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Old 31st July 2005, 06:35 PM   #1
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Join Date: Apr 2002
Location: Winnipeg Canada
Wink 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.
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Old 28th September 2013, 07:35 PM   #2
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Join Date: Jul 2005
Location: D-55629 Schwarzerden
Default Questions to the first Version, Model according Sticker: Mini-Teeny Brute

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.
Attached Images
File Type: jpg Polytone Mini-Teeny Brute front.JPG (45.8 KB, 82 views)
File Type: jpg Polytone Mini-Teeny Brute operating front.JPG (30.1 KB, 80 views)
File Type: jpg Polytone Mini-Teeny Brute sticker.JPG (47.8 KB, 81 views)
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Old 10th October 2013, 08:42 AM   #3
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Join Date: Jul 2005
Location: D-55629 Schwarzerden
Default More Informations provided and wanted

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:
MODEL: MINI/TEENY-BUTE - 120/220 VOLTS 50/60HZ 90 WATTS SERIAL-NO. "P. 7218" FUSE-NO. 1 AMP.SLO-BLO
On the device from mentioned us ebay auction I read:
MODEL: MINI-BUTE - 120 VOLTS 60HZ 90 WATTS SERIAL-NO. "P. 3041" FUSE-NO. 1 AMP.SLO-BLO
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:

Last edited by tiefbassuebertr; 10th October 2013 at 08:57 AM.
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