I did this, and it's good.

[Paul G.]

Paul's AB165

Maintenance/Repair
1. New F&T filter caps ([80/80]/20/20/20/10).
2. Replace drifted grid stoppers on 6L6s.
3. Replace screen resistors on 6L6s.
4. Clean all pots, switches, jacks.
5. Remove, clean, replace circuit backer board.
6. Replace bias capacitor.
7. Replace all cathode bypass caps (x5) Sprague Atom.
8. Replace all 100K plate resistors with 1W carbon film.

Mods
1. Remove output stage negative feedback circuit.
2. Convert bias balance to true bias
3. All brown turds replaced with Sprague Orange Drops.
4. Remove plate bypass caps on Normal and Bass Channel.
5. Convert to AA864 negative feedback scheme, 820R feedback resistor,
reverse OT primaries, reconfigure Phase Inverter (100K/82K plate resistors).
6. Put both plates of V3 on the same B+ node.
7. Reconfigure Bass channel to Trainwreck spec's
250pf silver mica treble cap/.022 mid cap/.022 bass cap/100k slope resistor.
8. Remove Deep circuit, replace with Bright switch (120pf cap).
9. Shielded cable input to grid, volume to grid V1, V3.
10. Add 1R cathode resistors to assist checking bias.
11. Grid stoppers (1.5k) added to V1b and V3b to isolate volume wipers.

Stuff the whole thing in a pine 2x10 combo cabinet with Eminence Lil Buddy speakers.

_6100009_DxO.jpg

_6100003_DxO.jpg

[Stevem]

Another mod you should make for the sake of 120 HZ hum/ buzz is to un-daisy chain the grounds on the power supply filters.

The 20 uf PI section filter should get its own ground wire back to where the 2 previous filters are grounded back at the PT, and the other 3 filters should atleast get grounded back at the brass strip under the pots, or back at the normal channel input jack.

If you want to get fancy then run 3 sperate ground wires for these 3 filters and ground them at there respective preamp tubes Cathode bypass grounding point.

[Stevem]

I would also play around with the 220k resistors off of the plate of each output tube back to the PI section .
I would start by almost halving the value of these 2 resistors while playing the amp loud enough to hear the output stage start to break up and then decide on what you want.

[Paul G.]

I would also play around with the 220k resistors off of the plate of each output tube back to the PI section .
I would start by almost halving the value of these 2 resistors while playing the amp loud enough to hear the output stage start to break up and then decide on what you want.

They are gone, that's what I meant by removing output negative feedback circuit. There is no longer a connection between the plates and the PI output.

I will try that grounding scheme for the filter caps though, it sounds intriguing. There is a very low but noticeable 120Hz hum I wouldn't mind getting rid of or at least making lower.

[Stevem]

So you still then have the feedback coming from the OT back to the PI input grid?

[Paul G.]

So you still then have the feedback coming from the OT back to the PI input grid?

Yes, I just reconfigured it to be like the AA864 or typical Fender AB763 format. It used to come from the speaker jack, through a 47K resistor and a .1uf cap directly to one of the grids of the PI. Now it comes from the speaker jack, the an 820R resistor into the cathodes of the PI. The top plate resistor is replaced with an 82K to balance the PI, and you need to remove the ground at the 22k resistor and replace it with a 100R tail resistor.

The AB165 is funny in that there are 3 different negative feedback circuits -- probably because as a "bass" amp, Fender was going for lowest distortion possible.

First is a 470K resistor connecting the grid and plate of V3B. Second is the normal OT to PI position, albeit oddly implemented, and the third is the Plates to PI.

I left the first alone, modified the second, and removed the third. I think the amp sound more lively and responsive, not that it was bad before, this just seems a bit better.

[Paul G.]

Inkedfender_bassman_ab165-fenderguru-mods.gif

[Paul G.]

Another mod you should make for the sake of 120 HZ hum/ buzz is to un-daisy chain the grounds on the power supply filters.

The 20 uf PI section filter should get its own ground wire back to where the 2 previous filters are grounded back at the PT, and the other 3 filters should atleast get grounded back at the brass strip under the pots, or back at the normal channel input jack.

If you want to get fancy then run 3 sperate ground wires for these 3 filters and ground them at there respective preamp tubes Cathode bypass grounding point.

Did this, well some of it. Grounded PI filter to the power ground point. Grounded the V3 filter to the chassis where the cathode resistor/bypass cap terminate. Left the other 2 alone.

Seems much better, tomorrow, I'll stuff it back into the cab and give it a good workout.

Thank you! Great stuff.

[Paul G.]

Nah, back to original ground scheme, no real difference in hum, so I'd rather keep the stock wiring.