Gibson G20 output stage issues

[Mars Amp Repair]

Hello folks,
This particular version of the G20 matches another on the forum that supplied schemo & parts list plus pix. The issue here is the driver xistor Q8 (BC1160) and the 2 power xistors Q2 & Q9 are bad. Q2 is a ER3055 (2N3055), BUT Q9, listed as a 2N4148, crosses to a thyristor. Since the ER3055 is fundamentally a 2N3055, I have to think Q9 must just be its complement. I tried using MJ15003 & 04's and am having issues with oscillation for which I'll just add snubber caps for, BUT, it still is acting weird-distorted. I used a generic amp driver of 2N5680 as it was the only top-hat PNP transistor I had.
Anyone have a successful experience using other xiistors for this particular configuration?

[52 Bill]

The 2N2148 is a germanium PNP transistor. Replacing it with a modern silicon transistor may require a resistor change to correct the biasing, so that it will not distort.

[x-pro]

1. BDW51C NPN & BDW52C PNP
2. 2N3055-- BD314
BDX18 has a small voltage Uc-e, so it will not fit.
I haven't had any experience with it, but after looking at the parameters, the first ones will do fine.
From Russian transistors are ideal (NPN-PNP):
2T819A-2T818A
2T819B-2T818B
KT819GM-KT818GM
(english letters are used in the title)

[Mars Amp Repair]

1. BDW51C NPN & BDW52C PNP
2. 2N3055-- BD314
BDX18 has a small voltage Uc-e, so it will not fit.
I haven't had any experience with it, but after looking at the parameters, the first ones will do fine.
From Russian transistors are ideal (NPN-PNP):
2T819A-2T818A
2T819B-2T818B
KT819GM-KT818GM
(english letters are used in the title)

Those Russian xistors appear to be of SI genus. Is that what you intended to replace the GE xistors?

[The Dude]

FWIW:

https://www.amazon.com/2N2148-BJT-10.../dp/B0861SLXHB

[Mars Amp Repair]

FWIW:

https://www.amazon.com/2N2148-BJT-10.../dp/B0861SLXHB

Thanx, I grabbed them. The NTE's are freakin' $40-55 depending on where you look. I'll take the chance on these for $10ea.
Also, I cannot find the NPN complement to the 2N2148. I'm more familiar with the Japanese transistors, so does anyone familiar with the Motorola xisotrs have any suggestions for an NPN GE T0-3 xistor?
probably be crazy, but I wonder if it's possible that they used the SI 2N3055 for the NPN side & the 2N2148 PNP GE for the other.
I'm going to try it as I have no other choice here.
Thanx again....glen

[x-pro]

I'm going to try it as I have no other choice here.

There is always a choice! For example, throw out those Si and Ge and put TDA2040. Of course, if you are good with a soldering iron.
https://www.st.com/resource/en/datasheet/tda2040.pdf

[Mars Amp Repair]

There is always a choice! For example, throw out those Si and Ge and put TDA2040. Of course, if you are good with a soldering iron.
https://www.st.com/resource/en/datasheet/tda2040.pdf

UH, Yeah...except generally folks who buy vintage amps, want them to be 'original' (;- )

[52 Bill]

probably be crazy, but I wonder if it's possible that they used the SI 2N3055 for the NPN side & the 2N2148 PNP GE for the other.

That's exactly right. When this amp was designed, they used a silicon NPN and a germanium PNP. In the early days of transistor amps, silicon NPN power transistors were easy and cheap to produce, while silicon PNP were harder and more expensive than the already existing germanium PNP power transistors. Germanium NPN power transistors were very expensive to produce.

I've seen Gibsons, Carvins, Heathkits that used this method. If you replace the germanium PNP with a silicon without adjusting the bias there will be a very noticeable crossover distortion at all levels of output.

[Mars Amp Repair]

That's exactly right. When this amp was designed, they used a silicon NPN and a germanium PNP. In the early days of transistor amps, silicon NPN power transistors were easy and cheap to produce, while silicon PNP were harder and more expensive than the already existing germanium PNP power transistors. Germanium NPN power transistors were very expensive to produce.

I've seen Gibsons, Carvins, Heathkits that used this method. If you replace the germanium PNP with a silicon without adjusting the bias there will be a very noticeable crossover distortion at all levels of output.

RIGHT-ON 52 Bill,
I had a suspicion about that mix. I do recall that the NPN GE's were more difficult and more $$ way-way, not THAT much, way back . I found a couple of 2N2148's online for cheap, so will give them a try.
Appreciate the info.
Glen