Strange issue with bias control:

[Stratz]

Hi all. I'm converting another PA/Hi-Fi to guitar amp and the strangest thing is happening.
I removed the cathode bias and installed an adjustable grid bias (ala Fender) I've done this many times with great success. This amp is a push pull EL84/6BQ5
AC voltage off the PT through a diode, cap, 10kL pot and a couple of resistors and I'm done.. but not this time.

Whats happening is the bias works fine with about 20 volt swing but when I put the output valves in the bias voltage goes positive rather than staying negative. I can watch it as I bring it up on the Variac. When I get to about -6v bias (about 70 VAC on the Variac) the bias voltage starts to go down to zero and then reverses to positive voltage.

The bias voltage has the same range and sweep as when the power tubes are removed but it's + voltage rather than - voltage as if the diode suddenly disappears.

I measured all components in the bias circuit and everything checks out good. I've tried different power tubes and it doesn't help.

This amp has an ultra linear output transformer (2 wires to each output tube) but I simply eliminate the leads from pin 9 which is grid 2/screen and add a wire from grid 2/screen to B+ as I've done with success on other amplifiers with this OT setup about 5 or 6 times.

I've attached a schematic of the amplifier.... it might help.
So far I've only re capped the amp (filter stage) removed the selector switch and tied both output tube cathodes to ground. I also moved the grid feed (pin 2) from node 1 of the power rail to node 2 on the power rail (node 1 =430vdc/node 2 = 360vdc) hoping it would help. There are some loose ends at the tone controls because I've removed the selector but I doubt that could be causing the problem.

I'm hoping that someone has run into this problem before or that somebody can figure out what makes the bias go positive when I put a set of power tubes in because after many hours of troubleshooting I'm lost and I'm in need of some help.

Thanks all.
Rob.


EL84/6BQ5 pinout diagram.

[potatofarmer]

I agree; as stated, I can't see how that would work. Can you draw up a quick schematic of the bias supply as you've installed it?

[g1]

I also moved the grid feed (pin 2) from node 1 of the power rail to node 2 on the power rail (node 1 =430vdc/node 2 = 360vdc) hoping it would help.

Not sure what you mean here, pin 2 is the control grid where you should be putting the negative bias voltage.
You need to revise the schematic with your changes so we can see what you have done. Trying to explain it is just confusing.

[Stratz]

Not sure what you mean here, pin 2 is the control grid where you should be putting the negative bias voltage.
You need to revise the schematic with your changes so we can see what you have done. Trying to explain it is just confusing.

Sorry G1, I moved the grid 2 which is pin 9 (I got pins 2 and pin 9 mixed up in my original post) wire to a lower voltage point on the power rail hoping it would help but it didn't.

I dont have any programs for drawing schematics so I apologize. I simply removed the second set of wires from the Ultra Linear output transformer that went to pin 9 and ran a wire from V3 pin9 (g2) to V4 pin9 (g2) to the B+
Thanks.

[Stratz]

I agree; as stated, I can't see how that would work. Can you draw up a quick schematic of the bias supply as you've installed it?

Hey P Farmer. It's a basic Fender ab763 bias setup (see attached schematic) but I started with 340VAC (no bias tap on the PT) so I used a 220k dropper R to the diode and cap.

I've attached a pic of the bias circuit in the amplifier (cant draw to save my life). It's a lot sloppier and more temporary looking since I started tearing things apart.

Thanks for the reply.

[Dave H]

Whats happening is the bias works fine with about 20 volt swing but when I put the output valves in the bias voltage goes positive rather than staying negative. I can watch it as I bring it up on the Variac. When I get to about -6v bias (about 70 VAC on the Variac) the bias voltage starts to go down to zero and then reverses to positive voltage.

The bias voltage has the same range and sweep as when the power tubes are removed but it's + voltage rather than - voltage as if the diode suddenly disappears.

That's impossible!
Where are you measuring the bias voltage? At the Pot wiper, junction of 220k resistors? Measure the voltage on the bias diode -ve. Does diode -ve stay negative with the power tubes inserted? If the answer is yes check the pot, it could have a high resistance wiper contact.

[SoulFetish]

Have you tested your tubes and/or wiring to the tube socket?
If your bias looks good before you put the output tubes in, it could be like there is some sort of short or leakage path to a +HV (essentially shifting the voltage divider your using to dial in your bias much more positive.)
Try removing the output tubes and disconnect the negative bias voltage and see if you get a DC voltage reading after C!5 and C19.

[Mick Bailey]

Regardless of the bias issue, there are no grid stoppers and as you describe your changes, no screen resistors either.

[Stratz]

Have you tested your tubes and/or wiring to the tube socket?
If your bias looks good before you put the output tubes in, it could be like there is some sort of short or leakage path to a +HV (essentially shifting the voltage divider your using to dial in your bias much more positive.)
Try removing the output tubes and disconnect the negative bias voltage and see if you get a DC voltage reading after C!5 and C19.

We have a winner!
You got it bud. The rectifier tube socket was bad, dirty and pin 7 was about to break off. When I started working on the amp again today I bought it up slow on the Variac and when I got to about 90vac I saw the rec tube flash. That tube got ruined (a beautiful RCA black plate too....) so I put in a used JJ rec tube and bought it up very very slowly, I saw a flash under the tube so I installed a new socket and all is well now.

I want to thank everybody that helped me with this problem and others I've posted.
You guys are the best.
Thanks again.
Rob.

[SoulFetish]

Cool. Glad you figured it out.
Mick brought up an important point though, you should put some screen resistors, grid stoppers, and also add some current limiting resistors before the plates of the EZ81. Check the data sheet for the recommended values( you can include the resistance of the power transformer primary and secondary winding as part of the limiting resistance).

[Stratz]

Will do. Thanks again all!