Sano 500R-12 output tubes

[alchemy]

Just picked up an old Sano 500R. Missing the power tubes which were 8417's. I dont really feel like spending $200 on a pair of old stock tubes so plan on converting to probably kt-88's. I cant find a schematic anywhere for this amp and it is not layed out as to make tracing the circuit easy. It is cathode biased which I dont think I have ever seen in an amp this size. Wondering what suggestions anyone has for getting up and running? Right now I'm thinking of getting rid of the cathode bias and adding a bias supply and rebiasing for the new tubes. However, I've also read that 8417's only needed a very small signal from the driver to produce lots of power and thus will probably have to do some work there as well in order to boost the signal for the kt-88's. My only reservation in doing this is that i will at that point have a different amp. Any suggestions as to how to change over to a different output tube with as little reengineering as possible? Anyone have a schematic for this oddball amp? Can a pair of kt-88's running 500v on the plates (or 6550's or 6l6 or el34) be cathode biased safely?
Any insight is most appreciated.....cheers!

[Chuck H]

You can probably get a pair of those "test good" or NOS for under $80. I might consider that route since your concearned about it being a different amp. And it will be. The specs for the 8417 look very different from anything else. Sort of a giant el84. Just the time you'll save in the work to mod the amp for different tubes is considerable. I think that amp is cathode biased for one or two reasons. One being that with such a low bias voltage there isn't much difference in clean performance moving to fixed bias so why not save the labor of including a bias supply. Another being that there could be some secondary usage of the voltage on top of the cathode resistor. I couldn't find a schem either.

[BlackAdder]

I have owned a Sano 500R for about 10 years. I wait until I can find a good deal on the 8417s and I snap them up. I have gotten a pair for $100 and another for $90.
A bit of history about this amp. Sano used to make amplifiers for accordians. That's right, this is an accordian amplifier. I play guitar and it's the cleanest amp I have ever played through. I was amazed when I came across this post. I have been looking for the schematics for 10 years and i have found schems for the 50R which is quite similar. The schematics for the 50R (google that) you may find. If anyone does any thing to their amp, please let me know. I LOVE THIS AMP!!!!!! Like I said, it is the cleanest sounding amp I have EVER played through. And I only have tried different tubes. I have built a few tube amps since finding this baby but the preamp is located in such an awkward position (if you have one you know what I mean). But I am ready to at least do a cap job but distortion or more headroom I don't think is what this amp needs. Just tender loving care, a cap job and a new "hum" pot for sure. Other things may include new sockets and a general good cleanup.

Please get back to me and we can chat some more about this wonderful amp. I even saw one on Ebay once. It went for around $400. I have 6 power tubes so I am not interested in changing over yet. The time will come though as these are very hard to find, as you all know.

Best regards, DT

[BlackAdder]

OK!!!!!!!!!!!!!!!!!!

Try here: Sano Diagrams, Schematics and Service Manuals for FREE

The 50wrt is the only other one to use the 8417. the others (500r not listed) use the 7591. But take a look!

Best regards, dt

[BlackAdder]

And alchemy, if you ever plan on getting rid of yours

[alchemy]

Ok so this got put on the back burner for a while, but now i'm back at it! I found the 8417 output tubes in the back of the shop that i got the amp from. They test good on my tv-7. After some cleanup and replacing all the preamp cathode bypass caps it works and sounds ok, super clean, but deffinately lacking in volume. Also, one of the output tubes is redplating slightly. I obviously have some more testing to do, but so far have discovered the voltage drop across the 125ohm cathode resister is 18v, plate voltage is 470v. A little high from what i gather. but still, it's only one tube thats red plating and it stays with the tube.
Questions for now (there will be more once I get in it a little deeper!):
Blackadder: Have you by chance taken any measurements of yours? Curious where it's supposed to be operating. Mine looks to have had a few mods, and without a schematic and ever having messed with 8417's i'm shooting in the dark a bit. Also, what is the switch on the back? Impedance I'm guessing, but what value? 4 and 8 ohm?
Thanks for any help guys, as always!

Edit: Changed the cathode resistor to 200ohms. This brought the current down a bit and the tube is no longer red plating. Some crackles but otherwise sounds really good. just seems like it should be louder.

[alchemy]

moved this to new thread as i have isolated the problem to the PI....