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did concombre 10th February 2013 05:40 PM

pignose conversion.
Hello to all.

I recently bought a pignose 7-100 in order to modify it to the ols specs according to this schematic :

I changed the polarity of all the caps, the diode, the poser supply and replace all the transistors.
I even bought 2 different types of output transformer (vintage radio specs 48:8 and 48:4)

My main problem here is that the output transistor are becoming really hot. I can have sound for about 5 minutes, then they are very very hot and the sound starts to distort heavily so I turn the power off before they explode (I guess).

I search all over the internet for informations about this type of power amplification (meaning pushpull with two germanium transistor and a transformer) but, as I read somewhere, this kind of information is somehow rare on the internet. So I was hoping you could help me a little bit before I start the back-to-the-factory-specs conversion.
Thanks a lot.

djoffe 10th February 2013 08:59 PM

Increase the value of the 4K7 resistor at the top of the input transformer voltage divider. With no signal present, measure the voltage across the 1 Ohm emitter resistor as you increase the 4K7. Something like 10 mV across the 1 Ohm resistor is probably about right.

On another note, I'm not sure what the component across the bottom resistor is. It's label is just "green" if I recall correctly.

did concombre 10th February 2013 09:10 PM

Unfortunately, the closest value I had was a 2 ohms resistor but I guess this is not that important, no?
As for the green component, it was a diode in my "new" pignose version wich I just reversed because of the power supply polarity change.
Anyway, I'll try to change the 4k7 value tomorrow (or maybe the day after tomorrow) and post the measurement here.
Thanks anyway, it's a good start for me.

JMFahey 10th February 2013 09:50 PM

Post what were the original transistors and what you are using now.

Also: why change the OT for anything else?


the green component, it was a diode
What diode?

sregor 10th February 2013 10:20 PM

I would guess that the original diode was a germanium and would have to be switched for a silicon diode. Looks like a change to silicon parts. You probably should have the diode on te heat sinks for improved thermal stability.(don't know how it was set up originally) (as an aside - you could have just changed the germanium transistors and diode to PNP silicon types and not have to bother with reversing the caps and power supply polarities) YMMV

did concombre 10th February 2013 10:41 PM

The original components were like in this schematic :

The diode (1N4148, sorry) was not on the heatsink. And from what I saw on other schematics, regarding the thermal stability, wouldn't the diode need to be between the base and emmiter?
Actually, I didn't have to change much from the silicon one to the germanium one. I thought it would work instantly essentially because of the relatively simple changes to do.
And to answer to JMFahey, I bought the two transformer just in case. I hadn't found a way to calculate the impedance (or resistance?) of the transformer for a pair of germanium transistors. I didn't find any information about the one pignose uses so I bought two that had the same ratio as in an old germanium radio with a 8ohms speaker and a 4ohms speaker (I was eventually going to reuse them in other projects though).

sregor 11th February 2013 12:24 AM

One correction Germanium diode would be less current on outputs, so that wouldn't cause the problem anyway.
DC-wise that diode is between base and emitter of outputs. (Base connected throuhh transformer, emitter and diode connect to hround (emitters with 1 ohm or 2 resistor)

Also, on seeing your drawing - you show emitter leads going to center lead of output transistors. Most outputs have collector as center lead. Tried looking for datasheet for TIP410 but came up empty, except for one picture which had a TO-220 body which has the collector in the center. Also, most power transistors the metal tab is also the collector - make sure it isn't shorted to the heat sink which is usually grounded.

AFAIK it should work.

JMFahey 11th February 2013 03:37 AM

1) there's no such thing as TIP410 , it's a TIP41C
2) the modern drawing is horrible and has mistakes.
3) the modern diode is silicon, good to bias Si transistors.
You replaced Si outputs with Ge, but kept the S1 diode= instant overbiasing and thermal runaway.
4) if you mod to original Ge specs, do it to the end, following the old schematic in full.
Do not mix old and new.

sregor 11th February 2013 11:43 AM

My confusion - you are going from silicon to germanium? Diode should be germanium. Silicon would cause the outputs to run too hot.

did concombre 17th February 2013 11:47 AM

So. I changed the 4k7 resistor connected to the center tap of the first transformer output for a 15k one and it seems to be better. The transistors still get hot but not as hot as before and only when I play with the guitar and amp volume at max. I couldn't try the germanium diode but replaced it with a green led as it was labeled green (just to try) and it seems fine. I don't have a germanium diode on hand but I am going to try it as soon as I get one. The amp has been on for half an hour without any buzz (it was doing buzz after about 5 minutes on) I haven't tried to play for a full half hour yet...
Thank you all. I'll post any new information.

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