Bias Problem

[robkeeler]

Hi, I am using the Francesca layout and all my voltages seem to be in range. However, I can't bias the amp. The tubes look normal when on standby, but when I flip the standby they red plate. Any suggestion on where to check/fix?
Thakns

[Richie]

I would pull the power tubes.. turn the amp on and see what the bias voltage is on pin 5.. with the bias pot turned up a bit.. you should see around 35 neg volts.. if its not there.. check and make sure the wire feeding the bias does not have a cold solder joint,going from the power board bias section to the preamp board..and check back through the bias sections for the voltages.. this should tell you where the problem is..

Richie

[UR12]

What do you have your bias voltage set to? It should be about -30 - 33 volts

If you used the 220k coming from the HV winding you may need to drop it down to 150k. Or just solder a 1meg in parallel with the 220k to bring the voltage into range on the pot. I got -28 with the 220k and that was as low as I could go and it will red plate your tubes.

[robkeeler]

I'm not getting any reading actually. I think it is so high my Weber Bias Rite won't read it.

[UR12]

Can you post some pics?

[robkeeler]

I will try to post some pics........ I did make one wrong connection early on in the build. I connected one of the PT secondaries to the single diode on the bias board (instead of the 2 diodes in series). So everything in that path I will replace and see if that changes anything.

[robkeeler]

Just to make sure I have everything hooked up right

OT
(toneslut)

6600 Blue -> Pin 3 EL34
5200 Blue/White capped and cut
C/T Red to standby
5200 Brown/White capped and cut
6600 Brown -> Pin 3 EL34

Gray 16ohm
Yellow 8 ohm
Red 4ohm
Black speaker jack ground


PT
(toneslut)
PT Primary
Black to On/Off switch

PT Secondaries
Yellow 300 to diodes
Yellow/Green cut and capped
Yellow/Red to ground
Gray cut and capped
Brown to heaters
Brown/White to ground

[UR12]

OT Red CT to ground???? Should be Red CT to B+

[mlp-mx6]

YIKES!!

[robkeeler]

Whoops sorry misprint! My Red CT is going to the standby

[UR12]

Unless I overlooked something the connections look right. Did you get the parts replaced on the PS board? What do your voltages look like with no tubes installed?

[jaysg]

I'm not getting any reading actually. I think it is so high my Weber Bias Rite won't read it.

Do you mean while it's red-plating? It's not going to read anything with the tubes out.

[robkeeler]

When I have the Weber bias rite and tubes in it is fine on standby. However, it won't get any readings for bias. When I flip the standby the tubes hit around 110 on the meter (bias). The plate voltage looks good though. What voltage should I be reading at the 220k junction on the preamp board? Thanks for all the help! I really appreciate it.

[UR12]

Which way is you white band on your diode pointing, toward the 220k resistor or toward 15k. If your getting positive voltage you have it in backwards. It should be pointing towards the 220k resistor. You need to measure the bias voltage at the junction of the 15k and the 47 k on the ps board.

[robkeeler]

My voltage at the 47k 15K junction is -.079

Any ideas?