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Circuit changes for the Pignose 7-100

Started by mark, April 10, 2013, 02:53:58 AM

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J M Fahey

Quote from: Enzo on April 13, 2013, 03:32:22 PM
I just wonder how he got past the membership committee...
Oh !! That one is *EASY* ;)
He just stood before the Committee and did what Enzo always suggests .... he SMILED.


mark

The Pignose is such a piece of junk, the pcb should be on the same side as the volume knob as this would mean shorter cable runs, the output cable is co-ax to prevent it being a source of positive feedback when turning the pcb around would have cured this problem.

There are good recordings of these amps on You Tube.

http://www.youtube.com/watch?v=Y4f76ng49wA

http://www.youtube.com/watch?v=4NnP-7ynCAY

Mark

J M Fahey

Hey, don't be so hard with it, it can work very well within its limits.
And when it appeared, it was revolutionary.
A lot of good Musicians used them .... musicians whose "other amp" was a Fender Twin or a classic Marshall, yet they even recorded with a Pignose.
Another proof that "sound is more in the fingers than in the hardware".
The first video you linked to shows a very cool clean sound.
Let's not speak about the Distortion, though ;)

mark

I don't think I'm being too hard on it at all. It is a mediocre bit of kit that has seen it's day and that should be recognised.
Surely if something like the ZT Lunchbox JR is a better option and we should recognise that?

http://www.youtube.com/watch?v=iNhjAhz6FZ0 

Mark

J M Fahey

You are comparing a 2010 AC powered design to the first battery powered guitar amplifier, designed in the late 60's.

Why not compare an F16 fighter to a WW1 Sopwith Camel, now that we are at it?






What are you trying to "prove"?

Enzo

I think he wants someone to agree with him that it is a crude design.

OK, it is a crude design.

I think when you start to talk better options, they need to fall within the same category of goods.   A slab of prime rib and a baked potato is a better option than a McDonalds burger and fries, but when you are driving down the interstate, there are not many prime rib drive-throughs.

J M Fahey

Oh, that?
Yes, of course, it *is* a really crude design.
:tu:

mark

The problem is in the output stage, I have to have a good look at how to effectively fix it. If that is possible of course.

Mark

Enzo

Of course it is possible.  You can rebuild most anything into something else.  A good first step would be to design in some bias to eliminate crossover distortion.   On the other hand, doing so would remove the grit that is part of the signature sound.

mark

Agreed there is precious little bias on the transistors. I did a rough and ready increase to the bias and the amp sounded much better, and I think the distortion will sound more pleasing too. The trouble will be not to exceed the specs of the TIP41's and not to effect the battery life too adversely. The clamping diode for the bias supply seems a bit crazy to me too.

I'm still having a bit of a think about how to do this. This amp is quite similar to the Deacy amp so there are a few choices available from the Deacy amp. I'm not worried about the amp without a signal, it is with a signal that will be the problem. I'll have to find out what wattage the TIP41's can endure without a heatsink.

Thanks again for your replies.

Mark

Roly

When under-biased this sort of stage operates in Class-B.  This means more crossover distortion, but it also means that the output stage idles at a very low current.  Increase the bias to Class-AB and you reduce the distortion but increase the idle current being drawn from the battery.

The point of the diode is that it provides temperature compensated bias voltage.

On the transistor data sheet there will be a maximum rated chip temperature, a thermal resistance (in degrees per watt) from chip to case and from case to ambient.  This is basically a series resistor circuit with temperature instead of voltage.  Germanium transistors should feel no more than warm to the touch, silicon transistors shouldn't boil water.
If you say theory and practice don't agree you haven't applied enough theory.

mark

#26
Thanks for the reply Roly.

QuoteWhen under-biased this sort of stage operates in Class-B.  This means more crossover distortion, but it also means that the output stage idles at a very low current.  Increase the bias to Class-AB and you reduce the distortion but increase the idle current being drawn from the battery.

This was a given, and I do expect this.

QuoteThe point of the diode is that it provides temperature compensated bias voltage.

I don't understand the point of the resistor in parallel with the diode as the voltage across the diode will be it's forward voltage (0.6VDC roughly). Agreed the forward voltage may increase or decrease depending on the operating temperature.
Mind you I still don't think it is a good idea as the transistors are pulling 5uV across the 2.2ohm resistor.

QuoteOn the transistor data sheet there will be a maximum rated chip temperature, a thermal resistance (in degrees per watt) from chip to case and from case to ambient.  This is basically a series resistor circuit with temperature instead of voltage.

Not sure about this. I'll include the datasheet in an edit. :dbtu:

Mark

Enzo

And the battery is the one part never to forget.

The common TIP41 is a 6 amp part with 65 watt dissipation at 25 degrees C.   But are we really planning to push 65 watts with this battery?   I think the least of our concerns is the power rating of the TIP41.  COntext matters. 

Enzo

The parallel diode doesn;t set the voltage across the resistor, it limits it to 0.6 or whatever.  If the voltage across the resistor is less than the junction drop of the diode, the diode doesn;t conduct.


mark

#29
Thanks for the reply Enzo.

QuoteThe parallel diode doesn't set the voltage across the resistor, it limits it to 0.6 or whatever.  If the voltage across the resistor is less than the junction drop of the diode, the diode doesn't conduct.

Agreed, but if the voltage across the resistor is less than 0.6VDC then it can be regarded as almost non-existant as it ceases to bias the output stage on. Thus it serves no purpose.

QuoteThe common TIP41 is a 6 amp part with 65 watt dissipation at 25 degrees C.   But are we really planning to push 65 watts with this battery?

Without the necessary heasink, these transistors won't get anywhere near 65 watts, in fact the datasheet rates it at 2 watts in free air at 25 degrees. The maximum rating appears to be 2.4 watts.
 
Mark