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I recently picked up a Polytone Mini Brute (model Baby Brute - Teeny Brute) that is wired for 110v. I wish to convert it to 220v. I have the schematic on how to do so (see link below), but I am having trouble locating the connector referred to in the directions. Anybody who has been thru this conversion process please help out! Thanks in advance.
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01-11-2015 06:33 AM
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Have you looked under the power amp circuit board?
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Trying to access under the circuit board looks like a complicated affair, and I don't think there is room for the 6-position wire clip described in the voltage conversion directions. The underside of the circuit board is boxed in. I would have thought the wire clip as described in the directions would be close to the transformer itself.
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There are 2 sections to the amp. You pic shows the preamp board, with the separate power amp in the bottom of the cab.
The power amp is in the bottom of the cabinet, and will contain the connector you need to locate. To get at it, you have to take out the vertically mounted pre-amp, showing in your pic, by unscrewing the 2 side bolts that hold it in place, and then disconnecting the 2 plastic wiring clips that are showing in your picture. You then remove the preamp, and can then remove the lower power amp from the cab by unscrewing 4 bolts on the bottom of the cab.
You may also need to remove the speaker which attaches to the front with 4 bolts, and the speaker wires.
From memory, the power amp board will be partly secured by the 2 large filter caps you can see- which are attached to the power board and also are secured with 2 compression clamps on the chassis. If you gently release the pressure on these clamps, you can ease the power board downwards and you should be able to see the connecting block referred to.
This sounds difficult, but it's not; it does take some time, so be prepared to work slowly and carefully.
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After posting this, I remembered that I have a UK polytone power amp in the workshop which is very likely to be the same era as your amp i.e. 78 onwards. I was wrong; the connector is in the power amp, as I said, but it's not under the board, it is at the side of the transformer. It is a light-coloured connector with 6 wires running into it, 3 from either side.
Notice that the wiring coding is NOT that given on the Murch Music site- maybe that coding refers to a later model.
This is the 378A power amp which was the power amp used on most if not all polytones with the red distortion knob.
On this model, one one side there are 2 green wires going into the outer positions on the connector, and a green/ black wire into the centre position. On the other side of the connector, there are 2 red wires going into the outer positions, and a blue wire going into the centre position. This works for 230 v operation.
Hope the pix below will show you how it's wired.
poly1.jpg
poly 2.jpgLast edited by Franz 1997; 01-12-2015 at 10:12 AM.
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sorry pix haven't loaded, haven't time to sort out why. PM me with yr email address and I'll send them.
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Franz, you are exactly correct with what I found once I pulled the amplifier apart. A connector wedged between the transformer and metal cabinet. If you could clue me in on what I need to re-wire in order achieve the 220v application I think I'm almost home on this one! My e-mail: fly_xc@yahoo.com
My connector looks just as yours does, one side is red-blue-red. Other side is green - green/black stripe - green.
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It looks as if yours is already wired for 230V; it looks the same as mine, which is 230V
Are you SURE yours is a 110v model?? Did you bring it in from the US??
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Franz, the amplifier came from Japan, which is of course 110v. I suspect that this amplifier does not have a "double wound" transformer that can be wired for 110 or 220v.
The amplifier works just great when I plug it into one of those voltage converters (220 down to 110v).
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I have just had a look through the 82 service manual and there's no mention of a double wound transformer, so I think your suspicions are correct, unfortunately. The 3 green wires indicate a centre-tapped, not double-wound, tranny - I checked mine with the meter.
bummer....
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Oh-well. My other option at this point is to install an additional transformer within the cabinet to convert the 220V to 110v. It adds additional weight but it's a better option than lugging around an external voltage converter.
In any case, thanks so much for taking the time to help me with the Polytone!
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You might find there's not a lot of spare space inside the cabinet - a step-down tranny needs to be well away from the preamp and reverb unit.
There are toroidal transformers like these:
VPT230-1090 - TRIAD MAGNETICS - TRANSFORMER, TOROIDAL, 2 X 115V, 250VA | Farnell element14
which weigh much less and hum less, but they are expensive. That could be packaged up as a light step-down unit.
A better option would be to find a UK poly mains transformer from that era.
There's yet another option, which I have taken on my 83 baby-brute. That involves ditching the poly power amp ( hence the old amp chassis I photo'd), installing an ALCO class D 180W power amp in the bottom, wiring the poly preamp to run off the new power amp's aux supply, and installing an 8" ( or 10") eminence beta speaker. That way you retain the sound and look, but have much increased headroom and punch, and reduced weight.
It all depends how much you want to spend. Strictly speaking, these mods are uneconomic, but since there is no modern equivalent to such a practical amp package, I felt it was worth it!
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That Triad transformer looks pretty cool. But as you said, a bit pricey. And I'm living in Cambodia at this time, so I'd have to deal with shipping which is always a hassle here.
Any major changes for the amplifier (i.e. changing the power amp) are beyond my skill set.
I think I'll just go for the step-down transformer option. I've converted about 10 amp's by this method, and so far, so good.
Can you tell me why the step-down transformer needs to be located away from the reverb and pre-amp?
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All mains transformers cause hum in preamp circuits if they get close enough, incl toroidal transformers. Also in reverb tanks, which why these tanks are usually positioned away from power amps. They can even affect speakers.
The early 60W version of the Hendriksen Jazzamp had a considerable hum caused by a mains tranny near the speaker; a simple relocation of an inch or so solved it.
The input side of the preamp is most sensitive ( obviously) so the tranny should go at the other side of the cabinet to the input.
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As you said, the Polytone has a very limited amount of free space to work with. I'll get around to installing the step-down transformer in the next couple of days, will let you know how it goes.
I have a Yamaha JX30 amplfier with an annoying hum- it also has a step-down transformer installed. I never thought of trying to re-locate the step-down transformer. I'll give that a try.
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Well I mounted a step-down transformer in the cabinet. Unfortunately the amp now has an unbearable hum/buzz, which is not present when plugged into 110v with no step-down transformer. Since it's such a small cabinet, of course there are few mounting options, all of which necessitate mounting the new transformer close to the pre-amp, the reverb box and speaker. Probably this is the source of the hum/buzz.
The step-down transformer doesn't have any markings on it, but it sure looks large enough to supply the necessary wattage.
Not sure what I'll do at this point. Maybe resign myself to taking the amp back to the the USA and using it there.
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Just a PS to this slightly zombie thread, in case anyone is facing the same issue in the future...
Early 115 v, 80s red-knob/ diamond covering polytones (378, 378A and 378B) CAN be converted to 230v, and vice versa. They do have double tapped transformers. However, the diagram on the Murch music site probably refers to later models, so you won't find the 6 pin connector they refer to, nor the same wiring codes, on the 80s red-knob models.
The wires that need changing are on the 8-pin, left-hand plastic connector that plugs into the underside of the pre-amp. The mains transformer connections are on the second pair of connectors from the front, looking from the back of the amp.
The black/white and white wires must be removed from this connector, soldered together, securely heat-shrinked, and tucked into a vacant space on the connector, not into a connector pinhole as the Murch diagram suggests. The red/white and black wires stay in place. This will convert the amp to 230v ac operation.
There are variances in the other wire codes even between the 378 and 378A models, but the transformer wiring codes stay the same.
Ok, back to L5s or big band amps or whatever's happening......my good deed for the day.
PS Polytone amps sometimes get a bad rap for construction, but the more I look at them, the more I think they are nicely designed and well thought out.
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Bumping this antiquated thread. And sorry for calling you out of blue for this one, Franz.
Originally Posted by Franz 1997
My knowledge and skill in amps are pretty much non-existing so I'm a little confused and mostly worried that things might go bad.
If you still have the Polytone, can you educate me on this little more, please? (possibly with photos?)
Mine is an 8" Baby-Brute from the 80s and, to my knowledge, it has the 378 power amp, too.
Or if anyone has successfully done this conversion, please help me!
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