Author Topic: Fender 400PS bias issues  (Read 310 times)

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Offline average joe strummer

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Fender 400PS bias issues
« on: October 16, 2019, 11:09:12 pm »
Hello all

New member here. I have been repairing tube amps for some time and understand the dangers and safety issues involved with working with high B+ voltages.

My question here is how to bias the 400PS correctly. I have been following the Ed Jahns 30 point setup

http://www.timeelect.com/400-upmod.htm

I can get the -39VDC on the bias pots and all components were verified to be installed correctly. I have a measured resistance across OT at 6.2 ohms and 5.5 per side. 692 & 695VDC per side plate voltage, but voltage drop of 0.519 & 0.304VDC per side seems to be drastically different. This is giving me 53.6 & 35.2 watts plate dissipation which I think is scary high for 6550's. I have installed new electro caps throughout the amp, all to spec, and new caps & resistors on power tube sockets.

What is the ideal plate dissipation for this amp, how can I balance the voltage drop of the OT, and does a radically different PD effect the performance? Schematic is in the link above.

https://www.jj-electronic.com/en/6550

Thanks to any knowledgeable minds who can point me in the right direction.

Cheers
AJS
 

Offline floobydust

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Re: Fender 400PS bias issues
« Reply #1 on: October 17, 2019, 12:23:01 pm »
An amp like this does need individual bias trimpots unless you are a rich man.
The 6550's are "special design" selected Tung-sol's and matched as an entire set. Pushing them pretty hard with a plate voltage of 700VDC.

"bias set for total ~210mA", is about 35mA each for 24.5W quiescent. With cathode resistor 10R, for 0.35V each. You're getting from 30.4mA to 51.9ma?
Those tubes aren't matched that great, well enough to work here or there are damaged screen/plate resistors. The OT asymmetry is not the problem, it's not going to make the Q-point that far out.

You have to at least get each side (set of three on OT leg) closely matched so you can work with the balance pot.
I would set the bias equal and measure then sort the tubes by cathode current, then shuffle them in each side so that is best matched, then do the bias balance to bring the entire set close together. Otherwise, you will just bake the tubes having higher transconductance and it will be in the shop again.

Having mismatched tubes between each side does affect distortion, but Fender does not allow any other way to set bias.
 

Offline Renate

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Re: Fender 400PS bias issues
« Reply #2 on: October 17, 2019, 06:57:41 pm »
Yup, floobydust has all the numbers that I wanted to say.

It wouldn't hurt to check the 10 ohm resistors just in case.

How many speakers are you using? Will you use?
If it's only one, you're only using a pair of tubes.
You can easily adjust the bias dead-on for that.
(The other four tubes can be stored away for later.)
 

Offline average joe strummer

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Re: Fender 400PS bias issues
« Reply #3 on: December 16, 2019, 10:44:50 pm »
Sorry guys, haven't logged in for a while.

Got it sorted nicely. Appreciate the help.
 

Offline floobydust

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Re: Fender 400PS bias issues
« Reply #4 on: December 17, 2019, 03:22:01 am »
What was it?
 


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