Help assessing mystery amp

[colebb]

hello all!

I just came into this amp that had belonged to my grandfather. he was involved in community emergency preparedness and amateur radio and it appears he purchased the amp in 2001 from an auction.

I'll include several photos below. ultimately, my goal is to have a great sounding stereo tube amp to power my DIY Klipschorn clones (see my post history for more info on those, if you're curious). obviously, this amp is mono and the condition looks pretty rough so I've got a pretty long way to go to achieve my goal. I'm also open to the possibility that this amp will not help me achieve this goal and I may end up needing to just buy an amp that works already. but if possible, I think it would be really sweet to modify / refurbish this amp that belonged to my grandfather.

it's definitely home built. the build looks pretty similar to some Thordarson amps I've seen, though the transformers (at least three of them) appear to be Stanco. the sales tag mentions four 6L6 tubes, though I could only definitively identify two of them (it's possible the other two are equivalents with different names).

The best case scenario would be that the included components are of a high enough quality (and in good enough condition) that it'd be worth modifying the amp to a single ended configuration, utilizing two 6L6 tubes, the included transformers, and the chassis.

I'm not here to ask for someone to design an amp schematic for me, but if anyone has any thoughts about the viability of the components of the amp, or about the viability of my goal for the amp, I'd love to get some advice from more experienced people.

Thanks in advance!
Cole

 

[Buck Rogers]

There are only two metal 6L6's. The bottom right are Gas regulators and the ones on the left are preamp tubes.

Top right is rectifier.

Been a while since I've seen a 22&1/2 volt battery for bias... cool...

Yeah, I'd strip it and do a basic 6L6 amp. The iron is probably good.

Fine guitar amp for sure.

 

[gadget73]

This would be single channel push-pull in stock form. You could probably strip it and leave just the power supply, then build a stereo single channel amp in the chassis but it would be a lot of work. Making the controls work would be challenging too, they would need to become dual pots for 2 channel use, and probably extra tubes to build a tone control circuit. There may be enough sockets, not sure, but unlikely the existing tube types will make sense for that.

Gas regs are probably for screen supply. The battery looks suspiciously not-original. I'd probably make this cathode bias anyway for a single ended amp. Depends on the voltages mainly.

 

[colebb]

There are only two metal 6L6's. The bottom right are Gas regulators and the ones on the left are preamp tubes.

Top right is rectifier.

Been a while since I've seen a 22&1/2 volt battery for bias... cool...

Yeah, I'd strip it and do a basic 6L6 amp. The iron is probably good.

Fine guitar amp for sure.

thanks for the feedback! would you say it might be worth trying to turn into a guitar amp but maybe not worth trying to modify into a hifi stereo amp?

 

[colebb]

This would be single channel push-pull in stock form. You could probably strip it and leave just the power supply, then build a stereo single channel amp in the chassis but it would be a lot of work. Making the controls work would be challenging too, they would need to become dual pots for 2 channel use, and probably extra tubes to build a tone control circuit. There may be enough sockets, not sure, but unlikely the existing tube types will make sense for that.

Gas regs are probably for screen supply. The battery looks suspiciously not-original. I'd probably make this cathode bias anyway for a single ended amp. Depends on the voltages mainly.

this is very helpful info! thank you for the reply.

I probably wouldn't need a volume control or even a tone control (I can control both from my preamp source) so that might simplify things.

the question, I guess, is do you think the metal 6L6s and power supply are good/useful enough to achieve an amp with great sound, albeit limited features?

 

[colebb]

another question would be:

if I could find a Dynakit ST-70 in good shape for $600, should I just jump on that and save myself from what might be a major headache? I can't exactly afford it but might be able to make it work if I'm creative. But if that's not such an amazing deal that it'd be advisable to jump on it quick, I may just sit tight and see if I can find a better deal.

 

[gadget73]

I kinda dig metal 6L6 tubes, but thats at least partly because I own an amp that runs them, and they are a pretty good value in the NOS tube world. They don't look cool so they don't command big money that some others do. Single ended you might get 7-8 watts from one in pentode mode, whether or not thats usable for you will depend greatly on your speakers. Khorns are pretty efficient so thats probably enough.

An ST70 is a safe bet, and there are vastly worse amplifiers for more money. If its in good shape thats not a bad price, particularly if the tubes are good and if its had any refresh work done already. The other thing about it, if you ultimately decide you don't want it, someone will buy it and if you aren't into it for a bunch of money you should have no trouble at least breaking even.

If money is the concern though, consider you'll need to buy at least a pair of output transformers and some passive parts to re-design the PA amp, and that can easily get into real money. If you don't have some familiarity with tube stuff, honestly starting with something as well documented as an ST70 is probably a better idea than trying to do a scratch build with a mix of used and new parts.

 

[Buck Rogers]

I Love the sound of 6L6 tubes!

They just flocking rock.

But it will be hard to find a matching output transformer.

Any amplifier like this that crosses my path will always find a home...

That said, If you want a HiFi... The ST-70 is the best deal on the planet!

Good Luck.

 

[albusark]

another question would be:

if I could find a Dynakit ST-70 in good shape for $600, should I just jump on that and save myself from what might be a major headache? I can't exactly afford it but might be able to make it work if I'm creative. But if that's not such an amazing deal that it'd be advisable to jump on it quick, I may just sit tight and see if I can find a better deal.

You can modify this to be a guitar amp and sell it for good money, then buy an ST-70

 

[Mr Boochie]

"306904" = Chicago Standard Transformer Corp, EIA 306, transformer. Made in 1949 or 1959, the 4th week — "904". I believe they were also known as Stancor.



These Aerovox paper caps look original and very old. I wouldn't be surprised if the unit is a 1949 vintage.

Vintage Stancor ST logo example.

 

[colebb]

You can modify this to be a guitar amp and sell it for good money, then buy an ST-70

that's an interesting idea. would you have any suggestions about where to begin looking at how to achieve that modification?

thank you for the reply!

"306904" = Chicago Standard Transformer Corp, EIA 306, power transformer. Made in 1949 or 1959, the 4th week — "904". I believe they were also known as Stancor.

These Aerovox paper caps look original and very old. I wouldn't be surprised if the unit is a 1949 vintage.

Vintage Stancor ST logo example.

this is very helpful info. thank you!

 

[colebb]

I guess one place to start could be making a schematic of the amp as-is, then comparing that to guitar amplifier schematics

 

[eiraved]

More info on that Chicago Standard/Stancor C-2308 transformer.

 

[gadget73]

PA amps often make good guitar heads.

 

[albusark]

that's an interesting idea. would you have any suggestions about where to begin looking at how to achieve that modification?

thank you for the reply!


this is very helpful info. thank you!

Uncle Doug - conversion of PA amp to guitar amp. The principles and practices in this video are applicable to just about 98% of all the PA tube amps ever made.

 

[GordonW]

That chassis has a very good "vintage guitar amp" vibe in terms of looks!

It could be very cool, converted to an early Fender Bassman circuit. Just use the "normal" channel (don't install the "bass instrument" channel or its pots).

That would require four pots (volume, treble, bass and presence). The on-off switch could be re-located to the fifth pot location (better in that recessed area, so the switch doesn't get broken off in tranport, like it might in its current location). Just put a flush-mount pilot light in the current switch location.



Regards,
Gordon.