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8417 vs 7409

Tone_Terrific

Silver Supporting Member
Messages
34,680
My son's friend was given an SG Maestro 100.
Apparently out of service for some time. The owner had died.
I am checking it out and it is foreign territory to me. They were always rare around here.
It is supposed to have a pair of 8417's but a 7409 is in there as with the 8417.
Good match? Both of these tubes seem to be rare and expensive.
Anybody sub in anything else?
 

Silent Sound

Member
Messages
5,598
Get rid of that 7409. I had an amp many years ago that ran off a pair of 8417 tubes. They're pretty rare as they were one of the last tubes developed and thus didn't have that long of a production run before they were discontinued. As I recall, there was no drop in replacement for the 8417's. I've never seen a 7409, so I've got no information on that. What I can say is that because of the scarcity and price of 8417 tubes, I had my amp converted to use 6550's instead. They're about the closest tube in spec to the 8417 that I can think of. KT88's might work as well. Anyway, because the 8417 is such a powerful and clean sounding tube, switching the amp to 6550's didn't make a huge difference in tone and was probably cheaper to do than buying a set of 8417's (considering they make new production 6550's these days for about half the price of 8417's).

And unless someone else with knowledge on these 7409 tubes chimes in and tells you it's okay, or better yet a tech who's looked over the amp first hand, I wouldn't use that amp until you get the right tubes. It may work fine, it may blow some other components inside the amp, it may throw the circuit breaker in your house and it may start a fire. Who knows? Either way, I wouldn't risk it. It'd probably be a wise idea to take the amp in to a tech regardless of what you decide to do with it. It may have been converted to run one 7409 and one 8471. That would be weird, and I don't know how they would have done that, but if they did convert it, it may mess things up to run two 8417's in there.
 

Tone_Terrific

Silver Supporting Member
Messages
34,680
^There is almost no info on the web, that I can find about 7409 although they do exist. Visually it is very similar to the 8419.
I have pulled the chassis and fired it up without tubes.
The plate voltages are only about 320vdc and the bias at -16vdc so the B+ is wonky somewhere. It is a voltage doubling style power supply.
It looks like there was prior work done in that area.
It's rarely easy, is it?
 

DefaultII

Member
Messages
16
^There is almost no info on the web, that I can find about 7409 although they do exist. Visually it is very similar to the 8419.
I have pulled the chassis and fired it up without tubes.
The plate voltages are only about 320vdc and the bias at -16vdc so the B+ is wonky somewhere. It is a voltage doubling style power supply.
It looks like there was prior work done in that area.
It's rarely easy, is it?
This guy has 8417s. No affiliation. http://lostartvintage.readyhosting.com/mainframe.asp
8417s were some of the last tubes designed and, while the bias voltage seems low, that is what the tube calls for. . With 530 volts, you can get 100 watts out of a pair.
 

drbob1

Gold Supporting Member
Messages
29,189
I have a Sano accordion amp that came with 8417s. It also came with the PT ripped out of the chassis and a few other UPS mods… I had it converted to KT88s by a tech that does great work but isn't cheap-total cost of all the repairs ended up in the $400 range IIRC. What I have now is a little amp that sounds fairly similar to a B12-25w, near class A, plenty loud and clear for keys or bass but interesting enough for guitars. If I had to do it again, I might just stock up on 8417s, as they're not THAT expensive!
 






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