Polytone Baby Brute - Low volume hum

[Nohhyuk]

Hi everyone,
I purchased a diamond tolex Polytone Baby Brute amp used about a year ago. It works perfectly except for a low volume hum. It’s only bothersome to me in my bedroom when it’s quiet. I don’t notice it when playing through it with others or ambient noise is loud enough.
I’ve taken it to two separate amp techs and they changed the caps and put more metal shielding in the amp, but that made almost no difference.
The hum is not changed by turning up the volume. It is always there even with no guitar plugged in. The reverb doesn’t affect it (hum still there with reverb all the way off).

Does anyone else have this hum with their Baby Brutes? I have a Mega Brute that doesn’t have this hum.

Does anyone have any ideas what next to try to eliminate this hum? I know it’s a tight space, especially in a Baby Brute. I’m wondering if this is common.

Thank you,
Ed

[Tal_175]

My mini brute from a similar era (with felt cover) also has hum with nothing plugged in and volume turned down. Of course every amp has some hum and hiss but my polytone's hum is louder than my other amps. It also picks up radio sometimes. I started a thread about it recently. I took it to a very competent technician (it also had another issue), he tried a bunch of things to fix the hum but he said the noise is probably due to the design. It's not shielded from interference well.

[Nohhyuk]

Thank you, Tal_175. That’s good to know!

[deacon Mark]

This is the problem with old Polytone amps. They were a great amp in there day and sounded generally very good. Problem now is much better stuff available that is much more reliable and consistent. I have given up my love for old Polytone amps. Repairing them is not easy when something goes wrong and it seems it does as age goes up.

[lammie200]

The problem may be that because the amp is so compact the reverb tank causes noise coupled with the power transformer. I am not totally familiar with Baby Brutes but if it has a reverb tank you might want to see what happens when you move the tank away from the transformers. If that helps you can try wrapping the tank in something like this product:

Amazon.com

I have a Polytone Mini Brute II. I haven't done the wrapping but I noticed that the proximity of the reverb tank to the transformers causes noise. I may do the wrapping at some point. Just spinning too may plates at the moment.

[vintagelove]

Well, if it makes you feel any better, mine's got a loud hum...


Interesting to hear all of ours have a hum (btw, mine is definitely higher than the average polytone hum). Interesting.


Anyone have an actual quiet diamond mini?

[Nohhyuk]

The techs didn’t think it was related to the reverb because the reverb didn’t affect the sound. I had read that sometimes grounding the reverb again to the chassis or turning the reverb tank might help. When the tech took the amp out of speaker cabinet, the hum went away. I’m guessing it was just a design error because it’s in such a small enclosure (even smaller than a Mini Brute). But, I’m curious if other Baby Brute owners have this same hum.

[lammie200]

The techs didn’t think it was related to the reverb because the reverb didn’t affect the sound. I had read that sometimes grounding the reverb again to the chassis or turning the reverb tank might help. When the tech took the amp out of speaker cabinet, the hum went away. I’m guessing it was just a design error because it’s in such a small enclosure (even smaller than a Mini Brute). But, I’m curious if other Baby Brute owners have this same hum.

If they took the amp out of the cabinet and the hum went away then the reverb tank was likely further away. I think that the tank is already grounded and there probably isn't enough room to turn it to get much relief from the hum. Try to move the tank away yourself and see what happens.

[Tal_175]

My polytone doesn't have reverb (or distortion) and has a larger cabinet than baby brutes but still has the hum.

[Nohhyuk]

I don’t think there’s any room to move the reverb tank, it’s so small inside. And why would this be a problem now, unless the hum has always been there before.

[vintagelove]

My polytone doesn't have reverb (or distortion) and has a larger cabinet than baby brutes but still has the hum.

Could be the main filter caps. My IV had an increased noise floor shortly before failure. After replacing those, it's pretty quiet. My baby, not so much. That bad boy goes "HHHUUUMMMMMM".


Fwiw, my noise I isolated to the preamp. When I unplugged the pre, the power amp was whisper quiet.

[Tal_175]

Could be the main filter caps. My IV had an increased noise floor shortly before failure. After replacing those, it's pretty quiet. My baby, not so much. That bad boy goes "HHHUUUMMMMMM".


Fwiw, my noise I isolated to the preamp. When I unplugged the pre, the power amp was whisper quiet.

The tech put additional filter caps. The noise in mine is also seems to be in the preamp. If I plug the preamp out to the effects return of my Quilter, I still get the noise.

[Nohhyuk]

Yes, when I do preamp out into my other amp, the hum comes out of the other amp as well.

[vintagelove]

The tech put additional filter caps. The noise in mine is also seems to be in the preamp. If I plug the preamp out to the effects return of my Quilter, I still get the noise.

Yes, when I do preamp out into my other amp, the hum comes out of the other amp as well.

I wonder if powering the preamp separately would do the trick. Perhaps it's a power supply issue?

[Nohhyuk]

Interestingly, the preamp-out-to-the-other-amp hum increases with increasing volume on the other amp. But, on the Polytone amp itself, the hum doesn’t increase with increasing the volume. Makes me think this is just how the amp is designed, and the hum is normal. ?

[vintagelove]

Interestingly, the preamp-out-to-the-other-amp hum increases with increasing volume on the other amp. But, on the Polytone amp itself, the hum doesn’t increase with increasing the volume. Makes me think this is just how the amp is designed, and the hum is normal. ?

I don't think it's normal. I have a couple other Polys that are quiet in comparison. What you just posted makes sense, because you're increasing the volume of the "power amp" (the other amp), therefore it would raise the noise floor. In the poly, the power amp is all the way up. So the fixed hum level in the pre wouldn't increase with volume.

[Nohhyuk]

I see. I hope other Baby Brute owners pipe in about their hum levels.

[Franz 1997]

I have an '82 diamond tolex Babybrute, but it has been modified by replacing the original power amp ( which proved unreliable) and reverb unit with a 200w class D power amp and a digital reverb circuit. The amp is completely quiet, you can hardly hear that it's on. This suggests to me that the low hum ( which the amp had originally) originates either in the original power amp, or in the reverb circuit via hum pickup through the reverb tank.

Also, any 40 year old amp that hasn't had the electrolytic caps replaced is going to be prone to hum anyway.

[Nohhyuk]

Did it have hum before all the mods? I just got a cap job, so that shouldn’t be the problem.
I took the back off and pushed around with a chopstick. I get a lot more hum with the back off. So, the shielding of the back panel makes a big difference.
When the chopstick is near the input jacks, I can make the hum louder (without touching anything).

[Franz 1997]

Did it have hum before all the mods? I just got a cap job, so that shouldn’t be the problem.
I took the back off and pushed around with a chopstick. I get a lot more hum with the back off. So, the shielding of the back panel makes a big difference.
When the chopstick is near the input jacks, I can make the hum louder (without touching anything).

Yes it hummed before, but not unusually so; most conventional ( ie not class D ) SS amps hum slightly. It wasn't too intrusive.
Anything near the input jacks can increase hum, that's normal. Also, removing shielding can do the same. Bear in mind I'm in the UK, where
a/c grounding is compulsory, so the problem is very rarely to do with the a/c power supply.

[Nohhyuk]

Any other Baby Brute owners want to let me know how much theirs hums?

[Nohhyuk]

I don’t know much about amps, but it seems to me that the input jack has been replaced in the past. This green wire looks suspicious. Do you think it’s possible an incorrect wiring of the jack cause my hum problem? Maybe, it didn’t get all connected properly, causing a grounding problem? The bare stripped part of the green wire might need to be connected, or the long wires need to be with better lead dressing?

[Herbie]

I don’t know much about amps, but it seems to me that the input jack has been replaced in the past. This green wire looks suspicious. Do you think it’s possible an incorrect wiring of the jack cause my hum problem? Maybe, it didn’t get all connected properly, causing a grounding problem? The bare stripped part of the green wire might need to be connected, or the long wires need to be with better lead dressing?

Congrats for having a legendary piece of jazz amplification!

I can’t see where the green lead goes to nor from. The ’stripped’ part seems like the original lead was too short and someone soldered a bit more to it to make it longer.

I have had 4-5 Polytones and some hummed more than others. Once I killed the hum by soldering a grounding lead to the reverb tank. Once I just turned the tank 180 degrees around.

Increased hum without back panel hints that it might be grounding problem. I believe that the back panel connects the ground of preamp and power amp?

Mostly the hum of Brutes comes from the tightness of the design, the reverb tank is too close to the power amp and supply and its transformers. The modernification mod that Franz1997 have done with his Brute would be a good idea for many of these!

[Nohhyuk]

Congrats for having a legendary piece of jazz amplification!

I can’t see where the green lead goes to nor from. The ’stripped’ part seems like the original lead was too short and someone soldered a bit more to it to make it longer.

I have had 4-5 Polytones and some hummed more than others. Once I killed the hum by soldering a grounding lead to the reverb tank. Once I just turned the tank 180 degrees around.

Increased hum without back panel hints that it might be grounding problem. I believe that the back panel connects the ground of preamp and power amp?

Mostly the hum of Brutes comes from the tightness of the design, the reverb tank is too close to the power amp and supply and its transformers. The modernification mod that Franz1997 have done with his Brute would be a good idea for many of these!

Thank you, Herbie. I’m going to take it back in one last time to see if the Jack needs to be rewired or the grounding of the preamp/power amp needs to be redone. I can deal with the hum, but it would be nice if it could be fixed.

[KirkP]

Interestingly, the preamp-out-to-the-other-amp hum increases with increasing volume on the other amp. But, on the Polytone amp itself, the hum doesn’t increase with increasing the volume. Makes me think this is just how the amp is designed, and the hum is normal. ?

That suggests that the hum is not entering the signal before the volume pot. If I have the correct schematic, this is the only part of the signal path between the volume pot and preamp output. It’s just an single op amp, but there are connections to the drive circuitry and reverb that could be the source of the hum. If I were diagnosing I might try temporarily disconnecting at each connection, one at a time. If you find the hum is coming from the drive circuit or reverb you could leave them permanently disabled. Use pedals for those features if you need them.