Fender solid state amplifier bias

[Loudthud]

Often the suffix letter doesn't mean much except a lower price. I worked at a semiconductor company that sold microcontrollers rated at 8MHz, 10MHz and 12MHz. The fact was they all had to pass final test at 12MHz. I needed a 25 amp bridge rectifier late one Saturday. The ones they have at Radio Shack were rated at 50V. I tested the one I bought on my Tek 576 curve tracer. It was good for over 1000V.

[Nigel Goodwin]

Originally Posted by dirkwright

I don't think power transistors come in hfe grades, like A, B or C, do they?

Yes they do, and always have - it's VERY, VERY common.

[Enzo]

From the semiconductor house or from the OEM?

[Nigel Goodwin]

Originally Posted by Enzo

From the semiconductor house or from the OEM?

From where ever you buy your semiconductors

As you mentioned earlier the A,B, C on TIP series is voltage rating (I've always only stocked the C highest version), but things like BC212L come in numerous gain groupings, if you have Towers International Transistor Selector (and any engineer should) it explains the meaning of the letters and markings.

I seem to have a vague recollections that BC212/182 etc. also used to have paint spots on them signifying groups as well?.

[Enzo]

Ah. Over here, I don't see the BC series parts much. Maybe a few like BC550. If a similar thing exists in the 2N or 2S serieses, I am not aware. I wear out old Motorola selector pages flipping through them, and don;t recall ever seeing such.

[sandip1]

Hi, I am hoping that one of you can help me. I have a Fender Deluxe 112 Plus. The output transistors are getting very hot even without any input and thus the thermostat shuts off the amp. I am suspecting high frequency oscillation but I may be wrong. Another symptom is that when you switch off the amp, it goes off with a "fart" noise. Can someone please point me in right direction?

[Enzo]

I'd tend to think not RF, but I too could be wrong. I am guessing you have no scope.

First, is there any DC on the output? Disconnect the speaker, and work without it until the amp is stable. If there is DC, that alone could explain the heat. Taking the load off it should eliminate the current draw that heats them.

Google "RF Probe". You will instantly see a number of very simple circuits to detect RF with a meter. All of them are more or less the same other than details. You can knock together that little circuit to see if there is RF voltage on the amp output.

When I see a solid state amp running hot just sitting there, my first thoughts go to the bias circuit. I suspect both polarities are turning on at the same time. You have four outputs, two and two. I'd expect about 1 volt on the bases of those, two of them negative of course. More than that will cause overheating. Your bias is four diodes - CR5,6,7,8. Do you get something like half a volt across each one?

Are all of your outputs getting hot, or can you tell that it is mainly the two on one end of the row? Not always easy to tell that. Or, look at those four 0.47 ohm 5 watt resistors by the outputs. Measure the voltage across each one. Are they all about the same? And what might that be? If one or two are a lot higher than the others, that means the associated transistor is conducting a lot more curent.

[Nigel Goodwin]

Originally Posted by sandip1

Hi, I am hoping that one of you can help me. I have a Fender Deluxe 112 Plus. The output transistors are getting very hot even without any input and thus the thermostat shuts off the amp. I am suspecting high frequency oscillation but I may be wrong. Another symptom is that when you switch off the amp, it goes off with a "fart" noise. Can someone please point me in right direction?

It could be?, high frequency oscillation often causes 'farting noises' when you turn it off.

You really need to stick a scope on the output, this will tell you in seconds if it's oscillating.

[sandip1]

Thanks Nigel. I do not have scope. So trying to figure it out with DMM.

[sandip1]

Thanks all the amp is fixed. Shorted one bias diode to get the bias right that made transistor run cool and changed the op amp that fixed farting noise.