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Old 22nd October 2007, 03:32 AM   #1
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Default Polytone Mine-Brute problem

Help! I'ma almost at wit's end over my Polytone Mini-Brute, made in April of 1981. Here's the shortie:

Noticed a distortion at the tail end of notes. I play an electro-acoustic Takamine, so purity of sound is utmost to me. Took it to a repair guy who seems like he knows what he's doing, but...
so far, he's replaced the speaker. Great sound, but distortion remained. (Did some other things, some cosmetic. Looks great).

Then, after taking it back, he checked the gain on the output ?resistors and found they were uneven. I'm not a technical guy, so exactly what he did goes quickly over my head, but it involved replacement of said transitors or one of output transistors. It has improved, but some distortion remains.

To make sure all was kosher at my end, I tried three different cables and two different guitars. Distortion seems more in the "mid-range of the guitar.

He seems sincere and he was really upset that "an amp came back.", but clearly he cannot hear this problem as each time it was there when i got the amp back. I will call him back, but thought you people might have an opinion.

is this the case of someone who is technically competant, but can't hear as well as a musician?

ANY THOUGHTS OUT THERE? Get a new amp?
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Old 5th March 2008, 06:47 AM   #2
fm4 is offline fm4  Austria
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Any news on this? I have exactly the same problem with my Mini Brute II and I narrowed it down to the power amp.

The preamp played through another amp did NOT produce any distortion in the clean channel. I have opened the amp and identified some parts that could be the problem but would like to get some advice on it before I fiddle around with the amp.

I tried to contact Polytone but they did not reply so far (should I be surprised?).

Any ideas or schematics? s/n is P-28074. Don't know which year unfortunately.

Thanks in advance for your help. Would be sad to through this amp away, sounds just great

fm4
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Old 5th March 2008, 11:59 AM   #3
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Anyone with the Polytone amps should search this forum for Tiger, Super Tiger or Dan Meyer.

Many of the polytone amps use the amplifier that was published in the 1973 Popular Electronics magazine designed by Dan called the Tiger. the amps polytone uses is the same circuit as one of the tiger variations.

There are a number of design issues with these amps. trying to successfully repair one takes a very skilled tech. Just replacing stock parts with stick type parts does NOT mean the amp should then work. These amps are prone to crossover distortion, Bias/Bias Stack resistor issues, Output device issues, stability issues. all kinds of issues.

The design of the amp itself was way way ahead of its time. and most of the problems stemmed from the devices of that time and the device to device variations between MFG's. some amps/kits came with Pecor brand devices, others motorolas. Only Motorolas should be used!


The polytone amps were cool, still sought after by many folks. But trying to repair one can be crazy!
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Old 5th March 2008, 02:04 PM   #4
teemuk is offline teemuk  Finland
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The only Polytone schematics I’ve seen (http://www.murchmusic.com/polytoneserviceinfo.htm) are those of Mini Brutes (I-V) and PA-60 and PA-120 PA amps. Aside generic stuff like CFP output stage, differential input or bootstrapped load they have no striking similarities to any Dan Meyer’s amps I’ve seen. Some tigers have their issues (e.g. no Cdom in VAS, output stages with voltage gain, crazy bypass capacitance in feedback loop etc.) but I don’t see any of these issues in the particular Polytone circuit. I don’t doubt what you said about the transistors, bias circuits and other issues, though – just that I fail to see the similarity to Tiger amps.

Your reference about 1973 article is also incorrect (I guess). That year Dan Meyer did publish a 4-channel power amp in Radio-Electronics (not Popular Electronics) and reputedly part II of the article had “Tiger .01 conclusion” (never seen this although I have the aforementioned article). Later, December article in the same magazine introduced the “Tigersaur”. Most “Tiger” and Dan Meyer designs were published in Popular Electronics magazine.

Here’s a list of Meyer’s amp articles I’m aware of:

Apr&May-68 PE MMM Instrument amplifier
Feb-67 PE The Brute
Jul-69 PE Tiger & Super Tiger (“Tigers that Roar”)
Dec-69 PE Lil’ Tiger
Oct-70 PE Universal Tiger
Oct-71 PE Plastic Tiger
Mar&Apr-73 R-E 4-channel power amplifier (cascode high power amp)
Dec-73 R-E Tigersaur (another cascode high power amp)

You can find all of them from here:
http://www.swtpc.com/mholley/magazines.htm

Anyway, I don’t know about other Polytone amps. Do you have some schematic links?
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Old 5th March 2008, 02:48 PM   #5
sreten is offline sreten  United Kingdom
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Hi,

For a 1981 amplifier in continuous use the first thing you should do is
replace all the electrolytic capacitors, which should be the power
supply capacitors, supply decoupling capacitors, coupling capacitors
and possibly some in tone control sections.

For most of the smaller values go for a higher voltage rating to
reduce possible distortion and extend life, for the supply probably
with modern ones you can fit bigger values of the same voltage.

Should not be expensive and just might fix the problem, it is unlikely
it would not generally improve things, not least the reliability issue.

/sreten.
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Old 6th March 2008, 12:59 PM   #6
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Those schematics are definitely not the polytone amp i worked on! but it does use the same output transistors.... I dont remember the model as it was many years ago. i thought it was the mini brute and not the mega brute as those sams are for.

Also the Dan Meyer article i have i think is a reprint from 73. it still have it. just need to find it again. i think it was one of those best of type reprints. But it was called the universal tiger. My father built an amp from that article around that time. always had crossover distortion at low volumes. i will see if i can find it. I posted about it here previously.
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Old 6th March 2008, 07:15 PM   #7
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acousticguy,

The first series (1970´s) and the second series polytone amp have a "Tiger" clone power amp, but the newer versions have a "National seimi conductor" clone with a IC/chip (LM391N) as a driver for the power transistors.

I think you can have to low quiescent current "bias" through the power transistor, so you hear a lot of crossover distortion.

If it is a "Tiger" clone, Polytone have done a strange bias adjustment modification, so it is very hard to set proper bias/quiescent current for the output transistors.
Some millimetre up or down on the preset, gives zero or to much bias, and it must be adjusted when the heatzink are in half-warm condition, to avoid "run a way" behavior.

--Bo
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Old 8th March 2008, 04:29 PM   #8
fm4 is offline fm4  Austria
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Dear All,

Thanks for the answers, they were very helpful......for my case, it is still not clear to me from which year my amp is (it seems to be a newer model). I opened it and checked if I can find the LM391N IC but was not successful in my first attempt. Do the newer polytones have the same chip or did I just oversee it? javascript:smilie('')

Thanks,
fm4
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Old 9th March 2008, 05:52 PM   #9
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I think they change over to the LM-391N-80 in the "PA-378" power amp units, with out any notice.

I have find this power amps in some Mini Brute IV.

--Bo
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Old 19th May 2008, 03:05 AM   #10
fm4 is offline fm4  Austria
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Default Bias adjustment Polytone

Dear All

Apologies for posting again on this old threat, but I would like to get some more information on bias adjustment for my polytone mini-brute II.

I am still facing this crossover distortion problem and before I start changing the elkos and other parts of the amp, I would like to get more information on bias adjustment.

I have to question regarding bias:

1) do you believe that bias adjustment can reduce/eliminate the distortion effect I am experiencing?

2) if so, can you give me a hint where are the measure point for bias current in the power amp and what the current should be? (I have built a tube amp and there bias setting is quite forward but with transistoramps I am not so familiar).

And one question regarding the caps

1) I found 8200 uF caps. If I am not mistaken, one can put in tube amps higher values for filter caps. I.o.w. can I take 10000 uF caps for the purpose or is this out of bounds (they are accessible here in Austria)?

Thanks a lot in advance for your help,

Frank
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