Supersonic Conquest radio's PR80 and PR80C

[Podbuilder]

Hello,  New user here.  Im looking to reproduce the two audio transformers used on the this radio.  Im pretty handy on the cnc and happy to have a go at wind them if I get some more info from the experienced people in this forum.  I see Kevin R has made some in the past and Im hoping he might still be around to provide some info.

[Podbuilder]

Very rare vintage radios (in the UK) are being destroyed in the pursuit of making these little "less than 1 watt" guitar amps.
I have first hand experience of random people contacting me via my website and offering 500euro and up to 1000euro for some of my radios which contain the same amplifier board.
I have finally got to the bottom of how all this started and I totally disapprove of destroying a radio to make a tiny guitar amp.
https://en.wikipedia.org/wiki/Deacy_Amp
Its a fad craze not un-similar to the audiophoolery that you hear about. In my view it should not be welcomed on a vintage radio forum.
Mike

Most of the components to this amp are freely available.  The only issue is the audio output transformer windings are a mystery so people are pulling them out of radios.  I've searched most forums only to find the guys that do buy them keep everything a mystery.  It'll just take one experienced person to carfully unwind their audio coil and release this info. Plenty of people would be happy with a non genuine circuit board and parts in their guitar amp but it's got to be the correct circuit with feedback.    Primary to Secondary winding ratio - Wire Gauge/s - layering of the winds in the correct order.  

Im going to have a go at winding a few to see how they sound so any info here would be appreciated.  And Im not destroying anything....  apart from a few cheap Chinese transformer for my blanks.

[Mike Watterson]

A regular transformer without the taps can sound the same with a suitable feedback circuit. Those are still available new or NOS. It's a niche fad.

It's fantasy that this exact transformer is needed to "recreate" the sound.

[Mike Watterson]

It's probably derived from the idea of some valve push-pull amp with taps feeding the screen grids by someone that didn't entirely understand what they where doing. It was stupid then and even more stupid now.

Queen's bassist John Deacon probably used a random radio chassis and speaker he happened to have. I've done the same a few times over the years for a little monitor amp when I had a spare junk pocket radio and speaker. Sometimes in a random wooden or plastic box. But unlike John Deacon, I never played a guitar and made living from working in Electronics, Communication Engineering, BBC, AV facilities, Computers & programming and have professional musician friends, so I know it was random and stupidity.

Edit:
In fact analysis was done about 15 years ago and replica amps were on sale in 2011.

It's a fad, and now a really old fad. See Prachett's Soul Music about wanna-be rock musicians getting exploited. It's the talent of the guitarist that's the import thing, and also the "luck" of getting famous. I've known loads of brilliant creatives (songwriters, musicians, artists, authors) and it's not the instrument, amp, writing tools, talent and how great the result is. The fact is that with music performance or writing less than 1% make even a minimum wage from it. The pop "stars" are not the best, nor is J.K. Rowling.

[Refugee]

It looks like Bush TR82 chassis has been stripped down to get the transformers for that circuit.
The output transformer has four windings that could easily be linked to work in that circuit.

[Podbuilder]

A regular transformer without the taps can sound the same with a suitable feedback circuit. Those are still available new or NOS. It's a niche fad.

It's fantasy that this exact transformer is needed to "recreate" the sound.

That's great to hear.  Might you have some examples of these regular transformers?  Fad or not....  collectors are going to collect..   Im just here looking for information.

[Mike Watterson]

A regular transformer without the taps can sound the same with a suitable feedback circuit. Those are still available new or NOS. It's a niche fad.

It's fantasy that this exact transformer is needed to "recreate" the sound.

That's great to hear.  Might you have some examples of these regular transformers?  Fad or not....  collectors are going to collect..   Im just here looking for information.

No, they will not be convinced. They are chasing a dream.

Altai / Eagle

LT44 AKA P631M, blue tape, is a driver transformer for push-pull 20:1 ratio.

LT700, red tape, is a push-pull output transformer

The two coils on the primary side consist of 420 turns each. Therefore, there are 840 turns in total with a centre tapping. The total inductance is 550 mH, and DC resistance 63 Ohms.

On the secondary side, the coil consists of 45 turns. The inductance is 2.25 mH, and the DC resistance is 0.38 Ohms

You can design for 3 Ohms or 8 Ohms speaker and silicon (NPN or PNP) or germanium transistors, or even FETs.

Using feedback from the secondary (at the speaker) allows you to simulate the taps. Having a driver stage for the feedback using an LT44/P631M allows easier simulation of the original.

Note this is about 300 mW max.

https://pattern.usefulcomponents.com/listing/1143368797

Loads of companies sell these parts as well as miniature mic transformers by Altai/Eagle, which can be used between an electric guitar pickup and a regular 10K to 100K line input for "colouration".

With push-pull the DC currents cancel, so you can use any centre tapped transformer without an airgap. A mains transformer with two primaries for 110-117 and 220-240 and a 6 to 12V secondary (depending on speaker) can make a 1 W amp to 100W amp depending on pair of valves (ECC82 even at 24V, Russian rod Pentodes, EF80, triode of ECH81 (=ECH83), not just 6V6, EL84 etc). There are also transistors that can work off 100V to 200V HT. A Shaver transformer is a "safe" isolated supply for 130V or 260V nominal up to 20W. Those are current limited, so make the amp soft limit at 5 to 10W.


Obviously for transistors off low voltage you can use two small 5W or 10W transformers back-to-back on the primary and connect transistor collectors to a centre tapped secondary. The 2nd secondary connects to the speaker. Or one transformer with two suitable secondaries. Lots of scope to experiment with feedback signals that have gone through the transformer.

The taps on the original are simply for negative feedback to give a cleaner sound with less distortion (colouration) from the transformer. There has been much nonsense written on the Deacy (Deaky) amp, some in the Wikipedia page. There is nothing clever or magic or special about the original, except in the minds of some people.

It's pointless replicating the the original transformer.

[Podbuilder]

So here's where Im at...  PCB done and donor coil/cores ready to wind.  3D printed bobbins and purchase most the components although I wasnt real happy with hFe readings on some of the germaniums.




Im thinking of using this speaker my father made in the 1970 when he worked at Rola.

 

Transformer parts.



Slowly coming together.

Still looking for winding ratios and DCR values.   Then I'll figure out which AWG wire best fits my coils dimentions to get these values.   Interestingly some people have wound 6.25 : 1 on the audio, yet the old papers say 4:1 inductance.   It been a bit of a learning experience so far... fun stuff.

[BusyBee]

So here's where Im at...  PCB done and donor coil/cores ready to wind.  3D printed bobbins and purchase most the components although I wasnt real happy with hFe readings on some of the germaniums.

It's a nice looking board

hFe readings on Germaniums generally are very low as compared to current silicon. Modern meters often don't cope with them either. I think it's probably their very high leakage, numbers of microamps, as compared to hFe's often of twenty or less. Power transistors can be even worse.

I have an old analogue meter which tests them Ok but when I tried some with a modern digital meter I had I got no sensible readings.

Tracy

[Podbuilder]

Thanks. Happy with how the board turned out. My little CNC machine does a great job. And I only missed drilling one hole.