Darlington Amps / Fender Stage SE

[tbonuss]

Howdy,

I've gotten myself into one of those problems where I just go round and round and don't get anywhere.

I repaired about a billion faults on a Fender Stage SE (schematic attd). Currently (and hopefully finally), I've found a fried darlington transistor 127 / 147 pair. Its obvious that transistors have been replaced a number of times throughout the power section. So...

Given that this amp isn't worth a whole lot, can I use the smaller pkg. 127T and 147T's I have on hand? I can physically make them fit, and the datasheets seem to be the same (in most but not all places).

Maybe reduce the collector current?

The amp as designed has way more power than anyone uses these days. They seem to max out between 3 and 4 on the volume knob. Cutting the power in half would not be terrible, and might even make the amp more usable.

Oh, BTW, it has a 16 ohm instead of the 8 ohm stock speaker.

Thanks in advance for any help at all.

Fender Stage 112 SE.pdf

[vintagekiki]

https://music-electronics-forum.com/forum/amplification/guitar-amps/maintenance-troubleshooting-repair/44469-fender-112se-bias-setting
Fender 112SE BIAS setting

https://www.eevblog.com/forum/repair/fender-deluxe-112-repair/
Fender deluxe 112 repair

https://www.diyaudio.com/forums/instruments-and-amps/209691-fender-solid-amplifier-bias.html
Fender solid state amplifier bias

1)
https://music-electronics-forum.com/forum/amplification/guitar-amps/maintenance-troubleshooting-repair/25865-
Fender Stage 112 se trouble

[J M Fahey]

Just use the proper TO247 ones.
YouŽll have to anyway, when the tiny replacements burn.
We are talking some $6 in parts

[Enzo]

16 ohm speaker instead of 8 ohms? it is already at half power then.

Please don't stuff tiny versions of the outputs into the amp. You don't feel the amp is worth much? Just putting smaller parts in it won't reduce its power, it will just make it less reliable. Without reworking the rest of the circuit, that won't make it less powerful anyway. And that rework will cost you more than any "savings" on a few transistors.

Besides, those larger TIP142/147 are used in many Fenders and are drop-in replacements for the BD65 or whatever they are the MArshall amps used. Just good to have on hand.

[Jazz P Bass]

If you look closely at the separate datasheets you will see that the smaller package power derating/ temperature is very small.
Meaning you cannot pump the same wattage through it as compared to the non T units.
As they may very well work, it is not a reliable fix.

[tbonuss]

Thanks Vintage Kiki--that is very helpful information on the biasing. I've never had to bias a transistor power amp that I can remember, so doing that and working with the Darlington transistors is a "good" learning experience

Thanks JMFahey--I've ordered the proper TO247's. Thank you for specifically identifying that--the TO247 packages look different at a lot of the vendors than the ones I have (the non-heatsink side is completely covered in some, unlike mine). BTW: If you know where to get them for a buck a pop please let me know.

Thanks Enzo--I didn't know that these were common parts, thank you for that--makes the added expense worth it. BTW: Sorry, I didn't mean to imply that I thought using a smaller transistor was going to reduce the power.

Jazz P Bass--thank you. I guess that is what I was originally trying to figure out. The datasheets that I found were inconsistent on that point--or at least I became very confused by the whole thing. I think they had the different packages cross linked or something, and the TO247's simply don't look like the ones I have. I don't know...at any rate, thank you!

As always, I appreciate the help and insight from you all.

Steve