Fender 400 PS bias

[pontiacpete]

Now the voltages have changed> interesting how this thing reacts:

V8 - 14vdc
V9 - 14vdc
V10 - .2vdc
V11 - .2vdc
V12 - 14vdc
V13 - 14vdc

something is wrong on the inside pair, V10 and V11
Not sure what. Got to check around some more and maybe try moving tubes around or try new ones.

[g1]

The cathodes for each set will only be connected when the load is attached for that set.
If the line draw does not increase by the appropriate amount for each set connected, adjust the bias.
The reason your draw did not increase from 1.5 to 2.1A when you connected the first load was because the bias had been set according to a voltage that was not really appropriate for these tubes.
Once the proper bias level is set, you adjust the balance pot for minimum draw (without touching the bias pot again).

[pontiacpete]

Thanks G1. Starting to see how this works.

with all jacks connected to their 4R loads, voltages are alot closer.

V8 .002
V9 .065
V10 .228
V11 .228
V12 .308
V13 .464

How does this look?

And the current draw is at 2.26A

[pontiacpete]

Today I am trying to match the output tubes working with the tubes I have been given.
Found that V8 wasn't conducting at all and finding some very weak.
This is what I have now with a full load on all outputs and the primary current at 3.03A
In their respective pairs
V8 .48vdc (48mA) and V13 .49vdc
V9 .42vdc and V12 .32vdc
V10 .19vdc and V11 .30vdc

[pontiacpete]

I tried adjusting the bias voltage to bring up the primary current draw to 3.2A, managed to get it up to 3.16A
and the voltages are
V8 54v V13 54v
V9 46v V12 35v
V10 22v V11 33v

I think this is best I can do with the 6550a's that were given to me

[TimmyP1955]

Check that the cathode switching contacts are open when nothing is plugged in to the output jack(s), and closed when something is plugged in.

[g1]

I tried adjusting the bias voltage to bring up the primary current draw to 3.2A, managed to get it up to 3.16A
and the voltages are
V8 54v V13 54v
V9 46v V12 35v
V10 22v V11 33v


I think this is best I can do with the 6550a's that were given to me

I'm assuming this is measuring at the cathode and you forgot the decimal place?
You can shuffle the tubes around and if the readings stay with the particular tubes, then I agree, that's the best you can do with these tubes. If the readings stay with the particular sockets, then look for something in the circuit.

[pontiacpete]

I'm assuming this is measuring at the cathode and you forgot the decimal place?

not again

yes there should be a decimal place, or the 'v' should have been 'mA'.

Took off the AC ammeter and the voltages at the cathodes were different today.
Anyway I wrapped it up with each set pulling between 30mA and 45mA.
They were within 5-10mA at the most per pair. Not the most matched pair. It sounds good.
Thanks for the input.