Mainline Bias Voltage Help [resolved]

[RdDgWest]

Hi,
This is my first Bottlehead build in over a decade.  Forgot how much fun this is.

My voltages were nearly spot on except for the b-side Kreg which I resolved.


+275vDC on the Power Supply Board (275V) 262V
+6.3vDC on the Power Supply Board (55-65V) 58.3V
-6.3vDC on the Power Supply Board (50-60V) 52V
IA on the A side C4S Board (275V) 261.5V
IA on the B side C4S Board (275V) 261.0V
Breg Regulator Board both sides (220V) (A-side) 217V & (B-side) 216V
-reg Regulator Board both sides (0V) (A-side) 0V & (B-side) 0V
Kreg Regulator Board both sides (8-12V) (A-Side) 10.4V & (B-Side) 10.5V

* If the B-side Kreg voltage is 1.3vDC check solder joints and connections on b6 or 22U

 

Now I'm struggling with the bias voltages and would appreciate some help. 
When I bias T20 for 145vDC I will get a 100vDC measurement on T30.  When I adjust T30 for 145vDC, T20 will drop to ~100vDC. 
What could cause this flip flopping of the bias voltages and were should I investigate?

[Paul Birkeland]

I would investigate the wiring of the center PC board and be sure to be mindful that the trim pots on those center boards control the bias voltage on the opposite side that you would expect. 

[RdDgWest]

Thank you Paul
I've done that a few times before posting and just re-confirmed the steps in the build manual.  I must be missing something obvious. 
I did some continuity checks to ensure all connections from the board to terminals and tube socket pins are good and all appear OK. 

This one has me scratching my head.  Not sure why adjusting pot A & B pull up or drive down the voltage on the other terminal.   

[RdDgWest]

Well I accidentally created a new problem. 

I was checking voltages on the tube socket pins for A5 & A6.  My probe slipped and shorted pins A5 & A6 which resulted in the B side LEDS on the C4S to no longer illuminate. 

Should I replace both the PN2807A and MJE5731A or only the PN2807A?

What a stupid mistake.

[Paul Birkeland]

Without knowing how those voltages changed, it's hard to know what to recommend.

[RdDgWest]

Hi Paul,
The Kreg (A-Side) is 5.4v
The Kreg on (B-side) is 10.5v

C4S over the A tube socket (Left side)
IB is 217v
OB 215v

IA is 261v
OA is 217v

Feels like the MJE5731 on the B side of the C4S board. 

Is this what the expert recommends? 

Thank you

[Paul Birkeland]

The drop in Kreg voltage on that side tells you that the regulator is now sucking down all of the current that would otherwise be available for the 6C45PI to use.

I would swap the high current C4S boards between sides.  We would expect that Kreg voltage and the high OB voltage to move with the board, and if it does you'll want to replace the board or the B side transistors at a minimum.

If those voltages don't move, then the damage occurred to the center board, which is certainly also a possibility. 

[RdDgWest]

Thank you, Paul
The issue appears to be the center board.  Swapped the C4S boards.  The issue did not follow the C4S board. 

Voltages
Kreg (A-Side) = 4.7V
Kreg (B-Side) = 10.4V

C4S
IA = 267V
OA = 217V

IB = 214V
OB = 215V

So consider a replacement for the 2N2222A and LM431A?

Appreciate the support.

[Paul Birkeland]

Be sure to swap 6C45 tubes as well.  The 431A seems to be working happily enough. On the center board feeding the offending side, what kinds of DC voltages do you see at the "O" terminal as you twist the trim pot?

[RdDgWest]

Good Morning, Paul
Swapped the 6c45 tubes.  The problem stays with the left side C4S board.  The B side LEDs still don't light on the left side C4S board.

The OB voltage for the pot near the power transformer stays at 5.3V.  I turned the pot 5-10 times in each direction.

The OA voltage for the pot toward the front of the chassis ranges from 1.7v - 3V

[Paul Birkeland]

I would replace the 2N2222.

[RdDgWest]

Thank you, Paul
Ordered. 

Let's see if this gets me back to my baseline issue so I can figure out why the bias voltages can't be set to 145v for T20 and T30 without impacting the other terminals bias voltage setting. 

[RdDgWest]

Replacing the 2N2222A solved the issue.  LEDs on the B side of the left C4S board are back on. 
Note to self, shorting tube pins A5 to A6 will damage the B side 2n2222A on the bias/regulator board.   


The issue with the bias adjustment measured on T20 and T30 remains. 
During the trouble shooting for the bias/regulator board I swapped them between the left and right side and the bias problem remains.

Unless I built both C4S boards wrong identically should I be focusing my attention to the C4S boards or the work on the tube sockets for the 6c45's? 

[Paul Birkeland]

I would very carefully go over the steps in the manual for constructing and installing the center board to be sure everything is where it should be.

[RdDgWest]

Hi Paul,
I looked the center bias/regulator board over until I was blue in the face.  After checking the component position, orientation, and connection to and from the board multiple times over two days, I was still experiencing bias voltage issues.  The T20 and T30 could be adjust to 145v with the other terminal at ~100V.

Although I did this numerous times, I rechecked continuity for all ground connection.  Although the ground terminal T8 looked good and measure continuity before, it seemed to be spotty today. 

I check continuity between T4 and T7.  Nothing.  T10 and T7 nothing. 

Although I had good continuity between T7 & T6 and T7 & T21. 

Touched up terminals T4, T8, and T10.  Now I had good continuity checks between T7 and all ground points. 

Powered back up and T20 and T30 were both measuring ~165v - ~170v.  Turned potentiometer on the B-side of the bias/regulator board for T20 and adjusted to 145v.  Moving over to T30 and adjusted the potentiometer on the A-side of the bias/regulator board to 145v.  Re-measured both T20 and T30 and they stayed at 145v. 

In conclusion a bad ground between T4, T8, and T10 caused my biasing issue.

Music is playing. 
High-Five to you and thank you.