Brian May's Deacy Amp secrets revealed  [traced]

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Re: Brian May "Deacy Amp" Secrets Revealed

Postby lolbou » 14 May 2010, 19:53

This from a 1971 radioshack catalog: thanks to an archive linked in a thread here...

Capture.jpg


A classic topology indeed! :D

analogguru wrote:Funny....here:
http://www1.gitarrebass.de/magazine/0307/vox.htm
we can read
that it was only 0,45W
... what was a normal value for two AC 128 running on 7,5V like in this early philips cassette players:


Now, would a transformer designed for 350mW would work ok with a AC128 output stage? I can't make myself a precise idea with this datasheet. Shall I get that you can expect 1W from an AC128? And the hand-written note on the schem above mentions AC132 for replacements, counting for lower power, so would it suit better to my transformers?

Would the output power be bigger than 0.45W with 9V, far too powerful for the transformers I have?
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Re:

Postby teemuk » 18 May 2010, 18:37

This amp is basically a Brian May boost > the schematic I provided (opamps subbed for transformers) > into a SS 10W amp.


Late in, but what does that mean? --- "Subbed", how?

The provided schematic is that of a basic old transistor radio, I've seen countless variations of the idea. You can tweak that basic circuit quite a lot just by varying the component values to alter gain and frequency response. Are you saing the component values are exactly that as in the schematic provided? Are the transistor Germanium or Silicon? What operating voltages are they using? What is the 10W output chip? Is it plain voltage feedback setup or are there other tricks as well? It also has a treble booster? Which of the various trebel boosters is it? Any mods in it?

Anyone actually have a schematic of the Vox Brian May amp?
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Re: Brian May "Deacy Amp" Secrets Revealed

Postby RnFR » 18 May 2010, 19:21

are you talking about the white vox amp? i have one here somewhere, although it's not in working condition right now. i'm rather busy at the moment, but i could try and wrangle it up one of these days and take a look at it if you want. i seriously doubt that it is actually anything close to the real deacy though.
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Re: Brian May "Deacy Amp" Secrets Revealed

Postby teemuk » 18 May 2010, 20:03

Yep. That's the one.

I'm also quite sure that's it not like the real, original Deacy amp but it's kinda interesting to see what they put in there.
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Re: Brian May "Deacy Amp" Secrets Revealed

Postby RnFR » 19 May 2010, 01:19

i'll see if i can find it, but it might not be for a bit.
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Re: Brian May "Deacy Amp" Secrets Revealed - TRANNIE SPECS

Postby moonbird » 19 May 2010, 23:37

Hello all --

I might have something to offer here. Please see attached a scan of specs for a series of vintage transformers for era-correct transistor radios. It is particularly helpful because is shows DCR values as well -- allowing someone with an unknown transformer to guess at its specs.

What is needed now is some guru to help us correlate these two tables or instruct us on how to select impedences for the OT / Driver trannie from the transistor spec sheet you have. :?

Can anyone help out???? thx.
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Re: Brian May "Deacy Amp" Secrets Revealed

Postby slim_blues_boy » 23 Jan 2011, 20:47

I finally got transformers from old radio for this amp.
how about gain of the transistors?
is it necessary to use transistors in certain gain range? any specific gain?
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Re: Brian May "Deacy Amp" Secrets Revealed

Postby CovingtonPedals » 01 Apr 2011, 08:44

Hi guys....Greg Covington here....from Covington Guitar Effects.

I'm the guy who builds the Fireplace Booster.

Ah yes the Deacy Amp....back in 2006 I teamed up with Mahoney and got the first working versions of the "Orchestrator" up and running. In Jan 2007 we recorded that version of "Procession" at his home in Nashville. It was done on my homemade Red Special with a Fireplace booster....Chris layed the backing track with a bass drum filled with towels.

The trick with the Orchestrator was not only the tone, but getting the right volume. I found it to be a tricky combo of the proper transformers and speaker. Ted Weber (RIP) helped me with a modded speaker and it fell into place. We sold quite a few of those amps. These days I am pretty busy with pedals so I turned the entire Deacy project over to Mahoney who is producing them in conjunction with RS Custom Guitars.

This is what the Orchy sounds like in the "raw"....my son and the Red Special: http://www.youtube.com/watch?v=osAxfzZUwZk
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Re: Brian May "Deacy Amp" Secrets Revealed

Postby RnFR » 15 Jul 2011, 00:13

found this:

col:
The transformers are Maplins miniature Audio transformers. Maplins numbers are HX82D (LT44 Driver)and LB14Q (LT700 output). The input transformer is 20kohm primary and 1KohmCT secondary, the output transformer is 1.2KohmCT primary and 3.2ohm secondary. If you follow the deacy amp schematic you can see where to put the centre tap. For the actual output I put one of the wires to ground (I think they were both brown on these transformers), I don't think it matters which one and a 100k log pot between ground and the other output wire, the centre of the pot being output. I don't have the latest catalogue but in the last one they were £1.73 each.

col:
It was a 1 meg pot not 100k, and here is a link to my drawing;

http://s159.photobucket.com/albums/t159 ... ent=BM.jpg

Hope it helps

brett:
Col
according to the Maplins site, those transformers have high impedance e.g. 12k:32ohms.  Is it the DC *resistance* that is 1.2k to 3.2 ohms?

In any case, for reasonable power output, you don't want to step the voltage down too much in the output transformer.  That 12k:32 or 1.2k:3.2 transformer has a voltage step-down of 19 times (the square root of the impedance ratio).  So even if the transistors are managing square rail-to-rail swings in the signal going into the transformer (e.g. 9 V), only 0.5 V would be coming out the other side.  0.5 V and 3 ohms is only 80 mW of potential output.

The output transformer that I used in this amp was 48 ohms : 8 ohms.  As you can see, the impedance ratio is much lower, so the the voltage step-down is about right (2.5 times).  It might be difficult to find a cheap, good transformer.  Mouser has this one, with a step-down of 6, and nice, low DC resistance (the lower the DC resistance the lower the losses): 42TM004 (see http://www.mouser.com/catalog/specsheets/XC-600134.pdf for details).  Using a reasonable supply voltage will help (e.g. 18 or 24V).  Most PNP germanium output devices will tolerate this amount of voltage.  With small heatsinks, two AC128s should produce 1W without melting, though one will probably do more work and get hotter than the other..
cheers
Steben:
Quote from: brett on September 24, 2007, 05:32:38 PM

With small heatsinks, two AC128s should produce 1W without melting, though one will probably do more work and get hotter than the other..
cheers


That's because the gains are different. This results in assymetric amplification and clipping a la Fuzz Face.
With a matched pair it is more standard distortion.

brett:
Hi
Quote

With a matched pair
and equal loads and equal bias and equal heatsinks, etc

Only likely with modern Si transistors and circuitry. In my experience, the DC resistance of the halves of the secondary of the coupling amps can vary quite a bit. But maybe that's coz I'm only paying $2 to $5.

The 42TM series have 200mW max power...
To achieve 1W I think you need Hammond 149's...
It is indeed very difficult to find an affordable transformer I guess.







it's from DIYS.

and references this schematic-

http://www.diystompboxes.com/smfforum/i ... #msg481408
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Re: Brian May "Deacy Amp" Secrets Revealed

Postby electricb » 30 Dec 2011, 19:11

Sorry to bump an old thread, but has there been any progress on tracing or finding a schem for the Vox VBM-1? Seems like it could be easily adapted to a pedal if we knew what transistors and opamps would be used.
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Re: Brian May "Deacy Amp" Secrets Revealed

Postby caspercody » 31 Dec 2011, 00:20

Deacy Amp Circuit Overview:

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.
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Re: Brian May "Deacy Amp" Secrets Revealed

Postby RnFR » 31 Dec 2011, 02:30

electricb wrote:Sorry to bump an old thread, but has there been any progress on tracing or finding a schem for the Vox VBM-1? Seems like it could be easily adapted to a pedal if we knew what transistors and opamps would be used.

is that the little white solid state amp? it's been in my closet for years. I could take some pics, but it's not that great. the treble booster is shit, actually.
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Re: Brian May "Deacy Amp" Secrets Revealed

Postby electricb » 31 Dec 2011, 17:07

is that the little white solid state amp? it's been in my closet for years. I could take some pics, but it's not that great. the treble booster is shit, actually.


I would actually love that if you have a chance. What I would like to do is adapt this amp into a pedal format. so if possible, I would like to trace everything except for the 10W amp part of the circuit. (The Vox manual has a good description of the circuit flow ). It's unlikely I would ever use the treble booster on its own, but I would be curious to see exactly what values they used. A real Deacy would be great, but its tricky tracking down the transistors and transformers. If I could make a version with easily available parts, I would like to do so. I am particularly interested in the transformer and speaker 'simulations' in this little guy.
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Re: Brian May "Deacy Amp" Secrets Revealed

Postby Jack5 » 11 Jan 2012, 15:53

Hi,

I've modded an old radio (Astor FSW - schem here http://www.kevinchant.com/uploads/7/1/0 ... 31/fsw.pdf) to accept a guitar input signal from after the volume pot. The circuit from there has the same topology as the Deacy and similar to the Deacy it needs a treble booster upfront to give it some treble. I'm presuming that its because the input impedance is low and sucks the treble. I've tried with a buffer upfront and the treble is there also, not as much with a treble booster though.

What I'm wondering is why the pignose 7-100, which seems to share a similar topology, does not seem to suffer from this tone sucking. Hopefully pignose schem attached correctly here.

I'm a paint by numbers guy for the most part but trying to improve my theoretical knowledge. The obvious stuff I can see that might be impacting this are the size of the bias resistors on the first stage of the pignose (increased impedance?), the vol pot at the input (??), the 10uf cap on the first trannie's base (not sure about this but would have thought this increases impedance for low frequencies vis a vis that same cap 2.2uf on the Deacy, ie doesn't help increase treble component overall) and the 1nf caps (??).

Any help appreciated
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Deacy amp PCB request

Postby valvusmusicus » 12 Feb 2012, 20:00

Hi All
These are very interesting pages
Just wondered if anyone nad made a PCB for the "Deacy/Pignose" amp and could share it with the group

I might just go ahead and do it myself-but day jobs, lager and families take up alot of time!!?
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Re: Brian May "Deacy Amp" Secrets Revealed

Postby bancika » 18 May 2012, 13:17

not sure if it was posted before, here's some info from fryer guitars website, including the schematic and layout, and cab plans.

http://diy-fever.com/wordpress/wp-content/uploads/2012/05/1w_ge_amp.zip

Also, here's the eyelet layout I drew, please take a second look.

Image

Cheers
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Re:

Postby bancika » 24 Aug 2012, 12:43

soulsonic wrote:Little bit OT: seriously, about germaniums; I really love AC128s, but almost all the ones I've gotten lately have been crap... low gains - around 30 to 50 hFE... and when they do have a decent gain, they have ridiculous leakage. One of the ones I tested from the last lot I got had an hFE of 130 - but a leakage of 1mA!


I got 6 of AC128K from ebay (german seller) and all of them have right gains (around 120-130) but leakage around 1mA. How important is leakage for output stage? What about using some other germanium transistors for this, like GT308V (from bonamassa fuzz, I bought 50 of these)? What makes a transistor good for output stage? Those ruskies are super low leakage but only rated 200mW.

Thanks!
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Re: Brian May "Deacy Amp" Secrets Revealed

Postby deltafred » 24 Aug 2012, 18:56

Without looking up the data sheet I can tell you that the AC128 is an output transistor so will have a higher current/power rating. No idea what the GT308V but I would guess that it is a small signal transistor so will not be suitable.

Download the datasheets and check the current/power ratings of each. If they are very similar you should be ok to substitute them.
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Re: Brian May "Deacy Amp" Secrets Revealed

Postby bancika » 25 Aug 2012, 10:45

thanks, looks like AC188 could be a substitute. They have them in the local store.
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Re: Brian May "Deacy Amp" Secrets Revealed

Postby pedalgrinder » 06 Apr 2013, 08:41

hmmm seems to be very few if no secrets revealed in this thread. What i want to know is what exactly makes this thing trigger the 2nd and third harmonic distortion to make it sound almost organ like. The topology is everywhere. How it exactly works not like how does a amplifier work thats everywhere also. What exactly causes the 2nd and 3rd harmonic distortion. In theory this thing should be clean as a bell yet overdrive it and it spasms. I am also curious if it majoritly board orientated so a possible pedal could be made. I don't know how much the speaker itself played a roll. If any one can shed some light on these questions i'd be rather interested. Cheers
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