tried swapping tubes to no effect; is this a concern?

 

[Paul Birkeland]

How close are you to 5V at terminal 16?  Does this voltage follow one of the 300Bs?

If the voltage follows a tube, you just have a ridiculously strong 300B, but the EH300B tends to be exceptionally consistent.  If the voltage doesn't follow a tube, then I would be suspicious that the volume pot/balance control are not properly grounded on that side of the amp which is throwing off the biasing a bit.

-PB

 

[joepiecuch]

I got 4.82VDC on 16 and 4.9VDC on 1; swapping gave me 4.92 on 16 and 4.82 on 1. That looks to me like it follows the tube, but is that a significant difference?

 

[Paul Birkeland]

Does the low plate voltage follow one of the tubes? (Plate is the 72V vs. ~100V readings)

 

[joepiecuch]

Nope.

 

[Paul Birkeland]

I would carefully measure the DC voltage at pin 3 on the offending side.  It may start at a few volts and quickly drop if it isn't well grounded (your meter will provide a little bit of a ground).

-PB

 

[joepiecuch]

I get .13VDC at B3 (the problem side) and 0VDC at A3. The .13 hovers a little, .127 to .132, say, but basically centers on .13 (I'm using a Fluke 77). The ground lugs and cases on the two control pots seems solidly grounded. 

 

[Paul Birkeland]

If you turn the volume pot all the way down, what is the DC resistance from A3 to ground and B3 to ground?

 

[joepiecuch]

VDC and resistance? Or resistance with DC voltage in place?

 

[joepiecuch]

With volume control all the way down, I get 0 VDC at A3 and .149 VDC at B3. With power off, i get 2 ohms at A3 and 8M increasing to OL at B3.

 

[Paul Birkeland]

With power off, i get 2 ohms at A3 and 8M increasing to OL at B3.

Yeah, that's a problem.  The grid at A3 doesn't see ground.  It gets ground from the volume pot, and this is supplied by all the black wires.  I would check that you aren't missing one and go over the instructions and solder joints to be sure everything is as it should be.  You can also post some photos of that area of the circuit and we might be able to see what's going on.

-PB

 

[joepiecuch]

Oh, the shame.  :-\  Mis-wire found, corrected, and good readings are in evidence. Thank you, sir.