Individual Bias Pots on a 440A???

[Kasanay]

Anybody know if it is possible (and worth the effort) to swap the single bias pot for four individual pots on a console amp like the 440A? I have such an amp running as a stand-alone and I think having the ability to adjust the tube bias individual would be a nice feature. I know this topic has been touched on in a few threads but I can not find any detailed info.
The file attached is my guess on how this would be accomplished. Take a look and let me know how far off base I am. I believe the new pots would need to be 20 ohms each.
I am amazed at the amount of wisdom folks on AK are willing to share.

Thanks

 

[dcgillespie]

That would do it, using pots that are 4X the original value. Also, you will need to install matched 10 ohm .25 watt resistors in the cathode circuit of each tube to be able to measure their current flow for adjustment purposes.

Dave

 

[gadget73]

Could the output stage be re-worked a bit like you did for the TA-600? That allows use of small value pots to tickle some voltage back into the grid vs a high power, low ohm version.

TA-600 Service Bulletin -- Early Version

Looks like the stage is overall pretty similar, though it uses a cathode resistor instead of the tube heaters. Might need to tweak the resistor values to get the grid voltage in the right range.

 

[Kasanay]

you will need to install matched 10 ohm .25 watt resistors in the cathode circuit of each tube to be able to measure their current flow for adjustment purposes.

Dave, sorry for asking what should be a no-brainier but, could you clarify exactly where I need to wire in the 10 ohm resisters? Do I connect one between each pin 3 to ground? If so, do I also leave the existing wire on pin 3?
Also, if I am installing 4 test port jacks, should I also install 1 for measuring plate Voltage?
Thanks,
Joe Y

 

[dcgillespie]

Disconnect all of the wiring from pin 3 of each output tube. Originally, they were all daisy chained together, and collectively connected to R125, C93A, and the new 270 Ohm 20 watt resistor. R125, C93A, and the 270 Ohm 20W resistor all still need to be connected together (as they originally were), but the point where they are connected together now forms what is called the "cathode buss". So

1. Install your four 10 Ohm resistors, one between pin 3 of each output tube, and the cathode buss.

2. Install a black colored test point and connect it to the cathode buss.

3. Install four red colored test points, and connect one to pin 3 of each output tube in the order of your choosing.

When you check the current draw of each output tube, the negative probe of your meter will ALWAYS go to the one black colored test point -- NEVER ground. Your red probe will go to the red test point of the tube being checked/adjusted.

I hope this helps!

Dave

 

[Kasanay]

R125, C93A, and the 270 Ohm 20W resistor all still need to be connected together (as they originally were), but the point where they are connected together now forms what is called the "cathode buss".

Dave,
A few more questions:
What happens with R104? Is it also part of the buss?
Will it make any difference if I use 1 ohm resistors rather than 10 ohm?
Is there a preference to install the resistors near the test jack or near the tube socket (jacks will be a few inches from tubes)?
Will the leads between pin 3 (each tube) and resistors need to be shielded?

Thanks again.

Joe Y

 

[dcgillespie]

R104 stays exactly as you have already drawn it: between the bottom end of the controls, and ground.

The only difference that 1 Ohm resistors will make is that your test readings will be 1/10th that of what 10 Ohm resistors produce. Using a 1 Ohm resistor will produce direct ma readings on your DVM, but also allows basic connecting leads to introduce potential error. A 10 Ohm value is as invisible as a 1 Ohm resistor is circuit wise, but will ensure more accurate readings, although you do need to convert the readings to determine actual current, which is nothing more than moving a decimal point.

Install the resistors near the jacks -- this to prevent the resistors from being heated by the tube pin. High stability resistors minimize drift in their value with regards to changes in temp, but minimizing how much heat they are subjected to to begin with reinforces this capability.

No shielding required.

Dave

 

[bhamham]

Install the resistors near the jacks -- this to prevent the resistors from being heated by the tube pin. High stability resistors minimize drift in their value with regards to changes in temp, but minimizing how much heat they are subjected to to begin with reinforces this capability.
Dave

Good to know Dave, thanks.

 

[Kasanay]

R104 stays exactly as you have already drawn it: between the bottom end of the controls, and ground.

The only difference that 1 Ohm resistors will make is that your test readings will be 1/10th that of what 10 Ohm resistors produce. Using a 1 Ohm resistor will produce direct ma readings on your DVM, but also allows basic connecting leads to introduce potential error. A 10 Ohm value is as invisible as a 1 Ohm resistor is circuit wise, but will ensure more accurate readings, although you do need to convert the readings to determine actual current, which is nothing more than moving a decimal point.

Install the resistors near the jacks -- this to prevent the resistors from being heated by the tube pin. High stability resistors minimize drift in their value with regards to changes in temp, but minimizing how much heat they are subjected to to begin with reinforces this capability.

No shielding required.

Dave,
Once again, I'd like to say THANK YOU! It is very kind of you to take the time to share your knowledge and answer so many questions. The amount of time you have saved people like me is immeasurable - not to mention the all the money and hard knocks.
I'll be ordering my trimmer pots and 10 ohm resisters today.

Thanks,
Joe Y