Fender Bassman AA864 - Plate current way too low

[robin6724]

Hello everyone,

I am currently working on a Fender Bassman AA864. I just replaced the tube sockets. Everything was rewired properly by myself.

Here's the issue I am facing now :

When I re-install the power tubes and try to bias them. The plate current will never reach over something like 11.6mA ... Using either the TAD's Bias master or measuring with the "Output Transformer Resistance Method". If you do the math, this value is waaaaaay below the 60% (approx. 40mA) or even the 50%...

I've checked the re-wiring, all the voltages due to arrive on pin 3, 4 (approx 445VDC) and negative voltage on pin 5 (between -52 and -60 VDC) look good. Screen grid resistors measure fine. I've even tried another matched pair of tubes, still facing the same issue.

The amp does make sound but it breaks up very early and doesn't sound good to me.

Does anyone has an idea about what's happening over here? I've checked on the internet, on forums, etc. And can't really find a similar situation from anyone...

Thanks in advance,

[pdf64]

Do the bias supply components check out out, particularly the 15k between the bias pot and 0V chassis common?
https://el34world.com/charts/Schematics ... ematic.pdf

[robin6724]

Hi,

Thanks for your answer. I won't be at the shop before the end of the week.

I haven't checked these.

Keep you posted.

[xtian]

Your bias voltage is too negative. The schematic spec's -44v. You need to adjust the value of the bias circuit's tail resistor, from 15K to perhaps 22K.

EDIT: Yes, what Pete says below:

[pdf64]

Your bias voltage is too negative. The schematic spec's -44v. You need to adjust the value of the bias circuit's tail resistor, from 15K to perhaps 22K.

If the 15k value is increased, then the ‘hottest’ bias setting will become more negative, and hence the 6L6 will idle colder.
If there's 8V across the 10k track of the bias trimmer, then that range limiting bottom resistor (should be 15k) may be up around 65k!

[robin6724]

I'll be at the shop tomorrow. First thing I'll do is checking the 15k. If it measure correct then I'll check the pot.

Thanks

[Stevem]

A heck of a great starting point will always be to refer to the schematic!

With a know good set of output tubes in the amp with a -45 volts at the junction of the two 220k resistors, the V+ voltage at that point on the center tap of the OT ( red wire) , or even on pin 3 of either output tube should be in the neighborhood of 425 volts .

[robin6724]

Hi all,

I've checked out the bias circuit tail resistor and indeed there was a 48k installed instead of a 15k like it says on the schematic.

Any guesses about why the last owner would do this?

Obv, the amp sounds way better, deeper, has greater dynamics...

Thanks for your help!

[robin6724]

A heck of a great starting point will always be to refer to the schematic!

With a know good set of output tubes in the amp with a -45 volts at the junction of the two 220k resistors, the V+ voltage at that point on the center tap of the OT ( red wire) , or even on pin 3 of either output tube should be in the neighborhood of 425 volts .

Indeed - I never know what tolerance I can refer to when measuring voltages.

[pdf64]

The 1st measurements that should be made are the mains and heater VAC. If they’re not at nominal then nothing else will be.
If the mains (and hence everything else) is running 7 or 8% above the schematic’s 117V nominal, it would be rather a bad idea to set the bias to the schematic nominal without verifying that the resulting anode current was ok.

[Stevem]

My guess!
The previous owner was either misinformed, or maybe was trying to reduce the amps wattage and run like 6V6 output tubes which would need that level of bias voltage.