Marshall amp tremolo w/ 2G374 transistor. Part substitutes?

[soapfoot]

I have a 1970 Marshall Bluesbreaker (1961; the 4x10 version) which makes use of the 1987T schematic (attached)

The tremolo hasn't worked in awhile, and I suspect the 2G374 transistor (germanium).

I have a hunch that a leaky old Ge transistor might be important in this circuit, since it's being used to short out the audio periodically... the old Marshall tremolo was weird, but it was still mostly a "tremolo" (not a repeat-percussion effect!)

Would an OC44 work? Other recommendations? (especially something widely-available).

Thanks in advance!

 

[Gus]

I wonder why they designed it that way?
There is no power to the transistor except from the AC coupled signal from the plate of the trem tube to the base node by the 68k, .1uf and 4.3k? (68k and 4.3k? are a voltage divider).
Also there is a pull down 150K from plate to ground
The base has a path to ground by the 4.3K? resistor
The collector has a path to ground by the 50k potentiometer and 33K
The transistor is .1uf cap coupled
So it looks like the transistor turns on with 1/2 of the waveform from the trem tube and the transistor and 10k and 50Kk control shunt the volume control wiper to ground.

Found this
https://alltransistors.com/transistor.php?transistor=190

https://www.radiomuseum.org/tubes/tube_2g374.html
http://forum.metropoulos.net/viewtopic.php?t=27961
I am guessing you tried another tube
Sometimes the caps in phase shift oscillator circuits can go bad have you checked the 4 caps in that section?
Have you checked the resistor values in the trem section?

Leakage is from reversed bias collector base
Maybe for fun if you can't find a replacement or good sub try a high beta PNP(2N5087 etc.) Si with a high value C to B resistor (100meg or more?)

 

[soapfoot]

Thanks for the thoughts!! A lot to investigate there; I haven’t checked the caps or resistors yet but have checked the tube.

When working it’s an unusual sounding tremolo—and I’m not often using tremolo with Marshall amps. I’ve let it sit non-working for a few years now because it’s been low-priority. But still, it’s nice when the amp is working 100%

 

[user 130681]

Germanium transistors, because of the way they were made, have a useful life of around 25 years.
The tin used to 'cement' the collectors to the case grow tiny "fingers" that cause intermittant faults.
I would fit a 2N4403. Standard pnp silicon transistor, if the oscillator is running as it should. Otherwise you are wasting your time and probably causing issues.
Scope the anode of the ECC83 to check.

 

[Whoops]

The tremolo hasn't worked in awhile, and I suspect the 2G374 transistor (germanium).

Did you remove the transistor and test it?

A Peak Atlas or even a cheap Chinese transistor tester will be able to test it and even do an indication of the leakage amount.

If it tests bad and it's shot I would try to replace it with a simple AC128,
easy to find. I can send you one for free if that's the case

 

[soapfoot]

Did you remove the transistor and test it?

A Peak Atlas or even a cheap Chinese transistor tester will be able to test it and even do an indication of the leakage amount.

If it tests bad and it's show I would try to replace it with a simple AC128,
easy to find. I can send you one for free if that's the case

Thanks for that!

I may ask that favor in the future... this project has been sidelined as I had an Ampeg B-15 fall into my lap that needs a total teardown and rebuild, and that's occupying my attention at the moment!

 

[Tubetec]

Dont forget you need a shorting footswitch to engage Trem on 1987T

 

[Gus]

Somewhat related
I looked at the schematics in the "The History of Marshall the illustrated story of "the sound of rock"" book
and noticed a number of the older amps used the circuit in this thread.
I also noticed a reissue 1962 std trem amp schematic has a JFET used with a different tube circuit driving it.

 

[CJ]

Did this ever get fixed?

I just fixed the same problem on a Marshall Super Tremolo by simply moving the grid (green) lead to a different place , finicky circuit indeed.

 

[soapfoot]

I ended up getting distracted by a million other more pressing matters and this one fell in the cracks.

I’ll get to it sooner or later!

 

[CJ]

me too,

 

[Whoops]

Is that a Line 6 Spider amp?

That one comes from factory with a really high power tube bias, you also should replace the stock rectifier tube.
Also replace all the 10 preamp 12AT7 tubes for 12AX7.
Then you will have the best Tube sound ever