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Author Topic: Circuit changes for the Pignose 7-100  (Read 6330 times)

mark

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Circuit changes for the Pignose 7-100
« on: April 10, 2013, 02:53:58 AM »

Hi

I'm fixing a Pignose 7-100 for a friend. The problem with the amp was batteries leaking and corroding the battery cases and wiring.

I initially liked this amp and compared it to my Champ clone, unfortunately it just doesn't hold a candle to the Champ. At the moment the more I play it the less I like it.

The clean tones are dreadful (the bass frequencies are very buzzy) and the distortion is so-so.

Initially, I thought of replacing the 50K volume knob which has to excessively load the guitars pickups. Though it also occurred to me that it was chosen to reduce hum (much the same as the 220K resistor after the first gain stage in a Boogie Mk 1).

I think more gain can be had from the first stage, though less gain and better tone might be a better option as nearly everyone has a decent fuzz that could be used prior to the Pignose.

As far as my bias goes, I don't have any particular loyalty to transistors and valves. If I record something, I'm likely to use Guitar Rig as it sounds pretty damned good.

Look forward to your opinions and advice.

Mark
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J M Fahey

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Re: Circuit changes for the Pignose 7-100
« Reply #1 on: April 10, 2013, 04:24:57 AM »

Not clear about what you are asking.
If it is "Mods to improve it" just don't.
There's no room for anything else, it is a very simple design, already stripped to the bones.
And there is no "first stage", it's a *very* simple (think transistor radio from the 60's) power amp.
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mark

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Re: Circuit changes for the Pignose 7-100
« Reply #2 on: April 10, 2013, 05:55:59 AM »

Thanks for the reply, but I beg to differ, I think a FET on the first stage is an obvious mod as it would increase the input impedance of the Pignose. Just thought of it then too. Thanks for the inspiration.

Mark
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Roly

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Re: Circuit changes for the Pignose 7-100
« Reply #3 on: April 10, 2013, 06:15:35 AM »

Circuit.
http://music-electronics-forum.com/attachments/2913d1218923726-pignose-model-7-100.gif
Thanks again to music-electronics-forum.com

If you are going to do anything with it I'd be inclined just to add a FET source follower ahead of that 10k volume pot.  Greatly increasing the input impedance for passive guitar will make more improvement than converting the first stage to FET unless you change everything in front of that as well.  While that 10k pot and bias network remain it doesn't matter what device is in the first stage.

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mark

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Re: Circuit changes for the Pignose 7-100
« Reply #4 on: April 10, 2013, 03:10:59 PM »

Hi Roly

Thanks for your reply. I have included the correct circuit diagram. The actual component values are in brackets.

I agree the value of the volume control and bias circuitry is a problem. I have read posts where people have used a buffer to improve the tone of the guitar.

My initial thoughts were to increase the volume control value to 1 Meg, and use a FET(naturally the 120K resistor to the supply would go.)

Mark
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Enzo

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Re: Circuit changes for the Pignose 7-100
« Reply #5 on: April 10, 2013, 07:04:14 PM »

It is utterly crude, the output stage alone is distortion waiting to happen.  But projects are projects, and if it is fun, go ahead and mod away.

You can hang all sorts of extra circuits on it, I imagine a motivated soul could wedge an FX loop in there if he tried, or a reverb.

I used to follow formula one racing, decades ago, and every now and then they changed the rules on the engines.  Displacement for example.  I recall at the time some discussion about what could be gotten from a single cylinder motor.   They discussed increasing the compression, adding turbocharging, changing the cam, and other stuf that I don't recall.  Seems to me they managed to get about 250 horsepower out of one cylinder, at least on paper.   Forward to something we see today - lawnmower races.  Guys hot rod their lawn tractors and race them.  SOmehow this Pignose upgrade stikes me the same way.  I don't mean that in a snotty way.  They made a whole Star Trek movie based on the same premise.  The first movie, where the simple space probe Voyager came back all modded up as "V-ger."
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Roly

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Re: Circuit changes for the Pignose 7-100
« Reply #6 on: April 11, 2013, 12:46:56 AM »

{There has to be a gag in here somewhere about turning a pignose into a silk purse, but it just plumb evades me ATM.  ;) }

I had a friend who was an aeronautical engineer who was fond of saying that "you can even make a brick s**thouse fly if you bolt on a big enough engine".

If you have a look at my work on pickups and input impedance here you will see that even 50k Zin is, well, pathetic, and your suggestion of 1Meg is more realistic, but again I say that simply adding a FET buffer/follower ahead of the existing volume control looks to me like your best option to get some "tonality" out of it.  The elemental NFB would make me loath to fiddle with the first stage.

Otherwise I'm inclined to agree with Enzo that this is a bit like lawnmower racing.

Have a go at FET-ising that first stage by all means, but I think you will find that the effort/reward ratio of adding a simple MPF102 or 2N3819 source-follower front end impedance buffer will be better.  JMO. 
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J M Fahey

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Re: Circuit changes for the Pignose 7-100
« Reply #7 on: April 11, 2013, 02:51:00 AM »

Thanks for the reply, but I beg to differ, I think a FET on the first stage is an obvious mod as it would increase the input impedance of the Pignose. Just thought of it then too. Thanks for the inspiration.

Mark
Sorry but you don't have a "first stage" there.
That 2SB172A is part of the power amp itself, look at the speaker out feedback being brought to its emitter .
And simply you can't slam a FET there, it won't work.
Now, you can add a Fet/op amp/transistor buffer before the volume control, but that's something else.

EDIT: just saw the Hemmo version you attached.
Sorry but for one, it has errors.
He writes what he measured ... but he forgot to lift one resistor end "in the air" to measure properly.
In some the difference was so amazing (as in 47 ohms instead of 68K) that he wrote both, just in case.
In others where he found it "normal", he wrot only the measured value (in circuit, which is wrong).
Sometimes he apparently can't read the color code, as in the output transistors emitter resistor.
He measured 3 ohms (probably forgot to discount multimeter cable resistance), but he quotes 2K7 which is ridiculous, reasonable value would be 0.27 ohms .
He probably read 27K or something on the resistor body, but in that case, "K" (or "J") refers to temperature coefficient, not "K" ohms.
He also misread a *printed* label.
Everybody who wants to build his own Pignose goes on a wild chase for "TIP410" which unfortunately, and to confuse things, do exist, just they are a very different transistor.
The one he has was TIP41C, which he misread as TIP410
As of the presumed "C9014", same thing as before, you can't just slam a FET there.
Leave that power amp as-is and add whatever you want before that volume control, you'll get better results.
Good luck :)
« Last Edit: April 11, 2013, 03:10:36 AM by J M Fahey »
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mark

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Re: Circuit changes for the Pignose 7-100
« Reply #8 on: April 13, 2013, 01:44:30 AM »

It occurred to me to use a Boss pedal has a buffer as they aren't true bypass but buffered.

There was a tonal difference with the higher frequencies being more apparent, there might have even been a bit more "drive" from the guitar due to the signal not being loaded down.

I was looking for an improvement in the bass and mids, in short it just didn't happen. I'll have a look at it with the CRO to see where the distortion is coming from, but I'm coming around to the idea that there isn't much to be done to improve this amp. It's a poor design that persist due to marketing. :loco

Mark
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Enzo

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Re: Circuit changes for the Pignose 7-100
« Reply #9 on: April 13, 2013, 02:05:13 AM »

It was never intended as a serious amp. It was a nifty little portable gadget, good for a gritty sound on a street corner.   I don't think marketing portrays it as competing with Fender for tone. It was always quick and dirty.  I want one for the knob,  The pig knob is worth the price of the amp.
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Roly

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Re: Circuit changes for the Pignose 7-100
« Reply #10 on: April 13, 2013, 12:20:06 PM »

Quote from: Enzo
I want one for the knob,  The pig knob is worth the price of the amp.

 :lmao:
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J M Fahey

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Re: Circuit changes for the Pignose 7-100
« Reply #11 on: April 13, 2013, 12:51:33 PM »

I want one for the knob,  The pig knob is worth the price of the amp.

I remember when reading the "Jaws" novel (yes, I actually read it, the film was not enough) the young "hero" wants to "blend in" better with the presumed "high class" people on holiday there, so he asks his mother to buy him a fashionable "Chemise Lacoste" French T Shirt .... to what she answers:
"forget it, it's just a $5 T Shirt with a $50 crocodile sewn to it":lmao:

Remember those were 1972 Dollars, multiply by 5 or 10 to get today's value  :(
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Enzo

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Re: Circuit changes for the Pignose 7-100
« Reply #12 on: April 13, 2013, 12:58:58 PM »

AH yes, the Izod crocodile shirts were THE item back when.   I used to have a similar shirt with a little crocodile holding a golf club embroidered on it.
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J M Fahey

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Re: Circuit changes for the Pignose 7-100
« Reply #13 on: April 13, 2013, 01:38:10 PM »

I bet the golf club added an *extra* $50 to the price. :lmao:
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Enzo

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Re: Circuit changes for the Pignose 7-100
« Reply #14 on: April 13, 2013, 03:32:22 PM »

I just wonder how he got past the membership committee...
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