Fender Bassman 50 Blackface AA165 - Bias Problems

[misterc57]

Hey Folks

I have a AA165 that came to me with a new Hammond PT and OT. According to the Hammond site they are the correct transformers.

The amp has no output according to the owner.

At this point I am trying to get the bias set properly before even getting to the "no output" issue.

For the bias the amp had an old 10K pot plus a 2K resistor to ground. My info shows I should have about -32V at pins 1 and 5 of the output tubes. My readings were much higher, over -60V and the overall amp current was above 1 amp.

For the Bias adjustment I installed a new 10K pot and clipped in a 15K resistor to ground. Once the tubes warm up, no matter how I set the pot, I end up with about -70 VDC on pins 1 and 5, and over 1 amp overall amplifier current draw.

I have tried two different sets of 6L6 output tubes and have the same results. I have tried different bias resistor values to ground with the same results.

I read -57 VDC coming into the bias pot from the bias supply.

Any suggestions please?

Fender_bassman_ab165_schematic.pdf

[misterc57]

If I set the bias pot wiper to -45 VDC I read -70 VDC at pins 1 and 5 of the output tubes

[Helmholtz]

According to the schematic the g1 (pin 5) voltage should be around -45V. A grid bias of -32V would be way too low..
Check if the 15k resistor has drifted high.
Pin 1 of a 6L6 is not internally connected, just used to support the grid stopper.
But what really matters for tube bias is not grid voltage but idle current.
So you need to measure idle/bias current to be sure. Something like -50V grid bias might just be fine depending on tubes.

What do you mean with "over 1 amp overall amplifier current draw"?
Is this mains input current?.

[Helmholtz]

If I set the bias pot wiper to -45 VDC I read -70 VDC at pins 1 and 5 of the output tubes

If the wiring is correct, pin 5 voltage must be the same as the voltage at the bias pot wiper.
This said, cheap meters often read a lower (i.e. less negative) grid voltage because of their low input resistance).
Check if the wiring of the bias circuit is original, i.e. provides output tube balancing.

[Tom Phillips]

Until you get the bias supply issue fixed there is no need to have the 6L6 power tubes installed. It's best to just leave the 6L6s uninstalled so they don't inadvertently get damaged.
I see from your photos that the original rectifier parts board has been replaced but I can't see the details due to a focus issue.
My guess is that you will find some strange wiring error.

[misterc57]

The 15K resistor and 10K pot are brand new. I was seeing same issue with the old pot and 2K resistor to ground.

I have a quality meter.

A lot has been done to this amp so it is very possible that something was done wrong.

The bias V at pins 5 is good until the warm-up period is over, then there is a sudden jump from -45 VDC to -70 VDC.

I am seeing about 1.4 amps at the mains current draw. Which seems high. I would think it should be close to and under 1 amp.

More pictures showing the rectification and bias supply. Seems to be wired correctly but it is using old 470 R resistor and diodes. Bias supply cap is new and 100 uF 100 V.

[Helmholtz]

Check if the chassis ground connecting of the 15k is reliable.
If it loses contact, bias voltage will jump to a more negative value.

[g1]

Check if the chassis ground connecting of the 15k is reliable.
If it loses contact, bias voltage will jump to a more negative value.

Or maybe if that lead of the resistor is touching the pot case, then stops touching?

[Greg_L]

Many of those solder connections look crappy at best

[Jon Snell]

"The bias V at pins 5 is good until the warm-up period is over, then there is a sudden jump from -45 VDC to -70 VDC." Check your meter and install the 27k properly. Never tag to old component leads, not even for testing purposes or you will get odd results or even catastrophic failure.

The first thing to do is rewire that peg board. Better still, start again with paxolin board and fit eyelets, like the original board was. Use the amplifier board as an example.
Don't use an old match box as a spacer, use standoffs.
Never solder onto a semiconductors business part, always use the leg length of at least 1/2", to avoid damage.

Check those anode load resistors for drifting. We call CC resistors 'Carbon Drifters' because they do.

[misterc57]

Updates

In the bias supply I have replaced the 470 R and diode, rewired, installed a new ground point. Also installed new pair of 220K resistors. Still seeing issue.

Installed a new pair of 220K balancing resistors at the in series filter caps. Still seeing issue.

I set the bias pot to output -45 VDC, I see a similar reading at both 6L6 pin 5 and around 450 VDC on the plates. Until the tubes warm up. Then I see the bias jump to about -70 VDC, and the plates drop to around 330 VDC,

The new output transformer has been setup with 3 output jacks. 4, 8 and 16 ohm connections.

With no load connected I do not see the V change after warmup. With a load connected I see the V change after warmup. I have tried both the 4 and 8 ohm jack connections with the same results.

The PS board has stand-offs which I removed to get to both sides of the board. Matchbox is a temp holder.

Thank you

[Helmholtz]

I set the bias pot to output -45 VDC, I see a similar reading at both 6L6 pin 5 and around 450 VDC on the plates. Until the tubes warm up. Then I see the bias jump to about -70 VDC, and the plates drop to around 330 VDC,

That seems contradictory as more negative grid bias means lower tube currents.
Lower tube currents should result in increased plate voltage.
An explanation could be HF oscillation.
Please try again with PI tube pulled.
Do you have a scope?

With no load connected I do not see the V change after warmup.

Never operate a tube amp without a load. Otherwise the OT is at risk.

[misterc57]

Yes I have a scope. With V4 (PI) pulled things are stable. I have mismatched tubes in there currently.

VD and R measured from OT center tap to 6L6 plate.

[Helmholtz]

Yes I have a scope. With V4 (PI) pulled things are stable.

That seems to confirm an oscillation problem.
Try to scope the HF oscillation at the output

[g1]

As it is a new OT I suspect the phasing is reversed, causing the oscillation.