What the hell is doing the 12AU7 tube in this amp - Schematic Here

[kagliostro]

A fried submitted me a question about the circuit of an old italian guitar amp

the question is about the 12AU7 that is in the circuit

At the first moment I was thinking to a particular PI (something similar to a paraphase)

Then I realized that the 12AU7 is used as a mixer to mix the signal from the previous part of the preamp with the signal from the ECC83 oscillator

But really I'm not able to understand how that happens, that portion of the circuit is very unusual and odd to me

Help please

Thanks in Advance

K

p.s.: Previously something was wrong and the schematic wasn't attached to the post (may be my fault) SORRY for the discomfort

[sluckey]

The 12AU7 should be the tremolo modulator, although I don't understand it. I think it's drawn incorrectly.

[eleventeen]

Well, the connection of the plate and cathode of the second half of the 12AU7 to the same place is certainly good for prolonging tube life, LOL. Or maybe I should say tube half-life.

Kind of interesting how the tone controls PRECEDE the volume controls.

[terminalgs]

Have a look at a magnatone 213 schematic. It is nearly a component for component copy of the magna vibrato section EXCEPT no varistors. They were improvising.  Accordion companies like panccordion and titano did a lot of manufacturing in Italy, other accordion CO's like imperial/tonemaster imported EKO guitars. All these companies sold rebadged magnatone 213s. They probably traded EKO their estey/magnatone schematics in all the dealings.

As for what they are attempting to do? mimick Don Bonham's patented vibrato (but without varistors)  (Google patents search for 2,988,706).

[kagliostro]

Many Thanks

Actually if you assume the "Z" as varistors there is small difference between the Ekosonic schematic and Bonham patent

look to fig. 3 on the patent, the cap attached to the plate of the first 1/2 12AU7 is missing

Yes, you are right, that seems to be really a Bonham circuit

---

I must say that I would have to recognize the circuit as Bonham, in my GEM 50 (another old Italian amplifier) the vibrato circuit is Bonham

Thanks again

K

[sluckey]

Actually if you assume the "Z" as varistors

That makes sense.

[HotBluePlates]

Actually if you assume the "Z" as varistors ...

This is handy, but notice also that there is a power supply node called "Z". So this is only telling you what supply voltage to expect.

Tell us how well the circuit sounds, assuming your friend actually has the amp. I'd be curious to see how well it works without the varistors.

[kagliostro]

My friend lives far from me, we encountered the first time at the biggest italian radio amateur flea market in Marzaglia some years ago then we have meet at other radio amateur flea markets in north italy and we begin to wrote each other via e-mail

He asked me about the Ekosonic circuit but didn't say if he has the amp, I think he didn't have one under hand, I'll ask him about

However I think that varistors and one cap are missing only as red herring, something like the mistakes all people say that are present in Mesa schematics

I don't think that to give a try in a Magnatone amp, shorting varistors and the cap will be a good thing to do, I think there will be easily damages on the circuit, or is what I suppose it may happen

K

[Shrapnel]

Have a look at a magnatone 213 schematic. It is nearly a component for component copy of the magna vibrato section EXCEPT no varistors. They were improvising.  Accordion companies like panccordion and titano did a lot of manufacturing in Italy, other accordion CO's like imperial/tonemaster imported EKO guitars. All these companies sold rebadged magnatone 213s. They probably traded EKO their estey/magnatone schematics in all the dealings.

As for what they are attempting to do? mimick Don Bonham's patented vibrato (but without varistors)  (Google patents search for 2,988,706).

There are some differences between the EkoSonic 120s and the Magnaton 213 "Troubadour" but it does have the feel of it. How the channels are set up is different, and this one the Vibrato is only on one channel (The Magnatone has it on both) Also the Magnatone has a single tone control for both channels, this one has one for each. The Magnatone uses a parallel triode for the oscillator, this one takes the second half and places it into use in the preamp (giving two full independent triodes per channel.)  The 12AU7 portion seems to be practically the same if you compare both closely, other than the Magnatone showing the Varistors, and where they would be on this one a solid connection with the "z" notes. I do note the missing cap between the first triode and the varistors as a difference as well.

Ak Kagliostro hinted at, these "missing" parts may be the official cover-up of a then patent rip-off.

[kagliostro]

My friend say he has not the amp, but he want to give a try to the 12au7 circuit

if he do something about that I'll report here the results

K