Polytone Baby Brute Reverb help on repairs

[tjmicsak]

I just recently bought a nice diamond tolex (looks like leather) Baby Brute with original speaker and code of 1980 something...
Amp works well but Im already in process of recapping all electrolytic caps in both the pre and pwr amps.
The only problem is with a very weak reverb. Once the caps arrive and I have deoxit all in hand the work will also include a full cleaning of all pin and RCA connection to give this gem a new lease on life.
My question is if this amp had any previous attempts to fix the reverb, how can I confirm it has the original tank?
This model has the reverb mounted top underside to top right behind the preamp so it is horizontal. The yellow fiberglass is all cut to fit so I assume this is original but can I measure the AMPs reverb and return RCA cable pins to get the relative impedance that should match the tanks send and returns on the RCA plugs? I want an option to verify what specs the tank requirements are AT THE AMP in the off chance I might need a new tank and if at the same time the tank in there is not a "good" number....

[jorgemg1984]

@Franz1997 is someone who I believe can help you, if he doesn't reply here, trying sending a PM.

[JazzNote]

My question is if this amp had any previous attempts to fix the reverb, how can I confirm it has the original tank?

My experience is that the original tanks in Polytone amps are not of best sound quality anyways. If you are unhappy with the sound it might be worth to try a 3 spring version accutronics, they come fairly cheap. https://www.stewmac.com/electronics/...s-reverb-tank/

[Herbie]

I just recently bought a nice diamond tolex (looks like leather) Baby Brute with original speaker and code of 1980 something...
Amp works well but Im already in process of recapping all electrolytic caps in both the pre and pwr amps.
The only problem is with a very weak reverb. Once the caps arrive and I have deoxit all in hand the work will also include a full cleaning of all pin and RCA connection to give this gem a new lease on life.
My question is if this amp had any previous attempts to fix the reverb, how can I confirm it has the original tank?
This model has the reverb mounted top underside to top right behind the preamp so it is horizontal. The yellow fiberglass is all cut to fit so I assume this is original but can I measure the AMPs reverb and return RCA cable pins to get the relative impedance that should match the tanks send and returns on the RCA plugs? I want an option to verify what specs the tank requirements are AT THE AMP in the off chance I might need a new tank and if at the same time the tank in there is not a "good" number....

Congrats for a great amp!

About ten years ago here was discussion about the cap job and one of The Polytone Experts Greentone said:

”Are you sure that the caps need replacing? It is routine to swap caps in a tube amp after 5-10 years (the 'lytics, anyway). In a solid-state amp, however, the power supply doesn't take the brutal beating that it does in a tube amp. The voltages being demanded by the finals in a tube amp are in excess of 400 volts. The electrolytic capacitors are frequently being operated right at their practical limits in tube amp power supplies. This is not the case in solid state rigs. Honestly, I haven't had caps malfunction even in quite old solid state amps.

If you are just replacing capacitors to freshen up the amplifier, or to make it more reliable...you might consider that many of us are operating old, to very old Polytones on their original capacitors without problem. I would simply never do this with a tube amp out of fear of wiping out the transformers--this gets expensive.”

Electrolytic capacitors in polytone babybrute

If You use Google You find also tons of information about the reverb tanks here in Jazzguitar.be. The forum’s own Search function does not always work (never?).

[bluenote61]

Some time ago I had a Polytone on my workbench which had a 'weak' reverb. There was nearly no reverb audible even at full reverb volume.
When I opened the cab I had first to pull out the reverb tank out of that mass of fiberglass which covered all walls inside the cab.

The original tank is a accutronic tank. On older accutronic tanks you may find a ink stamped date code on the housing which would allow you to see if that date corresponds with the date of the amp.

Check the reverb tank if the inner spring box is correctly fixed inside the tank. If not this may cause a malfunction of the reverb by blocked reverb springs. That was the root cause for the reverb issue of that Polytone I had for repair.
Also, I noticed that the tank code stamped on the housing - which indicates also the correct mounting position of the tank - did not match with the in situ position of the tank as originally installed by Polytone. If the amp will be pushed hard during handling that may cause the inner spring box bouncing and jumping out of the spring box lashing.

[Herbie]

Some time ago I had a Polytone on my workbench which had a 'weak' reverb. There was nearly no reverb audible even at full reverb volume.
When I opened the cab I had first to pull out the reverb tank out of that mass of fiberglass which covered all walls inside the cab.

The original tank is a accutronic tank. On older accutronic tanks you may find a ink stamped date code on the housing which would allow you to see if that date corresponds with the date of the amp.

Check the reverb tank if the inner spring box is correctly fixed inside the tank. If not this may cause a malfunction of the reverb by blocked reverb springs. That was the root cause for the reverb issue of that Polytone I had for repair.
Also, I noticed that the tank code stamped on the housing - which indicates also the correct mounting position of the tank - did not match with the in situ position of the tank as originally installed by Polytone. If the amp will be pushed hard during handling that may cause the inner spring box bouncing and jumping out of the spring box lashing.

My MBIV (felt era) had a harsh reverb when I bought it, at first I thought that it was very early digital reverb.

Then I opened the back and found out that it was a tank reverb and one of the tank's three springs was loosened from the other end. Somehow I managed to attach it back and the reverb sound got a bit better. But I use it only a bit, so enough for me.

[Franz 1997]

There are some useful comments above. Poly reverb tanks are notorious for breaking, so replacements are very common. The diamond-tolex era baby brute schematics don't give the reverb tank number, although it is obviously low impedance. So without this information it's difficult to know if the tank you have is original, unless any BB owner on the forum has had the same rev tank from new, and can give you the accutronics code .Considering that's over 40 years ago, it's unlikely.
If it's any use, i have replaced a friend's broken Polytone reverb tank with a generic low impedance tank, and it worked as well as the original. As pointed out above, broken springs and misplaced insulating foam can also cause problems, as well as corroded phono connector plugs, which can be easily cleaned. If you really want to calculate the optimum values for the tank by studying the driver and recovery circuits, the diamond-tolrex schematics are available on -line. However I'd try a generic low imp. tank first, after cleaning the input and output phone plugs to eliminate them as the problem; reverb is not Polytone's strong point and most jazz players use very little anyway. On my Baby-brute, I had the reverb replaced (by a digital reverb circuit) , along with the power amp. The amp has been completely reliable ever since.

[Herbie]

(…) On my Baby-brute, I had the reverb replaced (by a digital reverb circuit) , along with the power amp. The amp has been completely reliable ever since.

Interesting! Pics and schematics or it didn’t happen!

[tjmicsak]

Progress report.

Power amp filter caps replaced

All reverb RCA phono contacts and molex block pins/sockets were cleaned with deoxit red.

Tank was metered out at 33 ohm and 203 ohm inputs/outputs respectively which corresponds to accutronics basic DCR values relative to the impedance so the tank is good, springs all intact.

While the reverb is weak, I am getting a good amount of increase in white noise/noise floor when the reverb knob is turned up but the effect itself is not ramping up in proportion so Im getting the impression the issue is on the send signal. Another indication is if the amp springs are bumped they splash with much higher volume. Next try will be replacing the 8 preamp electrolytics, one being the final component coupler cap on the reverb send.
Reverb did get a bit better with all the contact cleaning so I hope the cap might restore it the rest of the way.

[tjmicsak]

One interesting thing I noted when looking up the tank specs on the Accutronics site-

My tank is stamped 1DB2C1C.

This tank in this amp model is mounted horizontal open side up, but the number indicates it is meant to be mounted vertical connectors side up. (?) so either Polytone used a poor choice of tank or this is actually an incorrect replacement.

[bluenote61]

One interesting thing I noted when looking up the tank specs on the Accutronics site-

My tank is stamped 1DB2C1C.

This tank in this amp model is mounted horizontal open side up, but the number indicates it is meant to be mounted vertical connectors side up. (?) so either Polytone used a poor choice of tank or this is actually an incorrect replacement.

I made the same observation (incorrect placement) in the Polytone I had for repair. It's also mentioned in my post #5.
Hence, the possibility is high that it's the original tank.

[tjmicsak]

As soon as the remaining preamp electrolytics arrive Im going to replace that last one on the send circuit to see if an old cap is choking it off input side. As I mentioned I get plenty of ramping up when I turn up the reverb knob but the noise floor ramping (background white noise) is far greater than the reverberated signal itself indicating it is something on the send side and the output is starving trying at its best to amplify a weak input. Spring splash is also up there near in volume to dry signal. If cap doesnt solve it Ill go for new tank.

[Durban]

My original Baby Brute has Accutronics 1DB2C1C reverb tank

they can can replaced ( so my electronics/valve guy said ) with other Tanks it is not critical, but delay/decay time varies with longer springs,

the reberb tank being very close to the amps/power can sometimes hum, you can shield them with tin foil i shall pull mine out and take the back off and peek.............................................. hopefully tommorow,


Baby Brutes sound amazing with archtops,

[tjmicsak]

All caps replaced and amp/reverb is working superbly now!!! I'm guessing that the 10uf/35V in the reverb driver was weak.



Just a quick note that the 5 1uf/50V electrolytics in the preamp while looking like typical polarized axial electrolytics are in fact non-polar ("NP") so that is important to be aware of...you don't want to use polarized caps as these NP work in both directions. One might also contemplate then just simply using regular Poly caps here, but the electrolytics will have a different ESR value (effective internal series resistance value) to be maintained to keep the amp circuit intact. The same holds true for matching original voltage ratings as well as capacitance.

[Hammertone]

I have found this to be the most effective way to repair the reverb in my Baby Brute.

[Hammertone]

This also works quite nicely.

[tjmicsak]

Thats a good backup. I initially have a Wampler Faux little green mini on hand but really wanted to get the amp "restored" at least to like new as far as the reverb. It still has a high white noise floor when up past about 60% but the strength of the effect now is enough to where I only need maybe 40% on the dial. These reverbs were never perfect or in the Fender camp by nature with a short length anyway but now it can be an all in one carry if desired, which is what I was aiming for. Mission accomplished.

[voyage]

There are some useful comments above. Poly reverb tanks are notorious for breaking, so replacements are very common. The diamond-tolex era baby brute schematics don't give the reverb tank number, although it is obviously low impedance. So without this information it's difficult to know if the tank you have is original, unless any BB owner on the forum has had the same rev tank from new, and can give you the accutronics code .Considering that's over 40 years ago, it's unlikely.
If it's any use, i have replaced a friend's broken Polytone reverb tank with a generic low impedance tank, and it worked as well as the original. As pointed out above, broken springs and misplaced insulating foam can also cause problems, as well as corroded phono connector plugs, which can be easily cleaned. If you really want to calculate the optimum values for the tank by studying the driver and recovery circuits, the diamond-tolrex schematics are available on -line. However I'd try a generic low imp. tank first, after cleaning the input and output phone plugs to eliminate them as the problem; reverb is not Polytone's strong point and most jazz players use very little anyway. On my Baby-brute, I had the reverb replaced (by a digital reverb circuit) , along with the power amp. The amp has been completely reliable ever since.

The accutronics linked earlier in this thread is a 16 inch long with 8 ohm impedance. Would that qualify as low of impedance as you describe? Thank you

[raylinds]

I recently bought a Mega Brute II for $350 that arrived with the reverb DOA. I was planning on getting a pedal as I don't like the Polytone 'verbs, but I got a $50 rebate from the seller. I bought a used SurfyBear Compact Spring Reverb and love it!