SE SS Power Amp questions

[mac]

I have a SE SS 36v amplifier on the breadboard.

Preamp is a fet copy of the Epi Valve Jr. I'm using BF245A, but MPF102 are ok too.
Amp section is a Tip 41c @ 300ma with home-made output transformer (tiny air gap), and a 4.7 ohm emitter resistor (no bypass cap)
A pot at the base to set the bias current.
Idle dissipation is about 10W. I can touch the plastic side without burning my finger so I guess heat sink is ok.

It sounds loud and tubey despite my lack of experience and the OT I made.
TS and RM push it very well.

But I have some questions about long term stability.

- Is a 4.7 ohm emitter resistor enough to control thermal runaway?
- Will you use fuses, where? OT wires are thick.
- Is second breakdown an issue? I tried power mosfets but prefer BJT
- Better power BJT? (2n3055, mje15007)

mac

[PRR]

> A pot at the base to set the bias current.

Show the actual whole circuit; all details matter to thermal runaway.

For older power BJTs, I would not cross 40V 1A. 24V will kick to 47V, but current will be way down, so I *think* you are on the safe side.

OTOH, how much does TIP41C cost today? Two for a buck? (I remember when it was 3 hours labor.) You could avoid brain-pain by beating it in a hot room to try to kill it.

[PRR]

A single power BJT with 5 Ohm emitter resistor has an input impedance like 250 Ohms. And parallel to whatever the base bias network is....

This is far-far below "Epi Valve Jr" (AA Champ) circuit values, even FET-ized.

[mac]

Show the actual whole circuit; all details matter to thermal runaway.

A single power BJT with 5 Ohm emitter resistor has an input impedance like 250 Ohms. And parallel to whatever the base bias network is....

This is far-far below "Epi Valve Jr" (AA Champ) circuit values, even FET-ized.

Paul,
I'll try to be as graphical as possible, no way to draw a schem right now,

There is a MPF102 fet buffer before the TIP41.
Like the Valve Jr, a 220k and a 1k5 "gate" stopper at the gate, and a 22nf coming from the preamp.
Drain to +B2, and a 10k from source to gnd.
The base of the TIP is connected via a 2k5 pot to the source.

The TIP is going to steal current from the source, about 300ma/(hfe:60)=5ma.
There are 300ma*4.7ohm + 0.7v + potValue*5ma at the fet source.
A fet with 3.5v and 4.5v is needed here. MPF102 are in this range. A mosfet can be used too.

A darlington can be used to neglect the third term but it's too much gain to my ears. Not an option.

The TIP collector goes to the OT, and to +B1 (+B2<+B1)
I wonder if a fuse here helps in case the TIP shorts.

Hope this helps.

mac

[R.G.]

- Is a 4.7 ohm emitter resistor enough to control thermal runaway?

How big is your heat sink??

[PRR]

If I am picturing your verbal description right-- that is a wonky circuit.

I repeat: these are dollar parts. Just abuse it and see if it fails.

[mac]

If I am picturing your verbal description right-- that is a wonky circuit.

It's based on this article.

http://gabevee.tripod.com/sstubepre.html

A booster I made long ago,



And a high gain distortion,

I repeat: these are dollar parts. Just abuse it and see if it fails.

I blew a TIP31C, unknown brand, and only at 200ma.
Now I have a ST TIP41C at 300ma, can be touched to warm my fingers... it's winter here

I'll draw a schem.

mac

[mac]

Schematic,

Fuse included at TIP collector in case it shorts.
A better place could be right after the bridge rectifier.



mac