1 Watt Mullard amp: form Ger to Si

[tca]

Hi,
I've just build a 1 watt Mullard amplifier from the Mullard book "Transistor Audio and Radio circuits" (1st Ed.) but replacing the germanium AC128 and AC127 transistors to silicon transistors TIP32 and TIP31.

The sound is ok, but can anyone suggest any modifications to optimize the use of the silicon transistors?

All the best.

[Elvee]

You should replace the interbase resistors by two 1N400x diodes.
Anyway, the circuit is not very safe without emitter resistors, it could very easily go into thermal runaway.

[toprepairman]

Compare to the 10W amp on page 122 Fig.76, and you have the answers.

[tca]

Actually on the first edition there is a 10W amp on page 39 (with AD162 ans AD162 also Germanium) and another 10W hi-fi amp on page 102 (fig. 71) with BD131 and BD132 which are Silicon.

The 1 Watt amp works in common-emitter configuration and the 10W that you refer is in common-collector configuration. Does the emitter resistors prevent the thermal runaway in both configurations?

I don't see how to adapt the circuit of the 1W amp, making it similar to the 10W, without changing all the circuit . This 1W amp has a relatively low part number, and that's the primary reason that led me into building it.

Cheers.

[mickeymoose]

Why are you wasting your energy and money on a design that is 40+ years old. Better designs with similar or smaller parts count and superior performance are abound. E

[tca]

I only have short answer to you question: for fun! My idea is to build a guitar amp from an old circuit, a transformer less amp, in the same spirit as the Deacy amp.

Could you point me some links to some "modern" 1watt amps with similar or smaller partsps? Thanks.

[toprepairman]

Hi TCA.
My Mullard book is the second edition so the page and fig numbers have shifted a bit, but the circuits discussed seem to be the same ones. The 1W amp using AC127/128 is clearly in common collector, the emitters are tied together and drive the speaker via capacitor C6.
The 'Deacy' amp you refer to is interesting. The name 'Deacy' is almost a certainly a simple corruption of D.C. as it's run off a battery, so as to distinguish it from the VOX AC30.
All I know 9of the circuit is what is implied from the text of the description.
It is a germanium design and uses one or two transformers.
There were a few similar circuits floating about in the early days of germanium which sort of mimiced valve designs. Some had a driver transformer and output transformer and some had just the driver with the output directly coupled usually via a capacitor.
The tonal (overdrive) characteristics derived from the use of the transformer(s) and germanuim transistors.
Replacing the germanium with silicon in a transformerless design is unlikely to give you the distortion characteristics you may be seeking, but thats no reasong to stop experimenting, keep going!
Regards
Henry

[poynton]

The crazy thing is that not many years ago, you could walk into any electronics shop and buy exactly what you want off the shelf.

I built several amps for schoolfriends with these ready-built modules.


Andy

.

[Elvee]

Originally Posted by tca

I only have short answer to you question: for fun! My idea is to build a guitar amp from an old circuit, a transformer less amp, in the same spirit as the Deacy amp.

Could you point me some links to some "modern" 1watt amps with similar or smaller partsps? Thanks.

Why not try something original for a change:

Here is an example based on a TL431 used as a supertransistor.
When built, it will look exactly like an ordinary discrete transistors amplifier, but with exceptional performances:

[toprepairman]

Ah, yes, the good old days.
They do say that nostalgia isn't what it used to be.......