the pimp / mod the Poor Man 660 thread

[mikeyB]

I said something about M/S hacking of the Edcors in several places, none of which I can point out in a quick manner.  I have not tried it, but it should work fine.   I've had to repair a number of Edcors straight from the factory; windings connected to wrong tabs. 

Is it about time that we all added our signatures to a wish list and send it to Brian at Edcor - I can't see it being too much effort to keep the windings seperate instead of centre tapped - look at the bobbin formers on the wsm/xsm txs. They have capacity for 8 solder tags - must add 2 cents for the tags ;D

New thread/poll ???

 

[emrr]

I said something about M/S hacking of the Edcors in several places, none of which I can point out in a quick manner.  I have not tried it, but it should work fine.   I've had to repair a number of Edcors straight from the factory; windings connected to wrong tabs. 

they're cheap so I'll just hack away. What exactly am I looking for in there, and any suggestions for a tool for the task?

two wires on the CT that you can move and wire elsewhere in M/S matrix mode. 

How do you determine wrong windings connection? By measuring DC resistance or inductance.. or something else. It will be great if you can give info about proper connection and how to measure that 

like 1/2 the sec and primary tied together on both sides instead of both pri and sec tied to each other.  so CT is clearly misbalanced.    Or one half of a winding wired out of polarity so it sounds terrible.    obvious easy stuff.

Remember, at that price there is no quality control.  No free lunch. 

 

[bluebird]

Alright so I went a little overboard on the pimping...

i was doing some sign wave tests on this thing and I kept getting just a hair of crossover distotion in the wave about halfway through the gain reduction range. I had strong matched 6BC8's and everything was calibrated right.

So I started to mess with the two bias pots and I couldn't seem to get the wave to look nice all the way through the GR range with out having the output pretty low. Useful compression was happening about 10 to 15 db down from where the input signal was.

To make a long story short I shoved in two 6BA6's wired as triodes and set both bias pots to -11 volts and my crossover distortion disappeared. I tried four to mimic the four 6BC8 triodes but I got oscillation at certain gains.

the two 6BA6's sound great!! Sound is tighter and doesn't break up as much in deeper reduction...I guess what I have now is a Sta Level front end and a Fairchild side chain...kinda ???

I'm REALLY happy with this now. I think I can put the cover on it for good.

Picture below is the 6BA6's tagged on to a 9 pin socket pigtail and stuck into the rear 9 pin socket.

 

[emrr]

Cool; a real hacker.  No other parts changes?  Only trimmer adjustments? 

Any personal theories as to why?

 

[[silent:arts]]

this is really creative looking ;D ;D ;D

 

[Moby]

i was doing some sign wave tests on this thing and I kept getting just a hair of crossover distotion in the wave about halfway through the gain reduction range. I had strong matched 6BC8's and everything was calibrated right.

Huh, that sounds like a real problem. Wiring two different tubes instead of 4 is a big difference.  Is the author of this project around? What about other working units? All have a same crossover issue? Volker, did you tried to measure your PM?

 

[Kingston]

What is the cross-over distortion here exactly? I'm only familiar with cross-over phase distortions in filters.

 

[Moby]

What is the cross-over distortion here exactly? I'm only familiar with cross-over phase distortions in filters.

http://en.wikipedia.org/wiki/Crossover_distortion

 

[nielsk]

Bluebird, any chance you would post a drawing of this mod, I would like to try it & compare, but I'm not good enough to get it from your verbal description....

 

[[silent:arts]]

Huh, that sounds like a real problem

a hair of crossover distortion

I don't think this sounds like a real problem

 

[Moby]

I'm still waiting for my metalwork to be finished so I still didn't connected my unit... So , I must wait to hear the sound :-(

 

[Kingston]

What is the cross-over distortion here exactly? I'm only familiar with cross-over phase distortions in filters.

http://en.wikipedia.org/wiki/Crossover_distortion

Doh! I'm just used to hearing the term "class A/B switching distortion" (probably not so common). Heck, I've even modelled a DSP plugin based solely on several types of these AB switching distortions! http://www.michaelkingston.fi/retroband/ (a result of my University dissertation)

Huh, that sounds like a real problem

a hair of crossover distortion

I don't think this sounds like a real problem

Doesn't sound like a problem. In fact, this parasitic can be highly usable in many sound sources. While I haven't built this unit, based on my measurements with this effect, it will be a big part of the "mojo" of poorman. I would not remove it.

 

[Moby]

Sorry I missed the word "hair" That can be a nice character if it's just a 'hair" 

 

[bluebird]

Sorry I don't want to freak everybody out. There is nothing WRONG with the sound of this compressor stock. I just felt I could make it suit my needs better.

I was getting the crossover distortion only at VERY HIGH INPUT LEVELS. My input transformers ARE NOT STOCK they have a bit more step up gain.

This all said, I felt the gain and compression range where everything looked right was a little lower than I would like considering there is not a lot of makeup gain inherent in this unit.

Other compressors using the 6BC8 like the UA175 have a separate makeup amp so they can get away with smaller signal levels on the input.

But remember, things are going to get weird if you push anything to the limit. I just like pushing things and I felt the 6BA6's could handle a little more.

Once again this is the PIMP thread NOT the "THERE'S SOMETHING WRONG AND I"M FIXING IT" thread.

So go and build your units stock..don't worry be happy..

Here is a quick hand drawn picture of the waves I was getting. I exaggerated the crossover distortion a little bit. I know its not to scientific. :-\

 

[smallbutfine]

So, if I understand it right, your higher step-up input transformers push things too far for the stock tube gain structure, creating too much IMD on higher levels, which you now compensate for with the 6BA6 replacement.
Correct?  ???
Kind regards,
Martin

 

[emrr]

Reading between the lines, I'm guessing we're talking about 1 kHz tone at 0 dB plus stock non-compression amp gain of about 18 dB plus additional few-ish dB of whatever input transformers you are using.    Seems things look normal at both light and maximum compression. 

On my 'to do' list is a more comprehensive set of measurements regarding distortion point of signal amp with no compression, etc etc.  Maybe you can beat me to it and save me the trouble.  Wonder if you are pushing that line with your input level. 

I have to wonder if your changes in the time constants have any effect on the distortion you were seeing, or if they affect the distortion when fiddling about in the highest distortion range of compression. 


So far you win the 'Mod the PM660' competition!  ;D

 

[bluebird]

So, if I understand it right, your higher step-up input transformers push things too far for the stock tube gain structure, creating too much IMD on higher levels, which you now compensate for with the 6BA6 replacement.
Correct?  ??

Yes and no...
I'm using this transformer http://www.jensen-transformers.com/datashts/6110kb.pdf So its not that much different. The 6BA6 is doing a bit more than just compensating for the high input level.

And I'm not pushing it so much as to skew the wave form with no compression...if that makes sense.

I have to wonder if your changes in the time constants have any effect on the distortion you were seeing, or if they affect the distortion when fiddling about in the highest distortion range of compression.  

I fiddled with the time constants and it didn't seem to have an effect over the wave form...

 
On my 'to do' list is a more comprehensive set of measurements regarding distortion point of signal amp with no compression, etc etc.  Maybe you can beat me to it and save me the trouble.

I really don't have the equipment (or brains ) to do a proper analysis. So I'll just leave it up to you.

 

[Moby]

I'm using this transformer http://www.jensen-transformers.com/datashts/6110kb.pdf

Sorry, what did you replaced with 6110k-b? 600/10k or 10k/600  transformer?

 

[emrr]

The input, using that Jensen backwards I must assume. 

 

[Moby]

Cool, and you are happy with the response?