AC568 Bassman - What is this mod?

[LarryTurtis]

I recently acquired a '69 Bassman, no drip edge. Seems pretty clearly an AC568 circuit despite the AB165 label. There appears to have been some modding. I was hoping someone could help me figure out some of the mysteries here.

There are two aftermarket pots on the back of the amp. One is a 10k pot that dials into the negative feedback circuit. Ok, that makes sense, maybe. The other is a 5k pot connected in series with the original bass pot on the front panel for the bass channel. Any idea why someone might add this? Some kind of coarse / fine control?

Another mystery-- there appears to be a solid-state rectifier and some big caps that were added to the heater circuit. Is that a common mod? Some sort of tube-life extender?

Finally, the amp has a not insignificant amount of hum when powered on, even with no external speaker connected. Is that normal for these heads? I have a '68 Princeton reissue and it has a little tube hum but you have to put your ear close to hear it. The Bassman I can hear clearly from 5 feet away. I checked and I *think* the filter caps were replaced. Would love if someone could confirm that as well.

Sorry for all the newb questions. I'm more of a synth / organ tech, really.

 

[corliss1]

Welcome to the forum!

Draw out on the schematic where it goes - you'll likely find it's a mid control.

The work in that amp looks a bit sketchy to me. I'd be tempted to just take it all back to stock.

 

[Lowerleftcoast]

I'd say a mid pot. Chances are the other is a variable NFB or a presence pot. The Bridge rectifier and capacitors looks like they have made a DC supply for some or all of the heater filaments.

 

[Lowerleftcoast]

The blue caps in the dog house will have a date code. Most likely 2 digits for year and 2 digits for week. All of the caps in the dog house have been replaced along with the dropping resistors.

Never run the amp without a speaker plugged in. If the transformer itself is buzzing sometimes tightening the bolts can eliminate the buzz.

 

[Peegoo]

@LarryTurtis The caps added to the heater circuit were probably an attempt to cut down on the noise after the first mods (mid control pot and NFB control pot) were done.

The noise is probably due to poor lead dress--which is where the wires to those added pots are routed over the other components. Return that amp to stock!

No need to apologize for "newb" questions. We all learned this stuff the same way

Welcome to the Crazyhouse.

 

[David Barnett]

In theory DC heaters, at least for the small tubes, should make the amp quieter. But at that voltage and current level, it takes a lot of capacitance to get it smooth enough. If the amp is noisy, it would seem the mod isn't helping, so that would be the first thing I'd remove and put back to stock.

 

[LarryTurtis]

Thanks all! Very helpful. Now, back to stock or back to AA864?

I have a couple projects ahead of this one but will likely have more questions soon.

 

[dogmeat]

The caps added to the heater circuit were probably an attempt to cut down on the noise after the first mods

I wondered what those were doing there. the one looks to be roached

 

[David Barnett]

Thanks all! Very helpful. Now, back to stock or back to AA864?

I'd start with the path of least resistance, put it back to the way it was made. After you get that working, then you can decide if you'd prefer to back-date it.

 

[Peegoo]

Finally, the amp has a not insignificant amount of hum when powered on, even with no external speaker connected. Is that normal for these heads? I have a '68 Princeton reissue and it has a little tube hum but you have to put your ear close to hear it. The Bassman I can hear clearly from 5 feet away. I checked and I *think* the filter caps were replaced. Would love if someone could confirm that as well.

Older power transformers often are noisy on their own because they loosen up over the many years of heat/cool cycles and the components can vibrate, making the PT hum or even buzz.

One way to fix this is to manually "pot" the laminations in an oil-based polyurethane varnish (Varathane, etc.), that you can get at your local True Value Hardware or even Wal Mart. If you examine transformers, you'll see most already have a varnish on them that's applied at the factory. Some look like they were dipped in honey. This varnish can, over time, break down and fail.

This is a similar process to potting guitar pickups to turn the coil wires and magnets into a monolithic structure to prevent oscillation and feedback. For pickups, plain paraffin wax is used. But for transformers that get hot in use, wax es no bueno.

Oil-based poly is the way to go. Don't use water-based stuff because it can activate any rust in the laminations and make the problem worse.

Be very gentle with the wires running from the transformer because the insulation on these old beasts is often crumbly; try not to flex the wires too much.

Terry shows you how:

 

[Paul G.]

I would return the amp to stock, then re-evaluate. I don't understand willy-nilly mods.

 

[Jasonpatrick]

Thanks all! Very helpful. Now, back to stock or back to AA864?

I have a couple projects ahead of this one but will likely have more questions soon.

Ab165 you mean which is closer the the original circuit since the preamp has an addition gain stage on the normal channel that the aa864 doesn’t have.