AA864 bass channel mod help - SOLVED

[bullkelp]

I see across many forums this is much discussed...so sorry if it looks like a repeat but Ive got some specific issues getting this to work.
Im trying to mod the bass channel. This amp isnt an original fender. Its a mashup build but both channels fairly identical and only two gain stages on each. The PI and power section follow the aa864 schematic. Normal channel is ab165 and I like it.
Bass channel is more or less teh same but with slightly different cap values.
Ive removed all the bass channel components and rebuilt as this:

https://www.thegearpage.net/board/index.php?threads/is-this-the-allen-mod-for-ab165-bassman.447295/

This issue im having is VERY low signal throughput. If I crank the volume I can hear a very low faint signal.

Seems to be voltage where it should be. Tubes are tried and tested. Normal channel still works as it should. Have been over and over it and im pretty sure things are wired per schematic.

What wold be the usual cause to such an issue?

Thanks in advance

[bullkelp]

Alternatively does someone have a schematic for an alternative circuit to try in the bass channel? Something to contrast the clean normal channel....

[stratomaster]

Have been over and over it and im pretty sure things are wired per schematic.

What wold be the usual cause to such an issue?

Unfortunately the usual cause is that you didn't wire things per the schematic.

Can you attach the schematic here and voltages you've measured? I was unable to see a schematic at that link.

[bullkelp]

Ok here is the schematic

[stratomaster]

Are those voltages in red your measurements? If not, please replicate those measurements and update with your values.

[bullkelp]

Yes they are my measurements

[stratomaster]

Cool. Do you have a way to generate a signal? Signal generator, phone app with a 1/4 cable, etc?

If so, put signal into the channel in question. Set your meter up AC volts, and increase the signal from your device until you read 100mV at the grid of the first triode.  A little higher is fine.  Set all controls to 5, then measure the AC volts at each grid and plate in that channel until you get to the cathode follower. Measure the AC volts on the cathode in that case. Finish out with the AC voltage at the junction of the two 220k resistors. Report those voltages.

[tubeswell]

Without the thing sitting in front of me, I’m just going to take some stabs in the dark.


Bad tube in that channel.


Dickey wiring behind the board- take a peek with dentistry mirror and a flashlight and/or test for unwanted dc-continuity



Leakey coupling cap(s) in that channel (don’t just shotgun them all.  Test for dc-leakage with your V-meter)

[bullkelp]

heya

I dont have a signal generator but presmably a single note keyboard loop on a pedal looper with a volume output will work ?

@tubeswell hi!
The amp is a turret strip job. Ill post some photos. The bass end that ive removed and rebuilt is just rebuilt with parts on hand, looks shonky but wanted to test it before getting new parts. The pot in the rear is a the mid pot.

Ive tried a new tube.

[bullkelp]

Original layout
As you can see the bass channel just has two triodes unlike the other blackface bassman builds that seem to have more gain stages there

[bullkelp]

Test circuit installed. Maybe looks goofy. Couldnt see a better way to do the voltage devider off the volume wiper (2 x 470k resitors) so I put one at the valve pin and ran another back to ground.

[bullkelp]

some better photos

Note i havent added the bright switch

[tubeswell]

Measure the coupling caps for DC-leakage. (Leaky coupling caps affect bias on the following stage, killing the volume).

When you do this, you should notice voltage changing across each cap when you power the amp up, but this should settle back to 0VDC when the amp stabilises to idle. If there is any voltage at that point, the cap will be bad.

Use insulated test clips and don’t put your hands in there with the power on.

Make sure you lift the lead on the ‘ac-side’ of each cap out of the circuit before testing each one to get an accurate measurement. (You’re looking for any dc that crosses the cap from the preceding plate).

[tubeswell]

Seeing as it’s a new build, another possibility is an incorrect value on a resistor that is resulting in the signal being loaded down too much or bias being too hot or too cold etc (depending on what job the resistor is supposed to be doing).

[bullkelp]

It’s a new build so far as I’m modding this channel
The amp , both channels , working fine previously but kind of similar topology so set about changing this channel

I’ll go thru the resistors and other values again for the umpteenth time.

 Would the bass pot value matter ? I think it might not be 1 meg . ?250k. I assume that would just change the bass frequency not the output etc.

[bullkelp]

OK!!!

Its working and sounds amazing!

Who can guess what I f'd up?

Ill tell ya

820k resistor to ground instead of 820ohm   DURGHHHHH
Pin 3 of the third triode

ill try and post some clips later on

Thanks!

im so dumb!