Poly-Tone mini brute

It has been quite a while since I've posted a thread on DIY Audio Forums.
The are 2 reasons for this a move to a new(to me)home &the associated
renovations & health problems.
I regret to say that the schematic of the mini-brute in referred to in
another thread, has disappeared in the packing & unpacking process
I hope to replace it, soon however to those of you who continue to enquire
about it I can only apologize for not getting to this matter sooner. When I do
replace my copy I will post it again.
To perform a cost estimate for an old version of this amp I want to have some informations:
1) Which transducer (Chassis) is in use? Maybe Eminence, FANE or Celestion ?
2) Which currently available transducer I must order for the case of replace?
3) Are there schematics available from this old version?
4) where is a service department of the brand "POLYTONE" here in Germany?

In the attachment there are some images of this device, which maybe I get in the next three weeks for looking.

Thank you for your advices.

In the meantime I have the version here, from which are you find the pictures in my previous post and the describtion by German ebay auction
Scheunenfund Polytone Minibrute 12" Basscombo | eBay
Therefore I will post some technical details, which I have find out by troubleshooting :
The voice coil DC resistance of the used transducer is 3 ohms and between power amp output and transducer I have found a serial resistor of also 3 ohms with 10 watts (ceramic version).
On the magnet of the used transducer are to read two numbers:
1) "12-0147"
2) "137 7942"
The voice coil diameter is approximately 45mm and at the basket I count only 4 windows instead 6 as mostly by currently available Eminence models.

Here some informations about the used power amp in my version (I have create a schematic and I measured the voltage on the supply rails).
In the front-End works a LM391N-80 (include Vbe multiplier for BIAS in the output buffer stage) and the output buffer stage is a Sziklai (compound) darlington - 2N5880/MPSU07 in the pos. half and 2N5882/MPSU57 in the neg. half. Supply voltage (measured without input signal, i. e. without sound from the speaker) = ±33VDC.
Idle (quiescent) current in the power transistors are only 1 mA.
The power amp main board is follow version: "PA378A" manufactured according imprint on the component side at 12.05.79 M

The Preamp with baxandall equalizer (bass treble) works with two pieces of RC4739DB (quad operational amp from Raytheon). Input voltage for the on-board votage regulators 7815 and 7915 are +/- 20VDC.

The differences between the version which I have and the version from ebay auction
item No 261299452085 about the URL
Clean 1978 Polytone Mini Teeny Brute Guitar Upright Bass Amplifier 10? Amp USA | eBay
(comes from the same time area - unfortunately pictured only with dust cover) are follow:

1) Sticker with serial No:
On the device, which I have for service, I read:
On the device from mentioned us ebay auction I read:
Additional I read follow: UL LISTED 427-S SOUND EQUIP.
2) Front-Sticker:
On the device, which I have for service, in the right corner at botton model MINI BRUTE
On the device from mentioned us ebay auction in the left corner on top, model TEENY BRUTE
3) Operating panel:
On the device, which I have for service I read: model MINI BRUTE
On the device from mentioned us ebay auction auction I read: model TEENY BRUTE

Sometimes there is to read "Mini-Brute", sometimes "Teeny-Brute" and sometimes "Mini/Teeny-Brute". This I don't understand. What means that exactly?

Concerning of my (just in the outline) not absolutely identical version with Serial-No P.7218 (to the version from mentioned ebay auction with Serial-No P.3041) rises up some questions:

Which replacement speaker from currently available Eminence or Fane acoustics guitar speakers would you recommend ?
I have heard from a guy from u.s., that WEBER - go to
Weber Speakers - Making the world a bit louder each day.
can build a replacement close to the original Eminence specifications from this first Mini Brute version, but an individual rebuild order is very expensive - so I think.

Which transducer are in use by the newer polytone Mini-Brute versions (MK-II, MK-III and MK-IV)?
Which material is the dust cover and where I can order this (maybe foam material)?

Are there test reports (test reviews) from this old Mini Brute versions, where is to find a frequency response?

Thank you for advices.

The only electrical difference between this both versions, which I can say, is the mains transformer itself - version from mentioned ebay auction is an us model with only 120VAC mains input voltage, while version, which I have, is suited both for 120 and 220VAC (mains transformer uses primary windings for both voltages).
The schematic from attachment is not the same, which I have - maybe a newer or older version - only the output buffer stage topology is the same (schematic is helpful for discrete replace of a defective and no longer available LM391N-80 (out of production).
Here the pictures from the ebay us auction:

no news?


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Hadn't seen it ;)

1) the regular Polytone speaker is an Eminence, think one from the Legend series, check which one is available in 4 ohms, shown by your reading of 3 ohms voice coil DC resistance.
Way back then they were made by CTS , of which Eminence is the "heir" , Mr Bob Gault used to make speakers at CTS and when the Company was in financial trouble, he continued on his own, that's why old speakers on both brands "are the same under the skin" .

That speaker was OEM/custom built , a very close modern version may be:
US Speaker - Eminence Texas Heat speakers - The Eminence Texas Heat is a 12" guitar speaker with tight bass and good top end bite from the Eminence Patriot Series. The Eminence Texas Heat speaker is an excellent upgrade or replacement for guitar cabi

This one is also very close to the original Polytone speaker:
Jensen C Series guitar speakers. The Jensen C12K 12" speaker. Great choice for lead guitar cabinets. 4, 8 or 16 ohms.

the codes you read mean:
On the magnet of the used transducer are to read two numbers:
1) "12-0147" <-- Polytone in-house part number
2) "137 7942" <-- 137: CTS EIA code ; Year "79" , Week "42"

EIA factory codes:
Speaker EIA Codes

The voice coil diameter is approximately 45mm and at the basket I count only 4 windows instead 6 as mostly by currently available Eminence models.
*Might* be 44mm diameter but that would be quite unusual in that time and age, because it's a typical "British" size: 1 3/4" or 44 mm, way back then popular "American " size was 1 1/2" (38mm) for almost everything and 2" (50mm) for high power speakers.

The series resistor you found is 0.3 ohms, and the intended purpose is to reduce amplifier damping, for a smoother looser sound.

Resist the urge to "improve" the speaker with a more "advanced/modern/exotic" one , the amp was equalized with the one you see there, by modern standards it would be considered more of a fat Guitar speaker than a deep Bass one, but a real Bass/PA woofer will be wasted there, the box is way too small for real Bass and the heavier cone and voice coil will kill mids and treble, you will lose definition big way.
Plus a heavy speaker will be hard to drive for the internal 90W amp .
Respect what the original designer intended.

And if the original speaker works properly, leave it there.

2) You seem to have the infamous LM391 driven power amp:

Polytone  PA 378.jpg

not because it's bad, far from that, but because LM391 are no longer made (for decades now) and very hard to find.
FWIW I think I have 1 or 2 , from the old days, which I keep in case a dead Polytone visits my shop.

But if yours works, fine, it won't die on its own, amp death is generally caused by shorted speaker cables or plugging 57 random cabinets, the toaster and the freezer in the poor output, not your case :)

3) as of Mini/Teeny/Maxi Brutes, all of them have the same basic preamp and 2 flavours of Power Amp (LM391 and discrete) and all sound the same, model names refer to different size and speaker complement, you will find from single 8" (Tiny ;) ) to single 12" (Mini) to 2 x 10" , 2 x 12" , single 15" , even a monster (for their standards) 2x15" , and some adding a couple Piezo tweeters for Voice or Acoustic Guitar use, but by far the most popular was the single 12" one.

4) it's a "Bass" amp but not the same as others, because of the very small size closed cabinet, it's meant for Jazz, very popular with Double Bass (acoustic Bass) players, where Bass has lots of harmonics and definition, not the typical Rock wall shaking Bass.

So you need very good midrange definition, reasonable Bass, warm lower midrange, the speakers suggested have it, an "improved" modern Bass specific speaker will have neither.

Please post your user experiences with it :)
My advice for people looking to replace the speaker in their MiniBrute/BabyBrute/TeenyBrute is to wait until OEM speakers come up for sale (as they do) on Reverb or eBay. There is a pretty noticeable difference in the sound between those CTS and Eminence (pre-1984, post-1984) speakers and the stuff you can buy from Eminence and other sources.
Hi there, I have a strange problem with this amp. It burned down: R17 100ohm resistor, drivers, LM391, one 0.15 5W resistor, power transistors. I replaced the resistors, the LM391 and the drivers. I checked the voltages with bulb limiter inserted and without the power transistors. I got:

MJE 182 (NPN)
E 0
B 0,6
C 29

MJE 172 (PNP)
E 0
B -0,6
C -29

I know the MJEs have a different pinout than PU57s.
The bias potentiometer works, I can set the -0,6V on the MJE 172.
On the pin 7 of the LM391 is cca 1,2V.

and now the strange thing:
When I put the power transistors the bulb starts to light and I have about 90mA on the power resistors (0.15 ohm). The bias potentiometer has no influence on it. The power supply voltage gets down from +-29 to about +-8V.

When I take the LM391 out and connect the pwr transistors on, I get these voltages:

MJE 182 (NPN)
E 28,9
B 19,8
C 29,4

MJE 172 (PNP)
E 28,8
B 28,2
C -29,2

Any suggestions please...? Thank you
I got a MbII from 1980.
I would ask if is normal to have hum, even with low volume.
No, it shouldn´t hum, not with nothing plugged in and volume on 0.
Is it normal hum, annoying but you can still play or **LOUD** hum, and you can not play, no instrument sound comes tjhrough?
Because in the latter case you may have a blown power amp.
Post a couple pictures, one showing the back top control panel, one showing the bottom cable/jacks/etc. bottom panel, specially showing the model/serial number plate for better identification.

Do volume and tone controls work?

Plus, the amp sounds flat, no dinamic, very very flat sound.
What’s the cause in your experience?
Polytones are designed for*Jazz* , so dull dark flat sound compared to "Rock" amplifiers.

What instrument are you using?

In my personal point of view, they are flat amplifiers, driving flat speakers, circuit is very similar to what you find in a Home Hi Fi amplifier, basically same tone controls, inside **tiny** closed cabinets, not much larger than the cardboard box speakers are shipped in, not kidding.
Such a cabinet tends to sound "boxy" (I wonder why ;) ) so Polytone chock fills them with tons of fiberglass damping ... that also contributes to typical sound.

That said, that "mostly midrange" sound is great to deliver acoustic/double Bass sound, better than a modern one:

Here they compare a very modern and powerful MarkBass amplifier, 300W RMS :eek: modern Kevlar cone speaker, Tweeter , a second "Acoustic" specialized amplifier, again with woofer and tweeter for wider range, to an ancient, 40 years old design Polytone, with a standard Eminence speaker in a closed box.
The Mark Bass sound fine, but the Polytone produces a better, more "acoustic"/realistic version of what the unamplified Double Bass sounds like ... and that´s the idea behind it.

No wall shaking bass, no glass breaking highs, just "same sound but louder".

Last edited:
Ok, do *all* the tests, as in:
1) no guitar plugged in.

2) all pots on 0
3) tone controls on 5
4) tone controls on 10
5) tone controls again on 5 , rise volume 0 > 5 > 10

6) plug guitar in, but set its volume and controls to 0
7) rise amp volume to 5
8) slowly rise Guitar volume to 10
9) slowly rise Guitar tone to 10
always touching guitar strings or bridge, so you are grounding your body to it

In all cases, describe Hum, I want to guess where is Hum appearing, by finding which controls affect it.
You might record and upload a short YT video showing all this.

Small doubt: you say setting EQ to Dark lowers Hum ... which should not be affected by it, are we talking the same?
* Hum: deep hard to hear , almost pure 50/60 Hz sinewave
* Buzz: annoying very dirty 50/60Hz wonky wave, mixed with "angry bees" sound, very easy to hear. Somewhat affected by deep switch.
* Hiss: like steam out of a kettle or compressed air out of a leaky coupling or valve, almost disappears when switch is set to dark.
Which one do you have?
Inn this case a YT video helps a lot.
hi JM
Do you know how to adjust bias ?


Carefully (it´s sort of jumpy) adjust bias pot until you read some 5mV across both emitter resistors, end to end (I measure them that way because they are very low in value) , which means an idle current of about 33mA .
It´s high enough to free you from crossover distortion.

Thanks for your answers.
I can live with hum.
What I can’t understand is the polytone II sounds flat...
I mean, no dinamics.
I would ask if there’s a sort of correlation between hum and flat sound.

the other polytone IV (15 inches speaker) has very dinamic attack.
No,hum is irrelevant.

But I strongly suspect the 15" speaker is the proper one ,which is a custom made Eminence *guitar* speaker, so: thin flexible paper cone, usually paper edge (no foam/rubber) ,snappy short and light voice coil, high sensitivity; while the other mayn have had the original speaker replaced by a cheesy (for this use) Hi Fi woofer: thick heavy cone, foam (or treated cloth) edge, very heavy long voice coil inside a largish gap (waste of magnetic efficiency), etc.
Or even worse: a Car type woofer or subwoofer, the worst Musical Instrument speaker available.

The amps themselves are basically the same.


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