Mystery amp from France (?)

[Vince]

Hello,
My friends amp, bought at French pawnshop.
Looks like custom order.
Specs:
Combo 1x12 in tweed,
2 x ECC83/12AX7 (first preamp like fender tweed and second very unusual phase inverter),
2 x 6V6 cathode biased, each on his own cathode resistor and capacitor
Rectifier GZ34/5AR4
Transformers by Hexacom.
I want more info about this amp and manufacturer before I mod it.
Pictures in this topic:
https://el34world.com/Forum/index.php?topic=27611.0

TIA

Vince

[pompeiisneaks]

I agree with sluckey's summary. Looks a lot like a tweed deluxe clone? I think? But without a schematic it would be hard to be sure.

~Phil

[Phil_S]

Can you give more information on "very unusual phase inverter?" If it is a tweed deluxe (5E3 circuit) clone, it will have a split load PI, which only uses one triode. This is not so unusual, just less common in guitar amps. Very nice looking amp. Congratulations on the purchase!

[Stevem]

That amp has a choke which a 5E3 does not, this will make for a play feel a good bit different when you rev it up!

What does it have for the 5 or so plate voltages now?

[Vince]

Not 5E3, not split load PI.
Visual appearance of this amp is a bit confusing - we all judge a book by its cover
Me too - my surprise after opening the chassis continues until now.
But split load PI is nothing unusual for me, I've been building amplifiers for years.
Tomorrow I try to post schematic of whole thing.

regards
Vince

[didit]

Hello --

Schematic definitely. Looks, at the glance, like perhaps a DC-coupled LTP PI?

Best .. Ian

[ChopSauce]

Sorry, I can't help but providing you with links to the transformers (*) even though you certainly already found them.

Nice (HiFi-style ? ) work by the way...

IMG_20210609_110045.jpg

This gutshot is pretty inspiring, yet not the usual guitar amp style.

If I were to do only one thing on this amp, that would be isolating these bare conductors (from a resistor?) over the bus, right in the middle of the chassis.
_

(*)
http://www.ciel-electronique.com/docume ... ME0003.pdf

http://www.ciel-electronique.com/docume ... ME0001.pdf

[Vince]

Schematic in atachement. Definitely NOT 5E3.
Amp got 16W on 8R dummy load when tone set to 5 and volume set to 2 (yes, two).
I mean sine wave not clipped.
All coupling caps 1000V MKP SCR, all resistors looks like Vishay metal.
Unusual phase inverter - I don't know how it works, there is open circuit between
inverters grid and ground. I check it by multimeter set on 220 Mohm.

[didit]

Hello -

Good schematic, so much much easier to understand what's what. PI is classic rendering of DC-coupled LTP. A web search should provide a few good references to review. Expect your PI plates actually are 68K, as 10x that makes poor sense. Overall this looks to be a straightforward somewhat HiFi design, and truly nothing like a '50s "tweed" that cabinetry might imply.

Best .. Ian

[Vince]

Plates ARE 680k, cathode 68k - when I saw it,
I don't believe my eyes, but multimeter
says it's true. Grid has no reference to ground,
but it works...
Not so classic rendering of DC-coupled LTP
Any clues?

Edit: Schematic with voltages added

[Lynxtrap]

Correct me if I’m wrong, but how would that even work as a phase inverter? With the same signal applied to both grids, the output from the plates would be in phase, making it some kind of parallell single ended thing?

[pdf64]

The same signal isn’t applied to both grids. Note that one is decoupled.

[sluckey]

Correct me if I’m wrong, but how would that even work as a phase inverter? With the same signal applied to both grids, the output from the plates would be in phase, making it some kind of parallell single ended thing?

The first triode's signal input is it's grid. But the second triode gets it's input on the cathode while the grid is held at AC ground via that cap. This is the same even for a more conventional LTP PI.

[Lynxtrap]

Yes, right! Sorry, I forgot the cap to ground. This is a textbook DC coupled differential pair, with unusual Ra values.

[pdf64]

It would be ok if it had a DC reference for the grids. As it is, their VDC / bias, and hence the stage’s operating point, will just float around.
Have a looky http://www.vintagehofner.co.uk/britamps ... ust15.html