Silvertone 1465 output transistors

[n8b]

I have an old Silvertone 1465 that has one shorted output transistor. Does anyone have experience replacing these? The markings on both transistors are 9158 274 6529 and I can't find a cross reference. I was thinking of swapping in a couple of 2n3055's since I have a few of those on hand. Any help is appreciated. I'd really like to keep this out of the dumpster.

Here is the schematic from the Jack Darr book:

[Jazz P Bass]

I may be off base here, but I believe the output transistors are germanium.
Test the forward junction drop of the one that is good.
Germaniuim drops .3 volts.
Silicon drops .6 volts.
If they are germanium & you decide to go with silicon, you will have to adjust the bias.
Google germanium to silicon conversion.
I did this to a Kay, but I do not remember exactly what I did.
Changed a bias resistor, if I do recall.
One such link: Redesign Germanium Transistor Circuits, Part 1

[n8b]

I hadn't considered that they may be germanium. I'll check the forward voltage tonight. I need to verify the pin configuration anyway.

The rest of the amp works as well as it should. When I initially powered up the amp, the top half of the waveform was missing at the output. One of the swamping resistors (0.27r) was open due to the shorted transistor.

[R.G.]

They aren't germanium, they're silicon. The bias network puts something like 0.544V on the bases, way too much for germanium. Also, they're NPN, and biased correctly with the power supply to be NPN. There were almost no NPN power germaniums; the few that existed were too expensive to use in a Silvertone.

2N3055 is probably not going to work well in this. The power supply is nearly 70V, and the 3055 has Bvceo of 70V. They may be a higher voltage selected 3055, or another higher voltage type. Put in a pair (never replace just one of an output pair) of MJ15015 or one of the ON Semi '"281" types with extended beta. These will be much more reliable. The higher voltage and larger second breakdown area will make this more reliable.

This advice is pretty much the same as for "Bulletproofing your Thomas Vox Amplifiers" at geofex.com, because the output stage is very much the same.

[n8b]

Awesome, thanks for the info. I have some MJ types in stock from other repairs, so I'll see exactly what I have and give them a whirl. I'll also check out your article.

Edit: I have MJ15024's, which should work. I'll give it a try this weekend and report back.

[n8b]

I replaced the transistors with MJ15022's because I had more of them and was able to get a pretty close matched pair. I still had a 1.5v dc offset due to a 10 ohm resistor that had drifted to 22 ohms (at room temp). After replacing both 10 ohm resistors with metal oxide types, the amp is putting a clean 19v rms sine wave into a 5 ohm load at 400hz.

Thanks again for the help!

[nsubulysses]

Anyone have a schematic for this amp by chance ?

[DrGonz78]

I will post a couple other's that I have to scan but here is the 1465 (chassis 185.12010) schematic that I already had on file.

[cjenrick]

maybe ohm out those 615/5 w and maybe replace the .27 emitter resistors to get lowest offset voltage,

what kind of driver xfmr, any part numbers on that thing? just curious,

six 10"s holy smokes, batman, how does it sound?

anybody remember the Marshall stacks with six 10"s? Robin Trower, Twice Remove from Yesterday

[DrGonz78]

Here is the other version of this amp Chassis 185.12011, hope it helps.

[J M Fahey]

replace the .27 emitter resistors to get lowest offset voltage,

They do not affect offset voltage.

[DrGonz78]

Emitter resistors are there to balance out any differences between crossover points for a pair of output transistors, right? That makes the transistors essential match up better. I have seen DC offset balance pots on some amps but I can't remember where in the circuit you sometimes find those.

[nsubulysses]

The amp has two burnt resistors that I can't read the color code. They are not the emitter resistors for the power transistors. They are most likely base transistors that set the bias, is that right? Kinda guessing. Anyway the schematic will help a lot thank you.

Amp makes no sound and has two burnt resistors that have something to do with the output transistors. Does not blow fuses.

edit: It appears R28 and R30 are the burnt the resistors. 10 ohm 1/2W

[cjenrick]

it is always a good idea to replace those emitter resistors if you had a shorted transistor since they get smacked pretty good,

cheap way to do a good job on the output section which is responsible for 90% of transistor amp failures.
and you have the chassis opened up, you have the board removed, so why not do it now?

those emitter resistors actually have about 6 different functions, and in this design, there is more quiescent current flowing than in your typical quasi-complimentary design which can be verified by measuring voltage drop across the resistors with no AC signal applied. since you have more current, bias will be affected by the emitter resistor values in a larger way than if no current, (which is quite common in today's amps) so if bias is different for the output pair, and one transistor is on more than the other, you may have a few milli-volts of DC offset on the output. i would expect about 30 ma idle current out of this circuit. (based on the Acoustic 150 circuit)