replacement / imporoved trannies for pre amp

[makaze808]

Hello, I have 70's marshall mercury which has a trannie pre amp as seen here
http://www.drtube.com/schematics/marshall/2060.gif

Any trannie replacemnts you can think of that might prove useful ??



Thanks.

[R.G.]

Hello, I have 70's marshall mercury which has a trannie pre amp as seen here
http://www.drtube.com/schematics/marshall/2060.gif

Any trannie replacemnts you can think of that might prove useful ??

Is there something wrong with the ones that are in there?

[makaze808]

It was very noisy with buzzing and hum so I'm renewing all the eleectrolytics, a couple of blown ceramics and lining the cab to cage the amp.
The preamp doesn't from I hear push the power tube much so I suppose I'm thinking I'd like more gain, if it still proves a little tame. Also I was thinking 40 years on there might be quieter alternatives to the bc180's.

Any help appreciated.

[PRR]

No, the circuit is brilliantly designed for transistors like these. The transistors do what the resistors tell them to do. The resistors were picked to make the most of a transistor and the other design decisions. I can't think of anything which would be better, and a lot of ways to mess it up with transistor swaps.

This amp is also the cheapest piece of work they could get away with that year.

> buzzing and hum

Yeah, cheap caps 30+ years old. Fix them!

Transistors don't hum or buzz.

C25 could perhaps be 47u 250V. I might take C17 to 100ufd 300V. C18 is cheap, but so is the rectifier, and I would not muck with that area without careful study. The EL84 should be fairly low ripple with the one diode to 32uFd. If it really won't idle silent, move the OT hot lead to C17, that will be silent.

> push the power tube much

Old cheap caps gone bad. C12 has to be healthy. With C12 bypassing T2 properly, there should be plenty of gain on tap.

Check the voltages on the collectors. W-I-D-E variation is fine (that's part of this amp's design brilliance). They should be above 20V and well below 40V.

[R.G.]

It was very noisy with buzzing and hum so I'm renewing all the eleectrolytics, a couple of blown ceramics and lining the cab to cage the amp.

Good approach. You may want to read the stuff on geofex and here about grounding. Wire routing and grounding will have more to do with hum and buzz than transistors.

The preamp doesn't from I hear push the power tube much so I suppose I'm thinking I'd like more gain, if it still proves a little tame.

Those preamp circuits depend much more on the resistor values to set gain than the raw gain of the transistors. You'll want to make the 6.8Ms and the 220Ks metal film; that will lower any resistor hiss as much as possible.

Also I was thinking 40 years on there might be quieter alternatives to the bc180's.

There are, but not terribly. The BC184 is still made, and with modern processing is very quiet. I'd have to check on whether the (few) quieter and newer transistors can take the voltage in the circuit. Special low noise transistors tend to be low voltage as well, so it's possible that the quieter ones can't take the power supply, or its variations.

But it may make sense to simply replace the BC184s that are there now. Bipolar transistors degrade over time if the circuit they're in lets the base-emitter junction get reverse-broken, even once. This circuit has one of the hallmarks of that, big capacitors on the emitters. So simply swapping for fresh ones *may* help for hiss, not hum or buzz.

[makaze808]

Thanks very much for that I'll  follow the advice and links.

[makaze808]

Would 1 watt metal films be OK for the 220k resistors or am I better with 2 watts?

[R.G.]

Given that the transistor stuff operates at low voltages, you're probably fine with 1/4 or 1/2 Watt.

[makaze808]

Thanks.

[makaze808]

Hi, i've done the cap and resistor swaps and put new trannies on via sockets.

Last job was to wire up the OT.  There are two wires, yellow and black going from the OT to the speaker terminal.
The soldering on these was very poor which makes me wonder if they were correctly positioned. 
On the terminal strip the black wire from the OT was connected to the red + wire for the speaker while the yellow was connected to the back - speaker wire.

Would an OT really be made with a black output wire for + and yellow for -  ?

Is there a way to establish the correct way other than using my ears?

thanks.