Polytone mini brute II

[glebert]

It would be good to see if the problem is preamp or poweramp. If it has an effects loop you can plug an guitar straight into the return jack and see if you still get distortion (that would be power amp). You can run the send jack to something like a mixing board to see if the distortion is in the preamp.

[J M Fahey]

You should scope it, only way to confirm what kind of distortion is it: crossover, oscillation, whatever and also whether only amp out is distorted, so amp problem, or it already comes dirty from preamp, in which case we will find where it originates.

Shotgunning the amp is a waste of time.

[52 Bill]

My first guess would be that the problem is in the power amp. The preamp tends to either work or not. I can't remember too many of them that had distortion problems. The power amps have lots of problems.

Which version of the power amp do you have? There were two around at that time one was all discrete transistors and the other used the LM391 driver chip.

[pipposalsiccia]

Hello everybody
It took time but a smart technician had fixed it quickly.
The problem was:
crossover distortion, microphonic brite normal dark switch, something else with power circuit (heat and drivers).
Now it plays much better and it has a better sustain and dynamic.
speaker is ok, caps ok.
Last thing, now I have to replace the reverb unit.
Which is the right replacement model of teeny brute 2?
Any suggestion about replacement operation?
cheers.

[glebert]

Do you have the original reverb tank? May have codes on it, otherwise you can measure the input and output resistance and figure out which replacement tank you would need. I don't know if Polytone was consistent with what tanks they used, seems like they were kind of loose on the design changes.

[J M Fahey]

And of course, if it works properly, leave as is.

Not much room for "improvement" since it already came with an American made Accutronics (Hammond) one; nowadays at best you´ll get a Korean made "Accutronics" ... really a Belton one (new company owners).

[pipposalsiccia]

And of course, if it works properly, leave as is.

Not much room for "improvement" since it already came with an American made Accutronics (Hammond) one; nowadays at best you´ll get a Korean made "Accutronics" ... really a Belton one (new company owners).

1db2c1c is the code.
the inside has been replacement by the previous owner, so i got only the tank as original.
It’s microphonic, and if it goes up to 3 l, hum is coming.
Not really a silent reverb.

[J M Fahey]

Mechanical reverbs are noisy, hummy, microphonic, by definition.

[52 Bill]

1db2c1c is the code.
the inside has been replacement by the previous owner, so i got only the tank as original.
It’s microphonic, and if it goes up to 3 l, hum is coming.
Not really a silent reverb.

Make sure that the tank is mounted with the output side as far away from the power transformer as possible and that it is covered with the original fiberglass insulation or vinyl wrap. The mounting points of the tank should have the rubber shock mounts and should not be screwed tight to allow for the tank to float.

One last thing is to be certain that the tank is plugged in correctly drive to the input, return to the output.

[pipposalsiccia]

Make sure that the tank is mounted with the output side as far away from the power transformer as possible and that it is covered with the original fiberglass insulation or vinyl wrap. The mounting points of the tank should have the rubber shock mounts and should not be screwed tight to allow for the tank to float.

One last thing is to be certain that the tank is plugged in correctly drive to the input, return to the output.

thanks.
the reverb tank is located upside, between the preamp circuit and speaker.
I just noticed today there was a noisy buzz when I was playing bass notes.
The preamp circuit buzzing like crazy, some foam sheet seems helps.

[pipposalsiccia]

Mechanical reverbs are noisy, hummy, microphonic, by definition.

yes they are crappy.
but I don’t really like digital reverbs.
so.. I will try to replace the unit and see if will improve or not.

[g1]

1db2c1c is the code.

You will have a very hard time finding that unless you come across a used one on ebay or something.
The commonly available equivalent would be 8DB2C1C

[pipposalsiccia]

You will have a very hard time finding that unless you come across a used one on ebay or something.
The commonly available equivalent would be 8DB2C1C

I think the horizontal version is 8DB2C1B

[pipposalsiccia]

another dilemma:
I found a belton 1BB2D1B which is very similar to original.
But i’ve read it’s an open side down.
the poly reverb is open side up

[J M Fahey]

Browse their catalog.

In principle basic same reverb springs are offered in two dozen different mounting and wiring configurations: open side up/down/to one side, and connectors grounded/floating/insulated/etc. plus all their permutations and combinations.

That said, one basic spring design will be as hummy/buzzy/boingy/microphonic as its exact electromechanical twin, even if made in a geographically different factory or in a different year, since all share the same basic blueprint and working principle.

Meaning: you won´t go from rusted iron to gold by replacing a 1979 spring with basically same, just made in 2018 .

The only difference a Twin Reverb reverb is not annoying while your is, is not "tank quality" but Twin one is driven balls to the wall with a properly impedance matched 2W RMS tube amp so *output* signal is huge, overpowering interference, and to boot is mounted inside a floating insulated bag at the cabinet bottom and some 24 inches or more away from the power transformer, while yours is bolted to chassis inside a shoebox and to boot is barely driven, if at all , but a cheesy flea power Op Amp, maybe getting 25 milliWatts of power.

Of course inaudible output signal can be brought up by a sensitive preamp. ..... together with any and all interference floating around.

Unless you improve that ... no tank change will solve your problem.