Unidentified Amplifying Object(s) - an unknown amp with unknown tubes

[christarak]

Greetings Earth creatures,

I have an alien amplifier which I can't identify containing some tubes which I also can't identify. How do you start with such an alien being? The only clue I have as to its origins is that it is from a deceased estate, which came with and old Aussie-made Maton jazz guitar (complete with the former owners Bow tie in the storage compartment).

With no amp model, and no schematics, and 3 unidentified valves (see photos) which I can't run on my B&K 707 tester, I'm stumped as to how to start. The unidentified tubes are all octals, but the tube with the anode connection is missing pin 3. I have to assume that the 2 TungSol tubes must be preamp tubes, as I see no others. I'm not sure of the function of the tube with the anode connection on top.

The other tubes I can identify are: a pair of 6V6 (smoked glass) and a 5V4G rectifier. There are also 3 very old caps. 3 x 8uF and 1 x 20uF. I would normally change caps over 20 years old, but I would appreciate your thoughts. What is your opinion on just getting this piece of history operational compared to getting this 65 year old amp potentially gig-ready again (in which I case I really have to solve the mystery of the tubes).

I have powered the amp up through my current limiter and all seems OK. The pots are very stiff and noisy but other than that, it works.

[glebert]

Don't know if it is helpful, but I had some tubes that seemed unmarked and by holding them over a steaming pan of water was able to make out the very faint markings. Wasn't my trick BTW, just found it on the internet.

[Justin Thomas]

Similarly, leave them in the fridge for a while, then breathe on them to see if you can see anything.

Justin

[glebert]

The amp has a bit of similarity to the Maton Super 6 in this thread. Not the same amp obviously, but looks kind of related.
guitarnerd.com.au » Maton nerd update

[glebert]

OK, I'm bored and have been watching "Sherlock" tonight so forgive my amateur sleuthing. From this thread: Maton01 a description of a flavor of a Maton AM1: "The valve line up is 5Y3 rectifier, a pair of 6V6's loafing at 13 watts output, 6SN7 twin triode which will be the phase inverter, and two metal valves that may be 6SJ7 pentodes, one for each input." I wonder if your Tung Sol tubes may be 6SJ7 and the other one a 6f8g, which is a coke bottle 6SN7 with a top cap. I found other accounts that had AM1 models that have considerably different configurations too.

[J M Fahey]

Tube functions don´t change, so follow each tube pin where it leads to, then draw a tube base and label pins.

Taking a 12AX7 as an example, "mystery pins" which show continuity to each other must be filaments, so pins 4 - 5 - 9 are so, chassis wiring will show the same, and with amp on you will measure 6.3V AC .
Plates will go to high voltage (+90 to +300V) through a 47k to 220k resistor, and a coupling capacitor will send its signal "somewhere else", in this case pins 1 and 6 .
Cathodes will be grounded or have low value resistors: 820 ohm to 4k7 and show +1 to +5 V DC to ground (or 0V if grid leak biased).
If Pentodes you will have a pin fed from high voltsage through a large resistor (usually >47k) and decoupled to ground with, say, a.1uF capacitor.

Once you made a "pin map" you can compare it to suspect tubes to see which is which.

In fact, now that you are at it, draw the full schematic and compare it to known Maton ones.

[christarak]

Similarly, leave them in the fridge for a while, then breathe on them to see if you can see anything.

Justin

Thank you Justin. I thought we were on to something there, but sadly, neither steam nor fridge, helped to reveal its identity.

[christarak]

Glebert, you are wasted in amp repairs. The Police Forensic science team need you. I'm convinced its definitely a Maton amp now, and it does look quite similar to a AM1...but not quite. I can't find any photos that are an exact match, but that's the closest. The tube with the top cap is missing pin 6. I will now need to trace the circuit as J M Fahey suggested below to see if it is the phase inverter that you suspect. If so, then my tube lost one of its plate pins during its long life and only one of the 6V6's is working. (This reminds me of myself. I think I lost a pin or two along my journey, and I'm sure only one of my output tubes is operating).

[christarak]

Thank you Juan. I need to now trace the connections from what may be a 6F8G to the 6V6's to see if its the PI with a plate connection missing. I will check those other pin connections you suggest, too.

[glebert]

Sounds like the founder of Maton really didn't push to grow the amp line, there are stories that some amps were done by Phillips in Australia but they wanted a certain minimum order that he wouldn't commit to, and that parts sourcing became an issue because they were making low volume but not at a boutique price point. I could totally see them saying "well, we ran out of 6SN7s and we can get these 6FG8s cheap, let's use those" and if you are doing everything by hand maybe it doesn't matter that much to change. I agree with Juan that you will need to just make a schematic for that amp since it may be the only one in existence exactly like it!

[mozz]

Tung sols looks like a 6SJ7 as was said. The tube with the top cap looks like a single triode or pentode , not a 6sny variant. It could also be a 6j7g? I don't think they would mix those 2 types but you never know.

[christarak]

Tung sols looks like a 6SJ7 as was said. The tube with the top cap looks like a single triode or pentode , not a 6sny variant. It could also be a 6j7g? I don't think they would mix those 2 types but you never know.

Thanks Mozz. I will trace the circuit after the Easter running around. That will at least help us decide if its a pentode or triode. How we determine the tube type, I'm not so sure.
Thanks again

[TimmyP1955]

Perhaps the top cap is used in lieu of pin 6?