Mystery amp from France (?)

[vvince]

Hello,
My first post, but long time lurker here :)
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.

TIA

Vince

[vvince]

More pics

[sluckey]

Those pics scream "home-made" to me. Doubt you'll be able to find any info. Neat workmanship. I would evaluate as is and draw a schematic before making any "improvments".

[brewdude]

Wow, that’s some nice work... I’d be reluctant to fix anything that’s not broken. 
How does it sound?

[PRR]

> very unusual phase inverter

What is unusual about it?

[sluckey]

I wonder what the builder was thinking when he put the speaker impedance selector right beside the input preamp. That's just wrong. Too tempting for the screaming howler gremlins to wanna play.

[shaun]

That's a very tidy build. Nice.
In all likelihood, it is a modified Fender of some type - 5E3 or Princeton maybe. I'd get a few Fender schematics and see which one comes closest, then change that schematic according to what's in the box. With a schematic, you'll get much better feedback on how to approach the circuit.

[vvince]

Schematic in atachement.
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.

[PRR]

Obviously it can't work that way.

But how does it sound?

[vvince]

Obviously it can't work that way.

But how does it sound?

I know, but it works.
Quite unusual.

[pdf64]

Given the schematic and layout, I’m at a loss how it isn’t oscillating
What’s the idle VDC on the LTP pins? And how about if the valve is removed?
It must be operating at very low anode current, which will tend to generate grid current, charging up the 47nF decoupling cap. If the schematic is correct, I guess it must be achieving some kind of equilibrium.
How stable are the LTP anode voltages if it’s heavily overdriven momentarily?
What is the AC voltage gain, between the anode side of the 0.1uF cap at the LTP input, and the output valve control grids ( dont want the input resistance of any measurement instruments to upset the LTP DC conditions).

[sluckey]

Quite unusual.

vvince, would you measure tube voltages and put them on your schematic? Especially the PI.

Merlin has a writeup on the DC Coupled Long-Tailed Pair. But as the name says, it needs to be direct coupled to the plate of the previous stage in order to establish bias. But your LTP is cap coupled. Trying to understand that. Voltages will help.

[vvince]

Lack of polarization LTP PI is a reason I show this amp,
I don't understand why it works.
All resistors values were verified by multimeter.
Lack of polarization LTPI was verified too.
I will measure tube voltages soon.
What you think about 1k8 resistor from top of volume pot to ground?

[sluckey]

What you think about 1k8 resistor from top of volume pot to ground?

I find that quite mysterious. Big volume killer. Maybe this is a bedroom amp.    

What resistance do you actually measure from top of volume pot to ground?

[pdf64]

... What you think about 1k8 resistor from top of volume pot to ground?

Haha, I was looking at the the LTP and power amp, and didn’t notice that, doh.
That’s what’s cutting the gain to a low enough level to prevent oscillation.
I guess there’s not much bass getting through though?

[vvince]

Schematic with voltages added.
From top of volume pot to ground is 1k8 :)
Definitely NOT bedroom amp - 16W measured on dummy load 8R
sine wave not clipped at volume set on "2" (on knob no "0" setting,
first is "1" :)

On grid PI is 0VDC, but there is some (45VDC) voltage going down to zero.

[pdf64]

Are you sure about that LTP cathode voltage?
If so, I wonder if it’s oscillating, due to the probes / meter; did you monitor for that, at the speaker output?

[sluckey]

According to the voltage drops shown for the PI plates there's a total of .71mA flowing through the tubes. This also flows through the 68K cathode resistor and should produce about 48 volts at the cathode.

[vvince]

According to the voltage drops shown for the PI plates there's a total of .71mA flowing through the tubes. This also flows through the 68K cathode resistor and should produce about 48 volts at the cathode.

I can check it by measuring another multimeter.

Strange - two "sets" of voltages:

1) 34,2VDC on cathode, plate 1 - 199VDC, plate 2 - 201VDC

then I try to measure voltage on grid 1 PI, about 40VDC going to 0 VDC

and then I try to measure  again
and set 2 is: cathode 5VDC, plate 1 - 343VDC, plate 2 - 345VDC.

Both multimeters shows these numbers :(

Maybe time to mod this mystery

[pdf64]

Ok, it’s kinda what to expect, as the operating point is totally undefined, left to float according to the vagaries of grid leakage current.