Bassman AB165, bass control is hardly working

[1ampman]

On the bass input channel, the bass control makes little difference. How could I get more boost from this control? I'm not using the deep switch. It's a stock circuit now. The normal channel is a little better, but still not a big change. I've tried a couple things with no change...

[pdf64]

http://bmamps.com/Schematics/fender/...b165_schem.pdf
Is the problem the same at different volume levels, eg the same at low volumes as at high?

[1ampman]

Just went a checked the amp. No difference at low or high volume setting. It has a 250K pot with an 8.2 resistor going to grd. Would the slope resistor make the pots more active? Or a 1 meg bass pot be helpful..????

[pdf64]

How long have you had the amp - have the bass controls ever worked properly?
Certainly the tone stack in the normal channel is a classic, its controls are very effective.
Are the controls for both channels similarly affected?
If so, they may be a common issue, eg HT cap failed.

[Justin Thomas]

I can tell you - both bass controls should be VERY effective at changing the Bass content. I've got one of these, stock except no 220k resistors on the 6L6GCs and a 100W Twin Reverb OT.

Though I'm guessing it's a typo, do you mean 8.2<k> going to ground? And does what's there match the schematic? Has it been nodded? These things are also kinda picky about lead dress, even though they're maybe blackface w. cloth wire...

Justin

[frus]

Check the pot value
A guy just brought me an ancient London City amp with bass pot "not working", it measured 20M instead of 1M

[1ampman]

thx for the reply's.... It turned out to be the .047 cap in the tone stack. I wasted a lot of time on this one... Geesh!

[pdf64]

Pleased you've fixed it!
Can you expand on the issue - what was wrong with the cap, eg shorted, open circuit?
The bass channel tone stack doesn't look to have a 0.047uF cap, so was this just the normal channel then?

[Philington]

https://cdn.shopify.com/s/files/1/23...97210715640943
https://cdn.shopify.com/s/files/1/23...97210715640943
I know this thread is almost 4 years old now, but I am pretty much having the same problem described by 1ampman. The bass control on the bass channel of my Bassman AB165 has no effect whatsoever - at any volume. I bought it used about 5 years ago - not sure if it ever worked and the amp does not appear to be modded. The treble control and deep switch work fine on the bass channel and all the control (including bass) works fine on the normal channel.

The bass pot on the bass channel seems to test fine.

I'm confused with the mention of the .047uf cap in the tone stack (as was pdf64) in the previous post. Doesn't the bass channel tone stack have three .1UF caps - two running off the bass pot and one connected to the deep switch?

[IMG][/IMG]

Sorry, I'm an amp nube - these are tone caps, right?
From what I've read these caps (ajax blue molded) seldom if ever go bad and there are no visible defects. I don't have any way to check the caps. However, would one or both of the caps running off the bass pot likely be the culprit?

[Justin Thomas]

It is possible that the OP's amp's Bass channel was modded at some point.

As to your "culprit," yes, if they were the culprit above they could be here. If you don't have a way to check them, it would only take a few minures to swap them out. You could use a cap from your Normal channel if you don't have spares. .1, .047, either will work for testing. Like I said above, either Bass control should be MORE than obvious if working correctly. You couod also have a bad solder joint or broken wire.

Justin

[Justin Thomas]

I didn't see the picture earloer, so yes, those are the tone caps. They may also be called coupling caps or blocking caps, depending on who you're talking to. Tone caps is the best description of that particular set, though, in my opinion.

So to "test" them you could swap a cap out of the normal channel one at a time for the 3 .1s in the Bass channel. The channels are completely independent so you won't hurt anything. Just make sure you can do all of this safely.

Read up at grofex.com's tube amp debug page about SAFETY. Your life may depend on it, if you truly are a noob. Don't be afraid, but always have respect. If you have any questions, this is one of the best places to learn.

Justin

[Philington]

I do know how to drain the E caps (I have a discharge probe) - and I'll be very careful.
Thank you very much for your assistance, Justin!
(Phil Engstrom - Burlingame, California)

[Philington]

No luck I'm afraid.
Swapped out all three capacitors and the same issue exists with the bass channel bass control. Frustrating. I can't find any reference on the internet for this issue (bassman or any other tube amp!).
Again, the bass pot seems to test fine. And there is continuity between the pot and these tone/coupling caps (no broken or shorted wires).
I'm at a loss - is there anything else to try before I take it to an amp tech (which is going to be expensive where I live)? It doesn't look like any of the caps have been overhauled replaced (including the electrolytic caps on the board or in the dog house).

[Justin Thomas]

Since the issue is limited to such a small section of circuit, it shouldn't be anything common to both channels. I say "shouldn't" because anything is possible. Try swapping the pot anyway. I've had a few that "test" fine but still are somehow "broken." Check the wiring from the plate of the tube to the cap. Get a layout & check for under-board wiring.

Others here will probably have more definitive advice; I just think that if you stick it out before taking it in, you'll get it & learn in the process. Don't take it in until you are going absolutely bonkers & everyone here is out of ideas.

Justin

[g1]

Make sure any board to chassis ground wires in that area are good. A resistance check from stack to chassis should do it.