abbey road d enfer said:
According to graph, current draw is about 30-35mA, so that would compute to about 1.5W. In fact, this circuit could be powered with a clean regulated 24V supply, omitting R12.
Actually, this is one of the worst designs I ever saw, clearly done by someone who does not have a good understanding of electronics.
1) it´s easy to make fun of a 50 y.o. circuit, specially if it´s one of the first SS reverb designs ... anywhere.
Should they have used an NE5532 to drive the tank?
2) Daniel Meyer didn´t actually design much of it, just slightly adapt it into a Magazine article form, design a PCB, build one, take pictures, write text, etc. ... the circuit itself comes
straight from a Hammond/Gibbs/Accutronics applications sheet.
If anything, blame them
In fact, it´s textbook classic design, step by step:
a) input stage Q1 is a properly biased FET gain stage. Care has been given to stability, drift, and to minimize Fet to Fet variation, that´s why it shows some *positive* bias which allows for a larger than usual Source resistor.
Fets were relatively rare and expensive, and were clearly selected to make Reverb compatible with Tube amps.
b) it drives an Emitter Follower Q2 so it can drive a relatively low impedance inductive load: tha tank coil.
c) reverb signal is recovered by a classic transistor gain stage, Q3, stable and properly biased.
d) Q4 is not strictly needed, but helps isolate Q3 from external loads, and it doesn´t hurt at all to have lower source impedance.
In all, quite a good example of late 60´s SS design.
3) funny thing, it actually works quite well, you will be surprised.
I built and sold a ton of them way back then ... yes .... 1969 on, go figure.
4) passing a few mA DC through the drive coil was actually acceptable, I think up to 6.5mA or so, it´s in the original datasheet.
The point being that to save an expensive driver transformer, some Tube amp makers straight drove coils from a Tube plate or cathode, in series, and obviously passing DC, so they better engineer it into the design or else .....
This circuit is the SS version of that.
5) since then current amp was typically a Tube amp, but first SS ones were appearing, and lots of people wanted to add Reverb (expensive and coveted) to their current amplifier, this excellent yet simple design covered all bases.
a) supply: actual circuit needs a few mA and around 25V DC, and it could be used on its own with such a supply, but since it was very often an add-on , they allowed to "steal" power from any available (amplifier) supply.
Tube-or-transistor
The Popular Electronics article was part of a series , spanning many issues, showing how to build a 60W SS guitar amplifier, complete with Tremolo, Reverb and Fuzz ... top of the line way back then.
It used a 60V single supply, so naturally the Reverb circuit showed how to drop that so some 25V .
What´s interesting is that it also showed how to feed it from a *Tube* type amp supply, and they show the Math on how to calculate needed resistor if you have, say, 300V DC available.
b) impedance/matching
To make it truly Universal, no matter what the amplifier but which would obviously have been designed without "adding a Reverb" in mind, this project was meant to be added between preamp Out and Power amp in .... any and all amps have *that*
So they relied on a simple resistive mixer: suggesting, say, 10k to 22k for SS amplifiers and, say, 150k orn so for Tube ones ... a prctical choice.
So in a nutshell: for its day and intended objective, it was a state of the art, very practical project.
Yes, a Fender Tube type reverb was better, and the Golden Standard, but was impossibly expensive for an average Hobbyist, requiring 2 extra tubes, punching 2 extra holes in chassis, having enough power (specially filaments) to feed them, a reverb driver tank (doubt they were available over the counter way back then in any case), etc.
And impossible on an SS amp.
Again: this was the Tank manufacturer "datasheet example" , so .....
