Brian May's "Deacy" transistor amp

dougsta

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I agree, that's the sound and the touch too! Great playing.

But I did notice that the guy doing the demo just in passing...

( note: and this always seems to be the case when folks are talking about other famous guitar players' signal chains- they glide over something that may be integral to their sound- like Angus Young or Rory Gallagher- it's NOT just guitar >amp...)

... says " Brian always used this with a Treble booster, which is what we are doing today.." ( right?)

This ( IMO) is no trivial bit of information, as doesn't using one of these greatly influence how the amp is going to sound?

What do things sound like w/o using the Treble booster?
I've never used one, but aren't they really powerful, and greatly influence sound?
Just a curiosity

On the subject of Rory Gallagher (he had an AC30 in the early 70s that was modded to use EL83 valves) I bought 4 of these NOS Tesla PL83 'Made in Czechoslovakia' to try out, only £10 inc shipping. The 'P' means 300mA and a datasheet voltage of 15v, they were used in TVs with a series heater chain, I've tried them on 14v wired in parallel in a modded JCA22H and they sound sweet. Also tried with some 50's Mullard PL83 valves too, equally sweet :)

Screenshot 2021-09-30 at 17.44.38.png
 

Chiogtr4x

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On the subject of Rory Gallagher (he had an AC30 in the early 70s that was modded to use EL83 valves) I bought 4 of these NOS Tesla PL83 'Made in Czechoslovakia' to try out, only £10 inc shipping. The 'P' means 300mA and a datasheet voltage of 15v, they were used in TVs with a series heater chain, I've tried them on 14v wired in parallel in a modded JCA22H and they sound sweet. Also tried with some 50's Mullard PL83 valves too, equally sweet :)

View attachment 904492

It's been a long time since reading ( could be wrong) but I thought Rory G. ran both the Vox AC-30 AND a '59 Bassman together. Wrong?

He sure could get a great 'Strat bridge pickup only' sound- what an entertainer!
 

dougsta

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It's been a long time since reading ( could be wrong) but I thought Rory G. ran both the Vox AC-30 AND a '59 Bassman together. Wrong?

He sure could get a great 'Strat bridge pickup only' sound- what an entertainer!

You're right, he moved on to Fender amps in the 70's

Like all great players he could make any guitar/amp sound good and died too young.
 

Greg70

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Back when I first started playing and didn't have a dime to spend on anything, I took apart my older brother's portable 8 track player, removed the leads from the tape head, and added a 1/4" input jack to it. I then ran a cord from the headphone jack of the 8-track player to the AUX input of my tube Sherwood stereo receiver. It made for a crude fuzzy distortion, but it worked and was pretty loud.
 

owlexifry

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Many thanks to the OP for sharing this last year.

this whole Deacy amp thing looked like something real fun to look into.
(especially after I checked out a demo with someone running a treble booster into one - sucked me in immediately)
so I started ordering parts and am planning to build that Greg Fryer layout.

and then this week i spotted a c1970 Philips 'MT10' on gumtree in my area:

(still operates according to seller)
1642727972505.png

1642727678610.png

1642728012170.png


looks like AC127(NPN) input stage, AC126 (PNP) driver, and AC127(NPN)+AC128(PNP) push-pull output.

Conversely to the Deacy operation (positive ground), this one looks like it is negative ground.

Also, I can't see any evidence of an interstage or output transformer... What's the go with that? not even an output transformer? 🤔
(i can see x2 transformers in the radio / antenna part of the circuit, but that's it)

The seller only wants AUD$45. The germanium transistors inside may even be worth more than that.
(N.B. everything is expensive and overpriced in Australia)

D'ya reckon I should grab it? Pretty keen to mod this into a Deacy style, but given the significant deviations mentioned above, I'm unsure if it would work that well or sound any good...
 

owlexifry

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I'd definitely pass that one by. $45?
There's no OT and this link says it uses silicon transistors instead of germanium.
Here's a good resource.
the link you shared indicates that the RF transistors are silicon.
(perhaps the AF116N, AF117N)
but that doesn’t refer to the amp transistors - which are specified as AC127, AC128, AC126 - germanium transistors. if you look at the images and schematic i posted, you’ll see this.

but yeah the lack of OT is bit of a deal breaker..
 
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printer2

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The transistors other than the output stage are Class A right? So it does not matter too much if they are si or ge, get the gain right and you are mostly there. The output stage, may not matter either. But look what is in between each stage, then compare it to the Philips schematic.
 

owlexifry

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The transistors other than the output stage are Class A right? So it does not matter too much if they are si or ge, get the gain right and you are mostly there. The output stage, may not matter either. But look what is in between each stage, then compare it to the Philips schematic.
true.
but no OT..
my limited understanding would have me believe that lacking would cause something to suffer somewhere..
 

printer2

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true.
but no OT..
my limited understanding would have me believe that lacking would cause something to suffer somewhere..
No, look at each stage. The Philips is all direct coupled in the amplifier section. The May amp has each stage isolated from the next by a capacitor or transformer. Just like most tube amps. Some have been built direct coupled to the next stage but those are rare. I have an old amp from the early 70's I built called the Li'Tiger that has three stages, capacitor coupled right through. The thing sounds great, only some local NFB in each stage, no global NFB. Is it the secret sauce? I don't know, I have not had time to play around with it. I also have a PA amp that has a transformer output to play with, one day.
 

owlexifry

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No, look at each stage. The Philips is all direct coupled in the amplifier section. The May amp has each stage isolated from the next by a capacitor or transformer. Just like most tube amps. Some have been built direct coupled to the next stage but those are rare. I have an old amp from the early 70's I built called the Li'Tiger that has three stages, capacitor coupled right through. The thing sounds great, only some local NFB in each stage, no global NFB. Is it the secret sauce? I don't know, I have not had time to play around with it. I also have a PA amp that has a transformer output to play with, one day.
ahhh yep. i see what you're saying. thanks for that explanation.
 

owlexifry

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so with that schematic.. i decided to do a layout for a tagboard
deacy.png

just finished it today, there might be errors - please let me know if you see any :)
 

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owlexifry

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48706771-D70E-4371-A18D-D24B9DC27C4E.jpeg
had a light bulb moment and drilled a couple holes in some steel brackets…
(plan is to screw it to the back of a speaker… turns out they make a handy stand too)
89860550-FF72-4F9F-89BD-D25D09966436.jpeg
works!
(initially had to sort out a negative feedback gremlin…)
sounds alright through this 6.5” 4ohm bookshelf speaker.
might try though a proper guitar speaker cab soon.
68DB13E9-86AE-4C09-9CA1-E66F904FB9AB.jpeg
fun build 🤟
 
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ArcticWhite

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View attachment 969762
had a light bulb moment and drilled a couple holes in some steel brackets…
(plan is to screw it to the back of a speaker… turns out they make a handy stand too)
View attachment 969763
works!
(initially had to sort out a negative feedback gremlin…)
sounds alright through this 6.5” 4ohm bookshelf speaker.
might try though a proper guitar speaker cab soon.
View attachment 969764
fun build 🤟
Neato!
Did you perhaps make a parts list?
What transformers did you use?
Where'd you get that board?
I'm just a monkey with soldering iron.
 

owlexifry

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Neato!
Did you perhaps make a parts list?
What transformers did you use?
Where'd you get that board?
I'm just a monkey with soldering iron.
the layout is a parts list… :cool:
i’ve updated and attached it here (to correct the NFB gremlin)

transformers sourced from here:

board:
also available here:

AC125, AC128 transistors - all ebay sourced.
 

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