Brian May Deacy Amplifier Build

dsgb

Hi everybody

I recently started a winter project to build a replica of Brian May's legendary Deacy amplifier. The goal is to make the amplifier replica, film and photograph the process, release a series of three YouTube videos and produce a project page on my website, http://dsgb.net/

In the introductory video I briefly discuss the origins of the Deacy amp circuit (salvaged from a vintage transistor radio that John Deacon found in a London dumpster in 1971) and what Queen tracks Brian used it to record on. I then move on to talk about the new Knight Audio Technologies (KAT) amplifier board kit and the other component parts required including suitable vintage and modern woofer and tweeter loudspeakers and the cabinet itself. I finish by introducing the only official Deacy amplifier replica which was made by Knight Audio Technologies between 2011 and 2019.

https://www.youtube.com/watch?v=Bdxc_GIqIfE
I will update this thread as the project progresses. I will be working on it at the weekends and will begin filming the first few sequences for part 2 "amplifier circuit assembly" this weekend. These will include 'unboxing' the kit and discussing relevant aspects of the key components. I have acquired all the props and test equipment I need to start assembling the kit and filming those sequences. In parallel, I am working up the CAD files for the cabinet and have taken delivery of a 1/2" thick chipboard sheet and two sheets of Sapele marquetry veneer for part 3 "cabinet, final assembly and testing". However, don't expect that the final video will be a masterclass in cabinet making!

Please check out my website and media on Instagram, Pinterest and YouTube. I am trying to grow my YouTube channel organically without begging people to like and subscribe to it as seems to be custom and practice. However, some of my videos on Brian May's guitars and gear (e.g. pickups) have been relatively costly and time-intensive to make so your support would be appreciated.

Doug

Chris.B

Very interesting part 1 video & looking forward to the further editions.  Now subscribed to your YT channel.

PeteC

Really interesting. I love recording using lots of different amps so I have always been intrigued by the story of Brian’s little recording amp and the amazing tones they got from it in the studio.   

  I have always wanted to build one to use for recording, but wanted to do it from scratch rather than using a kit.  I gave up when I couldn’t find a reliable schematic and sufficient info to etch my own pcb.    Will be good to see how it turns out.  

dsgb

PeteC said:

Really interesting. I love recording using lots of different amps so I have always been intrigued by the story of Brian’s little recording amp and the amazing tones they got from it in the studio.   

  I have always wanted to build one to use for recording, but wanted to do it from scratch rather than using a kit.  I gave up when I couldn’t find a reliable schematic and sufficient info to etch my own pcb.    Will be good to see how it turns out.  

Hi Pete.

I was accumulating and stashing components with the intention of doing the same as you - building my own and possibly etching a PCB. Much more satisfying than assembling a kit. But then Nigel made his kit available, kind of out of the blue. He normally tips us (BM forum community) off when he has things in development rather than just launching them on us and hoping they eventually sell.

For the last year my interest has been in making BM related stuff and making videos about the process to add to the content on my website and YouTube channel so the kit was ideal really. It reduced the workload and gave me something potentially more interesting to put in a video. Plus, for the few like you and me who are relatively content to make a thing from scratch, there are scores who just want to watch a video and buy a product shown in the video.

Doug

LastMantra

Interesting stuff. 

I made an amp from a valve radio my friend was throwing out once. It sounded quite sweet actually, not very loud.
Nothing to do with the deacy one though. 

dsgb

I have completed part 2 which covers unboxing of the components and assembly of the circuit board.



The video is 21 minutes duration with fully time-indexed segments in the description. It is aimed at novices in electronics assembly (as I am) so assumes no prior knowledge of transistor circuits, electronic components or indeed guitar amplifiers.

Next up is making the cabinet. This is likely to take me a while and with the holiday season fast approaching, I can see this not being complete and released until January 2023.

Doug

ICBM

This is very interesting.

Do you have a view on how close the Pignose amp circuit is to the ‘Deacy’, and whether it would be feasible to mod one? If I remember rightly the schematic isn’t too far away - it's the same sort of two-transistor preamp with a coupling transformer driving the push-pull pair, and I suspect both are derived from the same original (Mullard?) circuit. I don’t know how much effect the component type/quality affects it either.

dsgb

ICBM said:

This is very interesting.

Do you have a view on how close the Pignose amp circuit is to the ‘Deacy’, and whether it would be feasible to mod one? If I remember rightly the schematic isn’t too far away - it's the same sort of two-transistor preamp with a coupling transformer driving the push-pull pair, and I suspect both are derived from the same original (Mullard?) circuit. I don’t know how much effect the component type/quality affects it either.

Sorry, I'm not familiar with the Pignose amp circuit.

The Deacy amp circuit is extremely easy to replicate in theory but in practice, even if you can source the four transistors with those specific Hfe gains which is difficult enough, I understand from the most knowledgeable people about the Deacy amp in the Brian May enthusiast community (Manuel Angelini, Mark Reynolds and Lee Speight) and Nigel Knight that replicating the transformers is very important. This is why so many have failed to make a plausible Deacy replica from scratch. I measured the ones in the kit and, as expected, the inductances and DC resistances of the windings are out of specification from the ideal values quoted on the Supersonic PR80 radio circuit diagram.

Nigel Knight has a supply chain for custom components (speakers, transformers, screen-printed panels, etc.) so it wouldn't have been a difficult endeavour for him to get say 100 sets of custom transformers wound to the exact specification that are in Brian's own original Deacy amp. Others have investigated this and had quotes in the low £hundreds for one set.

I would say that it isn't worth bothering, but then I was lucky to get a KAT Deacy amp in mint condition at a substantial saving on U.K. eBay in 2014 so I don't have any yearning to replicate it other than for a project. If I could be bothered, it would be interesting to make a Deacy circuit with similar transformers to see exactly what difference to the tone it does make.

The way the waveform clips smoothly when overdriven is very specific and very pleasing with this amp. Even though I have an official one, I was still pleasantly surprised at how beautiful the treble-boosted tone of the kit amp is through a Red Special guitar. It's almost like it's just on the verge of breaking up even when kept clean, like the Gilmour thing. I tend to believe the more knowledgeable people that it's not easy to replicate this characteristic with any old similar components.

Doug

ICBM

OK, thanks for that - it sounds like getting another old Pignose and attacking the circuit isn't a short-cut to a Deacy then .

PeteC

Very interesting.  Thanks for the info.  

  The closest i have come to replicating the tone was by using a rangemaster clone into the front of a Marshall valve PA - but into the mic input which had a transformer input stage with an EF86 preamp tube.     It was very close indeed.    I dont have the amp now sadly but I do still have the mic transformer    

I am sure there are folks here who could wind pretty similar TXs given the skilled pickup winders we have on the forum.   I might experiment with this circuit a bit this winter and see whats what.  

dsgb

PeteC said:

Very interesting.  Thanks for the info.  

I am sure there are folks here who could wind pretty similar TXs given the skilled pickup winders we have on the forum.

Thanks Pete.

Replicating the transformers would of course require the items in Brian's original Deacy circuit to be evaluated which Nigel Knight has thankfully done but yes, winding something similar should in itself be straightforward for a skilled pickup winder. I have never heard a Deacy replica with generic type transformers to assess how different it sounds.

Mark Reynolds gave me some feedback (no pun intended) on my video. I prefer the lower boost level of a germanium transistor treble booster with the Deacy but apparently that's not optimal.

"By the way, with your KAT kit, use the [Fryer TB] Super or [Cornish clone] BC149 type booster. The Rangemaster type you used would be a bit weak for the Deacy. It's more to do with the sustain reaction and saturation."

"Also the flat [7.5 V] battery level is a must for that smooth edge"

This is why the Deacy Amp Battery Simulator (DABS) is a must-have accessory. Brian May prefers the tone when the PP9 battery is almost flat.

Doug

dsgb

An update on this project. I took a couple of weeks break over the festive season then resumed work on part 3 last week. I have successfully cut the cabinet panels from chipboard. The dry fit looks tight and I now need to make a jig to allow me to rout the rebates in the mitred edges to accept 1/8" thick oak splines. I have no particular woodworking skills so this will be somewhat laboured.

I'm aiming to get this third and final video complete by the end of February 2023 but there is still a lot of work to do including veneering, staining and varnishing the completed cabinet then final assembly. All this has to be filmed and a voiceover added so it might run into February. If so, that will be two months without releasing a YouTube video. I have approached Andy Guyton of Guyton Guitars to ask him to provide original high resolution photographs of the guitars he has made for Brian May to compile into a video so hopefully that will fill the gap.

Doug

dsgb

A quick update on part 3 of this Deacy amplifier replica build project.

I glued up the cabinet panels before taking a week's vacation. I'm aiming to get this done in the next week or two; it has taken a lot longer than I anticipated because I have had to make a jig to rout the rebates for the mitre splines and make some panel alignment blocks to assist with gluing up. Here is a quick animated gif of my final dry fit test before gluing.



Doug

dsgb

The cabinet scratch build is complete and only final assembly. Part 3 of my YouTube video series should be published at the weekend.

Doug

roberty

That's awesome. I am enjoying this build thread 

PeteC

Very nicely done.  Looking forward to hearing some soundclips/youtube content.  

dsgb

O.k. guys and gals, that's the winter-long project complete. Link to the Part 3 YouTube video below:

https://www.youtube.com/watch?v=5E_zpUDWBpE

Pete - I ran out of time and energy to do a demo on this video, although that would have been a suitable way to close it out. I tried it out once the unit was complete and, perhaps unsurprisingly, it sounded very similar in tone to the official KAT Deacy replica. It is louder and there is some detectable fizziness to the output when the input level is very low (i.e. plucking the strings gently with fingertips) but it sounds killer when played striking the strings normally. I consulted Nigel Knight and he replied:

"The difference in level is due to the transistors! The old TO1 can devices have far more current drive than the units we could re-tin for RoHS compliance! The fizz is almost certainly down to the relative efficiency of the two speakers along with the choice of crossover cap. Try putting a 500 Ohm pot in series with the crossover cap and back that off till the fizz level is acceptable!"

I had an idea to try connecting the KAT Deacy and my version to a TC Electronic G Major 2 unit and a treble booster of course to see how they sound in a stereo chorus configuration. That will have to be for another day and another video.

Doug