Help with bias circuit issue after modifications

[jlatrace]

I am trying to make the following mods and have run into an issue. I made the following changes and afterwards my bias circuit was disabled. I did the following:

Change:
C9 from .1 to .02uf
C13 & 14 from .02 to .01uf

My voltage readings across the blocking resistor (i'm using a 220K) are normal but after the diode the voltage is -0. The pin 5 voltage is also -0.

I noticed the schematics that used similar values (like A0 schematic) have a different bias circuit. So, my question is do I need to rework the bias circuit to match the A0 schematic (or similar) with a 470k (ish) blocking resister, a 100uf bypasscap to ground? Or have I made a mistake that I haven't discovered?

Thanks in advance.
les

[Phil_S]

I'm not exactly clear on the cap references, but it seems to me that you have either toasted or grounded something after the 220K dropping R. If you have nothing after the diode, then you know it's before the diode or it IS the diode. A 1N4007 is worth 5-10 cents. replace it and see what happens. Look around for something that's grounded when it shouldn't be.

[jlatrace]

Yeah it's a problem after the diode and it's probably a bad part. I'm going to pull them and replace them later today.

also i looked closer at the A0 schematic and saw that the bias system there was running off a bias tap and not a B+ tap.

It's frustrating because it's clearly something I screwed up and it shouldn't be hard to find.

[gearhead]

Look at this schematic, it's probably the most up to date:

https://tubeamparchive.com/viewtopic.ph ... ght=layout

Good layout there too.

[jlatrace]

Thanks for the schematic.

I've now checked everything to the schematic and checked continuity of each connection using a jumper all the way through the bias and the phase inverter circuit and I cannot find my issue.

Again my symtoms are ..... everything is normal except the bias circuit is diabled. Voltages read normal from B+ to the dropping resistor up to the diode. The diode has been replaced and is in good working order. However, after the diode the voltage drops to -0 (instead of -33 as it was before I changed the values of the coupling caps to and from the phase inverter) and stays -0 through the phase inverter all the way to pin 5 on the power tubes. There is no change in the symptoms with and without power tubes.

I get B+ readings at all other points in the circuit. I am going to put the spec coupling caps back in my amp later today and then try to find what I screwed up yesterday.

Does anyone have any further trouble shooting suggestions?

[Bob-I]

Again my symtoms are ..... everything is normal except the bias circuit is diabled. Voltages read normal from B+ to the dropping resistor up to the diode. The diode has been replaced and is in good working order. However, after the diode the voltage drops to -0 (instead of -33 as it was before I changed the values of the coupling caps to and from the phase inverter) and stays -0 through the phase inverter all the way to pin 5 on the power tubes. There is no change in the symptoms with and without power tubes.

Does anyone have any further trouble shooting suggestions?

Yes, you replaced the coupling caps and the problem started. It sounds like you may have a bad cap.

Disconnect the bias supply from the grid resistors, the 2-220K resistors labeled R16 and R17. Measure the bias supply (remove the power tubes).

If the bias is ok, reconnect and measure on both sides of the caps. you should have voltage on the side of the resistor and none on the other side.

report back.

[jlatrace]

Bob
Your troubleshooting idea of disconnecting the bias circuit from the phase inverter lead me right to the issue. I located the issue by continuing to disconnect parts of the circuit until the bias voltage returned. When I knew exactly where the issue was it was an easy fix. Thanks for the suggestion.
Les

[Bob-I]

Bob
Your troubleshooting idea of disconnecting the bias circuit from the phase inverter lead me right to the issue. I located the issue by continuing to disconnect parts of the circuit until the bias voltage returned. When I knew exactly where the issue was it was an easy fix. Thanks for the suggestion.
Les

Glad I could help.

When in difficult troubleshooting situations, always simplify and troubleshoot smaller circuits.