Deacy amp transformers

[sahib]

This thread reminded me of something that I had. A 10W amp that I had bought some years back on e-bay. Never been used. I'll use it at some point on something.

 

[Wavelength]

Given 7.43 V from batteries, the voltages are: Q2 emitter (R10): 6.56 V, Q3 emitter (R11): 7.41 V.
I also checked RV1: 7.37 V.

Yes, there isn't.

There are 3 schematics in this thread. The first one is of an earlier KAT Deacy-like amp (we should ignore this one). The second one is of the PR80 radio from which the original Deacy got it's PCB. The third one is of the KAT Deacy replica kit, which is the one I traced, and is supposed to be the exact copy of the Deacy.

Remember most of the circuit is still based on 9V and the lower noise sections from the dropped voltage 7.5V at R2.
I have like 10 versions of this schematic now. Some with the 4K7 with 47K, some without those, some with 220uF feedback and various feedback resistors. Basically v3 of the schematic with the 220uF feeding back to the emitter of Q2 would have 100% feedback, which is never a good thing.
I basically want to know the currents so I can get a better idea of good transformers for this.
I am not thinking of making an exact copy, just something similar with that sound.
Thanks,
Gordon

 

[rsasgtr]

This thread reminded me of something that I had. A 10W amp that I had bought some years back on e-bay. Never been used. I'll use it at some point on something.

That's interesting. Germanium transistors. The topology looks somewhat similar, although the feedback and speaker connections are different.
With the input impedance being only 50k you might want to put a buffer in front of it (or a treble booster ). Post some audio samples when you try it, please!

 

[abbey road d enfer]

That's interesting. Germanium transistors. The topology looks somewhat similar, although the feedback and speaker connections are different.

The topology looks more different than similar, actually. The only coincidence is that the stages are organized in a similar way, input stage, driver and output stage, and the use of transformers.
This one has a single global NFB path, when the other has two, and the auto-transformer output provides a lower output impedance.
I would bet the measured performance (BW, THD) would be very different.

 

[rsasgtr]

Remember most of the circuit is still based on 9V and the lower noise sections from the dropped voltage 7.5V at R2.

I find it strange that the guitar ground is connected to 7.5V instead of GND.
Still, I'm running the whole thing at 7.43V (from 6 rechargeable AA). According the Greg Fryer, 7.5V is the sweet spot (Fryer on the Deacy Amp). Brian May supposedly also liked it when the battery was partially depleted.
I should probably measure what the "7.5V" is in my case.

I have like 10 versions of this schematic now. Some with the 4K7 with 47K, some without those, some with 220uF feedback and various feedback resistors. Basically v3 of the schematic with the 220uF feeding back to the emitter of Q2 would have 100% feedback, which is never a good thing.

Not sure which specific resistors you are referring to, but the pictures I've seen seem to match the schematic I posted (the resistors and capacitor-only feedback).
The PR80:

PR80 PCB:


Original Deacy and a Doxy replica:

As far as I can tell, there are no new parts on the Deacy, so no extra resistor for the feedback.
I guess it's possible that John Deacon cut a trace and added a resistor, but I'd need a source for that

I basically want to know the currents so I can get a better idea of good transformers for this.

I've no oscilloscope or signal generators, only a standard meter with AC/DC V/A, R, C, and hFE (no L).
I guess I could breadboard a sine wave generator, or plug the output of my audio interface, then measure AC voltages at different points. I'm open to suggestions.
BTW according to the KAT story, they added more windings to the transformers to figure them out. From the KAT Deacy Amp Replica brochure:
"The biggest surprise was the transformers. As Greg mentioned in Part 1, Dave Petersen remarked that he had never seen units like this before, and to be honest, neither had I. This was really our first clue that the Deacy amplifier did not follow the same rules applied to the Mullard circuit.
The transformers themselves were simple enough and used older style laminate materials, but the windings made no sense whatsoever. Nor did the pin-outs. Simple resistance and inductance tests were not going to reveal anything of much use, so I took the board to some old friends of mine who design and make all our transformers for us. They also make great coffee there!
There was, to be brutally honest, a degree of head scratching initially because we had to find a way of testing the Deacy's transformers in isolation, yet without physically taking them out of the board. Greg had earlier stated that he did not want to be the quy known for killing the Legendary Deacy Amp, well, neither did I, so we scratched our heads further and had a few more cups of coffee.
Then the light bulb moment! "Lets put some extra windings around the existing coils and use that as the reference, that way we don't have to touch the actual units at all" said Dave (Transformer genius!) ! Ingenious I thought!
Basically, if you know the amount of turns you are applying in the new auxiliary coil', you can then go on to calculate the turns ratios to the other windings, the phase relationships between the windings, the inductance and DC resistance of the individual cons and even whether or not thov are sitlar wound
This was the perfect solution and so after a couple of hours of testing, and a few more coffees, we finally had enough data to know exactly how these transformers were built in addition to any foibles they may be exhibiting after many years of service.
There was no place to hide! We knew everything!. ..."

 

[rsasgtr]

The topology looks more different than similar, actually. [...]

I stand corrected!

 

[abbey road d enfer]

I find it strange that the guitar ground is connected to 7.5V instead of GND.

This was quite common with Ge transistors, which were PNP mostly. You may consider that the commonly accepted polarity, which was randomly selected, is actually wrong.

From the KAT Deacy Amp Replica brochure:
"The biggest surprise was the transformers. As Greg mentioned in Part 1, Dave Petersen remarked that he had never seen units like this before, and to be honest, neither had I. This was really our first clue that the Deacy amplifier did not follow the same rules applied to the Mullard circuit.
The transformers themselves were simple enough and used older style laminate materials, but the windings made no sense whatsoever. Nor did the pin-outs. Simple resistance and inductance tests were not going to reveal anything of much use, so I took the board to some old friends of mine who design and make all our transformers for us.

Shows that the basic knowledge of whoever wrote this was very limited. Indeed, multipath NFB was not very common at the time, but anyone with good knowledge of audio techniques would have figured that out. Multiple-path NFB was certainly known in HiFi designs (McIntosh being the main proponent).

 

[sahib]

That's interesting. Germanium transistors. The topology looks somewhat similar, although the feedback and speaker connections are different.
With the input impedance being only 50k you might want to put a buffer in front of it (or a treble booster ). Post some audio samples when you try it, please!

I did not know anything about Deacy Amp up until you gave the Deacy Amp link (I did not know who Fryer was either). Very interesting. However, sinc I play guitar, building a guitar amp was the first thing that crossed my mind anyway. I actually have some unused Blues Junior PCB assemblies and I'll use this as the power amp stage for a hybrid. I also have two pairs of yellow and green backs. It will be interesting to see how the whole thing will sound with my Strat and Tele. However, this will not happen until summer as the BJ boards and speakers are with my sister in Istanbul.