Old Guitar Preamp - First Stage - Please review for issues

[alexage]

Hi guys - I hoping a few of you could review this circuit and let me know if there are any obvious problems with it. I have an old (late 60s early 70s) guitar amp that is all tube except this circuit, the first stage.

The amp has been butchered and I'm attempting to get it back together and working. My concern is that someone may have changed things in this first stage. There are two channels that were probably identical to start with, it looks like the only changes have been small resistor values.

The transistor is a 2N1273 PNP in a TO-5 package. In a socket, no less. I found this data for it:

Ge PNP Lo-Pwr BJT
V(BR)CEO (V)=15
V(BR)CBO (V)=15
I(C) Abs.(A) Collector Current=200m
Absolute Max. Power Diss. (W)=250m
I(CBO) Max. (A)=14u
@V(CBO) (V) (Test Condition)=12
h(FE) Min. Static Current Gain=27
h(FE) Max. Current gain.=165
@I(C) (A) (Test Condition)=50m
@V(CE) (V) (Test Condition)=1.0
h(fe) Min. SS Current gain.=18
@I(C) (A) (Test Condition)=1.0m
@V(CE) (V) (Test Condition)=5.0
@Freq. (Hz) (Test Condition)=1.0k
Package=TO-5

The stage is powered by the same voltage that biases a pair of EL84 output tubes, I'm guessing about -15 to -25 volts. I haven't got the power supply straightened out yet, so it's not been fired up.

I created the attached schematic, since none could be found. I'm pretty familiar with tube circuits, but am a solid state moron. Hopefully the CBE leads are indicated correctly!

So, please let me know:

1. Does this schematic look okay (functionally)?
2. What might be a good replacement for the 2N1273 should they be bad?

Thank you!

[nigelwright7557]

A positive supply is missing.
I suspect it goes to the + of the 50uf capaciitor.

[alexage]

Can't PNPs be powered by a negative supply voltage?

[nigelwright7557]

Originally Posted by alexage

Can't PNPs be powered by a negative supply voltage?

Yes but it also needs a positive supply or a connection to ground.

[bob91343]

Not a stable circuit. The emitter should go to ground but the bias will wander with temperature.

I'd suggest gutting it and starting from scratch.

[alexage]

Thank you, Nigel and Bob! I made a mistake of course. there is a ground connection. I missed it because t was to the chassis through the input jacks.

Attached is a revised schematic.

Does this look more stable?

Thanks.

[alexage]

In regard to gutting it, I was hoping to get the thing up and running in it's as-designed state, sort of museum project. I can't find another example of this make/model anywhere, save for one instance where a used one was sold years ago, and for all I know that's the same one I have now. That may not explain why I treat it like an endangered species. Who can explain these odd obsessions?

One more question - supposing I was to gut and replace, can someone tell me what the approximate voltage gain of this circuit is?

[bob91343]

No, the stability problem lies in there not being an emitter resistor. So as the E-B junction warms up, its voltage drops, causing more current flow and the possibility of saturating. You need to swamp that with a resistor in the emitter lead. That will reduce the gain so you should bypass it with a large capacitor.

As it happens, the 100k resistor to the neg rail will limit the current so there is no danger of burnout, but it takes only a few microamperes to saturate the transistor.

You don't indicate the magnitude of the V- rail. If the transistor approaches saturation, the C-E voltage will drop and thus reduce the current, but frankly it's not too good a circuit. It will most certainly add plenty of distortion, which will change with temperature.

[djoffe]

This kind of stage could work just fine...the collector-emitter voltage is about 30 times the nominal Vbe, probably about 30*-0.6=-18 V. Whether or not it will be happy depends upon the V- voltage. If there's enough V- voltage, then as the Vbe changes with temperature, the change in collector voltage won't be a problem. Some of the values are a bit suspicious...It might help to look again or to have some confirming information about expected/spec'd input impedance and sensitivity or gain.