Care and feeding of germaniums (transistors)

[Pixel]

So I was looking at my SS PA amp with the idea of possibly removing the output transformer because I only ever use one tap on it. Turns out that some kind of NFB voodoo is happening via one of the output taps so forget this idea.

But it leads me to wonder a couple things:

1. What is T2 for?
2. Why are the thermistors so far away from the germanium transistors?

When I got this amp the thermistors had come loose from the kapton tape that was sticking them to the case. Lacking the ability to restick the thermistors to the kapton, I instead epoxied them to a chunk of aluminum and bolted that where the thermistors used to be . Works fine so far as I can tell. But this location is quite far away from where the germanium transistors are heatsinked to the case. Would it be better if each thermistor were closer to its corresponding transistor?

mpa-20_schematic.pdf

[Enzo]

T2 is essentially your phase inverter, it drives the output transistors.

[Helmholtz]

The output stage can't work without the OT - even apart from the NFB loop.

I assume the position of the thermistors was carefully/deliberately chosen for best thermal stability. Adding a "chunk of aluminum" may slow down thermal response.

[dmeek]

The output transistors are silicon.

2SB337_SavantIC.pdf
2SB407_SavantIC.pdf

[Pixel]

The output transistors are silicon.

[ATTACH]n932404[/ATTACH]
[ATTACH]n932405[/ATTACH]

I've seen datasheets showing these as silicon, others as germanium. This amp has Hitachi parts in it which I believe to be germanium. Lots of things I believe are wrong though...

[The Dude]

E-B voltage drop is approx. .3V for germanium and .7V for silicon. If you want to know which you have, just measure the junctions with diode check on a DVM.

[dmeek]

Yikes! You're right. At least 2SB337 can be germanium.

2SB337_2SB471_2SB472.pdf

and 2SB407

2SB407 ge.pdf

[Pixel]

E-B voltage drop is approx. .3V for germanium and .7V for silicon. If you want to know which you have, just measure the junctions with diode check on a DVM.

I gave it a shot but it looks like I can't get a meaningful reading without unsoldering so I'll just have to learn to live without REALLY knowing.

[loudthud]

Judging from the Voltage numbers on the schematic, +18 for B+ and +17.8 for Base Voltage, you must assume the output transistors are germanium.

[Pixel]

Judging from the Voltage numbers on the schematic, +18 for B+ and +17.8 for Base Voltage, you must assume the output transistors are germanium.

AHA! This is probably gonna be my lesson of the day. Thanks. How are things at 221B Baker Street?

It never occurred to me to read a schematic that way. I figured the temperature compensation was a clue but couldn't get much further than that.

[mozz]

I am seeing both as germanium, 337 listed as being made by Hitachi and the 407 being listed as made by Sanyo.

[loudthud]

How are things at 221B Baker Street?

Apparently, it's a Museum ! I didn't understand the reference until I googled it

[dmeek]

Elementary, my dear Watson.

[Pixel]

Elementary, my dear Watson.

In this case the great detective was able to deduce the engine in my machine from long ago measures of its fuel pressure. Not bad. Not bad at all.