Mullard-"Deacy" amp redrawn for you! (schem)

[Steben]

For your pleasure! Supposed to be 1W and brian may-like with treble booster in front.
I redrew the schematic to clearer numbers as found on soulsonic's gallery.
I'm not proclaiming this is THE magic amp. I haven't built it. But it sure is "sexy": germanium, push-pull transfo power, solid reputation...

[gnognofasciani]

Thanks Steben!

What about t1 and t2 specs?

[Pedal love]

I built this it sounded ok. I was surprised at how low the volume was, but I might have used the wrong transfromers.

[Steben]

I built this it sounded ok. I was surprised at how low the volume was, but I might have used the wrong transfromers.

What did you use?
Tried overdriving it?

The Deacy Amp circuit is a 1950s audio style Germanium transistor push-pull circuit, utilising in its front end an AC125 and AC126 respectively, with the push-pull Output stage comprising two AC128 Germanium transistors.
It also features a Driver Transformer and an Output Transformer and is similar to several designs of the time, which can be found in the Mullard Reference Manual of Transistor Circuits (see pages 168, 170 and 171).
John Deacon slightly altered the front end of the amp to allow it to better suit guitar frequencies, and it will be amusing to many people to learn that this design had been originally created to run cleanly and undistorted in audio use - a very far cry from the way Brian May turbo-charged the little amp for guitar use!
The speaker box contains a 6.5" 4 ohm twin cone driver speaker and a small tweeter speaker which is no longer working The Deacy Amp is powered by a large 9 volt PP9 battery and its power output is approx one watt.

interesting...

[col]

I built an NPN version of this using AC141 transistors and the audio transformers from Maplins as suggested by others on here. Mine had plenty of volume and a HUGE sound, I just haven't boxed it up yet. I posted a stripboard layout for the npn version but my posts are digital photos of hand drawn layouts, it should be readable. If I was to build another I'd put it on a much bigger board so that I could mount it properly, my usual (very strong!) double sided sticky pads will stuggle with this one due to the weight of the tansformers. A search should get it

Col

[MetalGod]

what spec are we looking at for the transformers? - I'm gonna have to try this one.

[km-r]

what difference would it make with modern SS amps?

[soulsonic]

what difference would it make with modern SS amps?

What do you mean?

[km-r]

why should i prefer this circuit than a modern LMxxx amp?

[soulsonic]

Because it uses transformers and germanium transistors, therefore it will sound very much different than the usual amp-on-a-chip things you see nowadays in this power range. The transformers alone are enough to give a HUGE difference in the sound.

[Steben]

I guess the build-up of six elements (4 stages and 2 transformers) gives a higher transition between clean and "turbo", resulting in wider range of sounds. The LMxxx series are one-stage.
Germanium has smaller bandwith(less high at hi gain), bad for hifi, nice for smoother guitar.
But never underestimate those output transformers. They distort too. Even "tube sound" often is determined by those.

[Steben]

I built an NPN version of this using AC141 transistors and the audio transformers from Maplins as suggested by others on here.

search for...? hammond xxxxx? 

[joelap]

I'm curious now about a pedal implementation of this... I wonder if using a step-down transformer in place of T2 and using no speaker (just having an out jack) would make this suitable to be used in a pedal form, into your own amp?  Or take the output off of T1 and skip the poweramp stage maybe?

[Pedal love]

What did you use?
Tried overdriving it?

I think I used a 1:1 interstage transformer of some sort. and I'm not sure of output transformer. Obviously the wrong ones. Its been years.
Saturated it sounded pretty good. What can I say? All of those germaniums.

[soulsonic]

I'm curious now about a pedal implementation of this... I wonder if using a step-down transformer in place of T2 and using no speaker (just having an out jack) would make this suitable to be used in a pedal form, into your own amp?  Or take the output off of T1 and skip the poweramp stage maybe?

T2 is a step-down already.

The easiest simplest way to make this a pedal is to put a little resistor in place of the speaker and then a simple little voltage divider with a volume control for an output  - basically the same way you'd slave the output of a big amp.

[Steben]

I guess the best way to make a small-signal version of a transformer is a band pass filter in combination with a sort of tape saturator (pot+diodes). Simply omitting the transformer will surely sound different.

Im' not sure how important the push-pull stage is.

[Steben]

I've looked up some transformers: Hammond 149H(driver) and 149Q(output) could do it, but they are not cheap.

Listening to sound samples, I wonder how close the old "Fat germs" idea comes to it.
http://www.angelfire.com/droid/burvenich/fat_germs.htm (sorry for my playing 3-years ago)
Maybe I should refresh this thing... I'll rebuild it...

[brett]

http://www.diystompboxes.com/smfforum/index.php?topic=53860.0
Vanessa and others discussed this some time back.

I built one, including the transformers, and thought this sounded good, even when built with silicon transistors.  However, a clip I posted only got one comment, so it didn't excite too many people.  But maybe my poor playing or a poor recording didn't help?

To me, there was some compression (maybe due to output transformer characteristics?), a nice hump in the mids (probably due to the limited bass thru the coupling tranformer), and some nice harmonic distortion (output transformer and distortion cancelling?).  The hardest parts to get will be the transformers.  The coupling transformer must have a fairly wide frequency response (ideally 160 Hz to 8kHz).
cheers
If anyone is interested, I'll check Mouser for suitable part numbers for the transformers.

[Steben]

Fat Germs!....
redrawn and up in Aron's vaults... 



Has a lot of deacy-in-a-box...

[rocket]

I built an NPN version of this using AC141 transistors and the audio transformers from Maplins as suggested by others on here. Mine had plenty of volume and a HUGE sound, I just haven't boxed it up yet. I posted a stripboard layout for the npn version but my posts are digital photos of hand drawn layouts, it should be readable. If I was to build another I'd put it on a much bigger board so that I could mount it properly, my usual (very strong!) double sided sticky pads will stuggle with this one due to the weight of the tansformers. A search should get it

Col

WHich transformers do you mean exactly?