There's so little inside a Polytone Mini Brute

**GuyBoden** · 06-04-2015, 12:30 PM

There's so little inside a Polytone Minbrute. Mini indeed..

Preamp

Power Amp

**Franz 1997** · 06-04-2015, 02:26 PM

so little, i can't even see it..

**Greentone** · 06-04-2015, 03:56 PM

Are you just looking at the preamp? In the bottom of the cabinet there is a power amp. The power amp is a pretty standard, well-designed amplifier. The preamplifier (in the top) is also a fairly standard and well-designed circuit. The entire circuit was modeled after a circuit that first appeared in Popular Electronics back in the 1960s. I think it may have been a monophonic version of the "Tiger" amplifier circuit that appeared in 1967.

Polytone amplifiers have always been quite well made amps. They were designed for the professional market. If they had an Achilles heel, it would be the Molex connectors used to couple the preamp and power amp circuits. Molex connectors are notorious for corroding when they get even slightly damp--humidity is enough to make them become intermittent. Secondly, the reverb tanks on the Polytones can get damaged--their isn't much isolation between the tank and the cabinet. The tumbling that a Polytone can take in the trunk of a car is enough to make _some_ reverbs go south. (Mine have always worked, though.) Finally, some of the early, first-generation ICs that Polytone used in the preamp circuit have been known to let moisture into the circuits through the legs. Some of those ICs have begun to fail. Not all of them, by any means, but some of the amps that were stored in basements or other damp environments have become unreliable. Too bad, because the IC--popular in the 70s--is now long out of production. It is a simple op-amp circuit. It can be substituted, but not with a direct, pin-to-pin replacement. What has to happen is that you make what is called a "dead bug" repair. You epoxy a smaller op amp, upside down, on top of an IC plug in adapter that has the right number of pins as the original IC. Then you make small wire connections from the new op amp legs to the correct legs of the IC adapter. Plug it into the preamp and you are back into business. The whole mess looks like a dead bug, hence the name. It is ugly, but it is a viable fix. (Some people complain that the new ICs aren't as gritty and noisy as the old ones.)

You will note when you look inside of a Polytone cabinet that the entire thing is well baffled with fiberglas. Try not to get that on your skin or breath any loose fibers. It's not asbestos, to be sure, but we don't really know what loose glass fibers do to your innards.

Meanwhile, enjoy your Polytone amplifiers. They are great amplifiers.
There's so little inside a Polytone Mini Brute-polytone-mini-brute-jpg

There's so little inside a Polytone Mini Brute-polytone-mini-brute-jpg

**Franz 1997** · 06-04-2015, 04:01 PM

I can see a power amp now...

**Greentone** · 06-04-2015, 04:02 PM

Originally Posted by GuyBoden

There's so little inside a Polytone Minbrute Preamp. Mini indeed..

That's the underside of the Polytone Mini Brute _power amp_. You are correct...there are not that many discrete components in it. There are about as many as there are in a Fender Twin Reverb power amp, minus an output transformer. Everything is miniaturized, of course, because of the use of transistors instead of tubes.

**Longways to Go** · 06-04-2015, 04:05 PM

I wish somebody would recreate the classic polytone like they do with all the boutique Fender & Marshall knockoffs. It would simplify the tone search for lots of jazz players . . .

**Greentone** · 06-04-2015, 04:10 PM

I have given this a lot of thought. I used to make boutique Fender and Marshall knockoffs. I have been thinking about resurrecting a "Polytone-ish" amp. Almost certainly I will make myself one or two, just to see how they will sound. Everything looks very straight ahead, to me.

I believe I will begin with the Baby Brute--my favorite--then build a "II." That was always a popular amp. OTOH, I think an amp with a cabinet the size of the III/IV, but a 12" driver might sound really great.

**GuyBoden** · 06-04-2015, 04:26 PM

I've been looking at this part of the Polytone's Tone Stack/Preamp (See below), and thinking that maybe I could put it into a pedal. Then plug the pedal into my power amp.

There's so little inside a Polytone Mini Brute-poly_tone_mini_brute_tone_stack-jpg

There's so little inside a Polytone Mini Brute-poly_tone_mini_brute_tone_stack-jpg

**Longways to Go** · 06-04-2015, 04:33 PM

I've got a diamond tolex 1x12 minibrute -- very nice tone on many setttings.
Only thing lacking for my tonal bliss is reverb!

I would love a baby brute.
A newly designed baby brute with "the tone" plus line out & digital reverb would be ideal IMO.
I'd buy one

**vintagelove** · 06-04-2015, 04:44 PM

Originally Posted by GuyBoden

I've been looking at this part of the Polytone's Tone Stack/Preamp (See below), and thinking that maybe I could put it into a pedal. Then plug the pedal into my power amp.

There's so little inside a Polytone Mini Brute-poly_tone_mini_brute_tone_stack-jpg

I have actually been trying to do this for a while. I am having trouble finding a builder to go through with the project.

**GuyBoden** · 06-04-2015, 05:08 PM

Originally Posted by vintagelove

I have actually been trying to do this for a while. I am having trouble finding a builder to go through with the project.

In my opinion, I don't think it would be a difficult task for a good Pedal builder to build the Polytone's ToneStack/preamp into a pedal, if you gave them the schematic:

Polytone schematics.
POLYTONE SERVICE INFO

**GuyBoden** · 06-04-2015, 05:15 PM

I'll be building this in a few weeks, I'm interested in the sound of Baxandall tone stack and using the RC4558 opamp for a preamp, I will keep you posted.

**jorgemg1984** · 06-04-2015, 07:17 PM

An old forum member, real eletronics wizard, gave me his Polytone pedal project files and a friend of mine is building me it. It's based on the old pre-sonic circuit Polytones and I went with OPA2134 op amps - I will post when I have it. I would love to share the files but I don't have his permission and he's not replying private messages any more!

**jorgemg1984** · 06-04-2015, 07:18 PM

Originally Posted by vintagelove

I have actually been trying to do this for a while. I am having trouble finding a builder to go through with the project.

If I get permission from the forum member I can give you the files and / or the contact of a Portuguese guy who will do it and ship anywhere in the world.

**Longways to Go** · 06-04-2015, 07:20 PM

+1 on the bandaxall EQ!

**jorgemg1984** · 06-04-2015, 07:34 PM

For people interested in Baxandal Eqs - comemrcially available there's the RC Booster, Fromel Shape EQ, most acoustic guitar preamps, Barber Linden EQ, VFE Rocket EQ, etc.. The Fromel is brilliant but lacks a volume control (I moded mine to have it).

**Herbie** · 06-04-2015, 07:36 PM

Could it be possible to make a tube amp version of that circuit? I have always thought that Polytones are the most tube amp sounding solid state amps I have heard. With real tubes the sound could rise to the next level!
The Polytone pedal is wild idea too, I am most interested in it!
I have a 70's Mini Brute IV and I think that an essential part of the sound is the "impossible" 15" speaker in that small cabinet.
Interesting amps anyway.

**jorgemg1984** · 06-04-2015, 07:53 PM

Ampegs are Baxandal preamps with tubes... and I bet you could do that with Polytones too.

I think I have his e-mail, if he's fine with it I will share the files!

**Archie** · 06-04-2015, 08:07 PM

Originally Posted by Longways to Go

I wish somebody would recreate the classic polytone like they do with all the boutique Fender & Marshall knockoffs. It would simplify the tone search for lots of jazz players . . .

Funny cause I have never played a pre 80's polytone that I cared about in the slightest.

I prefer the 90's ones.

**jorgemg1984** · 06-04-2015, 08:19 PM

Mambos kind of resemble Polytones, a though they have their differences.

**vintagelove** · 06-04-2015, 10:09 PM

Originally Posted by jorgemg1984

If I get permission from the forum member I can give you the files and / or the contact of a Portuguese guy who will do it and ship anywhere in the world.

I would appreciate that. I would really like to work with an engineer as I have a couple changes that will be necessary for it to work best.