Hi

I'm fixing a Torque T100K amp. It is working, but the pots are shot.

In fact one of the treble pots can stop the audio dead in it's tracks. I tried spraying the pots with DeOxit, but I suspect the pots are beyond it.

The pots are about 18 to 20 mm wide.

Mark

Hi Enzo

That is the trouble with pasting on sites such as this one, if people don't know you well enough they will take things the wrong way.

It's not the first time nor will it be the last time.

Getting back to the thread I wasn't implying a mojo with the TIP41C transistors, in the past the transistor was either dead or alive, in this case it wasn't feeling well. It's all very tube like. :duh

Mark   

Enzo, you are assuming the TIP41 transistors were working correctly, and I certainly don't think you can make that assumption. The difference in response was not subtle the most noticeable was the difference in distortion, the TIP41's were like a 60's fuzzbox while the TIP41C's were probably like a 50 watt Fender on "5".

Actually I remember the distortion increased in my RAT pedal prior to it blowing and I had to replace the IC.

Getting back to the Pignose, that's just how it happened.

Mark

The outcome of the Pignose was that it was fixed and returned to my friend. I decided to replace the TIP41 transistors in the output stage with TIP41C transistors.

This helped the overall tone of the amp though it didn't have as much gain, I found the trade off was it was much more touch sensitivity which was a great trade off. I can't explain why changing the transistors helped so much, my friend was very happy with the amp.

Here is a link to the various Pignose circuits I found during the repair process.

http://www.ssguitar.com/index.php?topic=2985.0

Mark

There are a few component differences and of course the pre 86 version uses pnp transistors.

I found Hemmo's version closest to the made in Hong Kong version.

Speaking of memorable SS amps.

http://www.youtube.com/watch?v=Z1SEDuWLtd4

The great unknown.

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

Mark

Thanks for the replies.

The watts angle should be approached conservatively, this Total Guitar review of the Lunchbox Juniour is a good reason why; http://www.musicradar.com/gear/guitars/amplification/instrument-amps/guitar-combo-amps/lunchbox-junior-571263

Quote"You've got a good 35 watts to play with, but solid-state power is a very different thing to valve power, and this combo won't fill a room like the 15-watt Eagletone Raging 15R we looked at a few months back, or even a five-watt valve combo, for that matter."

I dare say the whole development of the notion of SS watts and valves watts is another thread in itself. It did giving the guitarists the notion that SS amps are inferior to valve amps, noting down classic SS amps such as the Roland JC-120 again is another thread in itself.

The question I am now asking myself is why does the bass response improve with more bias current?

Mark

Decided to get the CRO onto this thing. I inputted 1Khz and the output with a 7.5 ohm load and a 12VDC supply was roughly 2 watts before distorting. I guess it comes as no surprise the touted spec of 5 watts is no where near the mark.

Prior to distorting the waveform looked smooth without any crossover distortion. I suppose this should come as no surprise as the issue I have is with bass frequencies.

I think I'll need to stop using RG Keen's quick and dirty oscillator and use one that can input lower frequencies. I'm expecting to see the output to drop off with the lower frequencies.

Mark

Thanks for the reply Roly.

QuoteThe parallel 220 ohms works in conjunction with the dynamic resistance of the diode to shape the thermal characteristic.

Please explain how?

The effective resistance would seem to be 81 ohms including the 220 ohm resistor. It doesn't really seem to perform much of a purpose to me. The current draw is about 7mA which is well within their specification.

It would seem to me the purpose of the diode is to turn off the output pair. I'd imagine the output pair conduct and then a voltage becomes present across the 2.2 ohm resistor, as that becomes large enough to exceed 0.6VDC the output pair switch off.

Mark

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

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

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

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

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

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