Got a problem I hope you guys can help me with. You can if anyone can.
I recently acquired an SG-215 amp (at least that's what it says on the back panel) and I can't find a schematic for it. Gibson sent me a couple of drawings, neither of which represents the actual circuit lying on my bench.
According to the SG-215 drawings, this amp is completely solid state, while my specimen has a 12AX7 PI and two great hulking 8417 output tubes! I also got a spec sheet that says the only SG amp with such a configuration is the SG-200, so I asked for this schematic. Long-suffering Bob sends it to me and it's got six output tubes!
I'd deeply appreciate any info on these beasts that you can provide. I'm tearing my hair out and that's in short supply!
Dave
The denunciation of the young is a necessary part of the hygiene of older people, and greatly assists in the circulation of the blood. - Logan Pearsall Smith (1865 - 1946)
Watch out if you try to re-tube the 8417's. They were one of the last power
pentodes to be developed and have had some issues. The NOS can be a bit
of a crap shoot with this type.
That seems to be the one, alright. I happened to have some Sylvania 8417s (which is what prompted me to get this amp) that I understand to be pretty good, on the whole.
Are you saying there are problems with re-wiring the sockets for KT88s etc.? Anything specific that I should be wary of?
Thanks again!
Dave
The denunciation of the young is a necessary part of the hygiene of older people, and greatly assists in the circulation of the blood. - Logan Pearsall Smith (1865 - 1946)
Nope, KT88 should fine, they have a very similar plate dissipation.
8417's were late comers as tube development goes, There were some reports that they were quirky.
I gutted and reused an SG amp 10 years back. The power supply has a
voltage doubler, the screen supply is taken from the half voltage point of the
circuit. Don't see that too often. I think I remember that with 8417 you
need to handle the screen grid current and voltage with care. It's not all that
common a tube type, But the plate characteristics are pretty close to
6550 and KT88.
I've begun to wonder if, in case I can't get it working as an SG, the xfmrs would be good candidates for donor iron- for a serious bass amp. Those things are massive.
Dave
The denunciation of the young is a necessary part of the hygiene of older people, and greatly assists in the circulation of the blood. - Logan Pearsall Smith (1865 - 1946)
I've actually got an SG with a single 15 inch lansing speaker. My son used it for a while. Great cleans but didn't seem like you could overdrive it. Noticed the other day one of the inputs was loose because the nut had fallen off.
Didn't know you could use KT88s. At one point someone had given me schematic to convert it to use EL34s. I'll have to see if I can find it.
Haven't ever looked inside it but that thing is HEAVY so I'm sure the transformers are huge.
The trannies are hefty. I'd try to measure the OPT for a turns ratio so you
know what your loading is, increase the screen grid resistors, and tack a
1 ohm resistor on the cathodes to get a current measurement to find plate
dissipation and set bias. I think I remember that with the amp I cobbled together, I scooped the
power side, and completed the socket dress to use 6CA7, a fender
style inverter and front end. I didn't trust the PS at the time, but ten years later, I'm still
in contact with its current owner, who uses the amp all the time, and he's
never had issue with it.
I'm trying to find time (and money ) to finish up a JTM50 style amp with metro and ceriatone parts and Marstran transformers. Maybe I'll have a look when I get done with that. One project at a time right
Curioius to see what size screen grid resistors on it. 1k or 1k5 you think?
I really think I'd opt for converting it to use EL34s but with a speaker change and 6550s it'd probably make a pretty decent bass combo.
The trend for "fidelity" is to use rather small screen resistors, 100R.
You can do this if you know exactly what your drive voltage is and do not
overdrive the power tube. This is probably what you'll find. Guitar is the
opposite, your tone comes from overdrive and the resulting harmonic distortion,
so you need to protect the screen grids with a much larger resistor when
the tube bottoms as a result of large signal swings. But it colors tone and
alters the valves response, which is why there's so much argument around
which value is best. KT88, KT90 would cover both bass and guitar nicely. I did
notice there's zener's on the inverters input to place a limit on input signal.
Ive been liking 2.2K on the screen grids. 470r, at the very least, 4.7k is too much,
between 1k and 3k to taste. Pull out the zeners and increase the screen grid resistors for guitar,
bass could be as simple as new tubes and bias.
Thanks ! I'll give it a shot at some point and let you know how it turns out. Probably give the Kt88s a shot as long as the son's playing bass now and has a few other amps for guitar.
I've been thinking about making a pair of monobloc power amps for stereo using 8417s and running a Jolida CD player into them.
The 8417 was designed for hi-fi audio use and it is designed so it needs about half the drive of a KT88/6550 to put out the same power. There are issues with this tube in quality control, but I believe the Sylvanias at least are reliable in a properly designed (ie conservative) audio circuit.
Putting it in a wild and wooly guitar circuit is probably asking for trouble, but I agree in a bass amp it should be OK. This tube is known for its high impact dynamics and extended bass and in audio circuits it allegedly kicks butt against NOS KT88s tonewise, which is not easy.
