Lab Series L5 Replacement Output Transistor and Bias Question

[parentheticalfact]

Hello!
I have a L5 Lab Series amplifier on the bench - shorted finals led to a replacement.
I don't have a lot of experience swapping transistors and knowing the sort of voltage drops I should be seeing across emitter resistors - I always refer to service manuals at this point for knowing the right way to set things.

I had a set of MJ15022 transistors and thought to give them a shot. They perform in the amp with good output, heatsinks get warm to the touch after 7-10 minutes but not hot. Original thermal grease replaced of course.

My 2 questions are
1) Would the MJ15022 be considered a good replacement for the original in this circuit?
2) Would replacing the output transistors with these (or another more ideal suggested part) affect the bias procedure?

My understanding as far as the bias procedure for the L5 is to measure 300ma across F21 (with fuse removed) but when that number is achieved (after warm up) the current begins increasing more and more which I understand to be a bit of a runaway situation.

Would it not be appropriate to bias this amplifier to read between 10ma and 20ma worth of voltage drop across the emitter resistors?

Thanks in advance for any insight here, I've been repairing tube amps for over 15 years but solid state I've only been diving into solid state power amplifier circuitry for 5 years and my training with these is just a bit less than mathematical at the moment.

[J M Fahey]

1) those transistors are excellent.
Basic bias procedure will stay the same.

2) that said, they give 2 bias procedures in the same sheet, in case of discrepancy IŽd go for the safest/most conservative one, always.
a) "300mA idle current for the full power board", what you mentioned.
That includes current through R304/316/331/Q309 , PLUS power transistors themselves.

Maybe it works as a quick and lazy way but looks like a too broad brush to paint with any precision.

b) in the very same schematic they show a more precise and specific value, just read the text by R338 and R345: ">3mV across each emitter resistor" which I trust way more.
Although it includes the "more than" sign, which is probably showing minimum value to kill crossover distortion, it does not follow that WAY more is fine.
3mV across .33 ohms is exactly 10 mA so 10 to 20mA which are standard values must be fine.
That would show as 3 to 6mV across emitter resistors, any of them, by their own admission.

Amp should be very stable under those conditions.

[parentheticalfact]

1) those transistors are excellent.
Basic bias procedure will stay the same.

2) that said, they give 2 bias procedures in the same sheet, in case of discrepancy IŽd go for the safest/most conservative one, always.
a) "300mA idle current for the full power board", what you mentioned.
That includes current through R304/316/331/Q309 , PLUS power transistors themselves.

Maybe it works as a quick and lazy way but looks like a too broad brush to paint with any precision.

b) in the very same schematic they show a more precise and specific value, just read the text by R338 and R345: ">3mV across each emitter resistor" which I trust way more.
Although it includes the "more than" sign, which is probably showing minimum value to kill crossover distortion, it does not follow that WAY more is fine.
3mV across .33 ohms is exactly 10 mA so 10 to 20mA which are standard values must be fine.
That would show as 3 to 6mV across emitter resistors, any of them, by their own admission.

Amp should be very stable under those conditions.

Thank you very much for that Juan!

Lastly, if the emitter resistors for Q311 and Q315 read closer to 30mV while the others all read right about 5mV - should that indicate to me a mismatch or other issue with those particular transistors? I should also mention that these resistors in the amplifier are .39 ohms, whereas the schematic calls for .33 ohms. Not sure if this is a common discrepancy or anything I should consider too different other than that it will make my current reading a little different.

[parentheticalfact]

Could this perhaps be a DC voltage leak from the driver transistors imposing upon the emitters of Q311 and Q315? These have not been changed and are the original RCA units. Perhaps these could be changed as well with the same MJ15022 transistors.

[Enzo]

All eight output transistors MUST be the same type. Do not mix them.

0.33 ohm or 0.39 ohm, doesn't matter, pick one, but all must be the same.

The four parallel output transistors on each side of the circuit have to share current, if you mix types some will hog current while others won;t turn on, so they will not conduct evenly.

From a service standpoint, if one transistor fails in a row, I often will find that is all we need to replace. But when I find all but one in a row shorted, I will replace them all because whatever stress took out 3/4 of them most likely stressed the remaining one.

[parentheticalfact]

For clarification, all 8 have been changed as a set. The original units that remain are the drivers, Q310 and Q309.
I made an effort to pick MJ15022 units that had matching numbers but I have not checked their data on a tester before installation.

[Enzo]

They will be close enough. Same part number/vendor, they didn't say date code.

I thought you said Q311/315 were remaining originals.

Note that Q311/315 emitter resistor shares current with the driver transistors. The remainder do not.

[parentheticalfact]

Enzo and J M Fahey, thank you, I've learned a lot here.
Amplifier is running smoothly