8417 Substitution

[RicharD]

One of these days I'll get to service a normal amp with typical tubes and a schematic.  Right now the patient's name is SG-215 made by CMI.  No schematic to be found anywhere (thus far).  One of my searches revealed a guy with the same problem and the schematic Gibson provided him was all solid state.  So.....

From what I've read, the 8417 came late in Sylvania's production and has QC issues.  Here's the data sheet:
http://www.mif.pg.gda.pl/homepages/frank/sheets/168/8/8417.pdf
I'm thinking the best route to take is to substitute a pair of KT88's and rebias.  $75.00 a pop for NOS doesn't melt my butter.  I'd greatly appreciate peoples thoughts on the subject.

Thanks!
-Richard

[FYL]

I've used 6550As as replacements in a few amps. No problemo, so KT88s should be fine.

[jjasilli]

Usually 8417's are repalced by EL34's.  6550's seem OK to me too, but check votages in this amp against max tube specs for plate and screen.  8417 are rated for 550 screen volts (though your specs say 500).  The other tubes are rated for 425 - 440 about 100 or more volts less than the 8417's.  How are your screens supplied:  UL tap; separate winding; dropping resistor???

[RicharD]

That's all the info I have on the amp at the moment.  I have not gotten under the hood yet.  Just assessing the obvious symptom which is a cracked & gassed tube.  Practicing the 6 P's:

Prior planning prevents piss poor performance.

[sluckey]

I like that!

[John]

http://www.kvraudio.com/forum/viewtopic.php?t=265173

This is all I've been able to find, but apparently Gibson will email you the schematic.

Because Gibson Guitars and CMI wre both owned in 1974 by Norlin, Gibson has inherited the old SG Systems documentation.

Gibson customer service can provide owner's manuals, schematics, parts lists, and service manuals. See Gibson's website.

SG Systems really only made three amps. They made a PA system, a bass head (100B) and a guitar head (100G). The speaker cabinets were just different sizes. The SG-215 was a cabinet with one Eminence 15" and two Eminence 10" speakers. My SG-215 has the 100B bass head.

The bass and guitar heads have a "notch shift" feature that was used to simulate other amps. For example, the 300 setting was for Fender and the 450 setting was for Standel.

The guitar head has a Maestro phase shifter built into the amp.

[Steve_P]

A buddy of mine on the Guild forum rebuilt a Guild bass amp to 6L6's from 8417's. He had to rebuild the bias supply to 6L6 specs as the 8417 needs around half the bias voltage.

[RicharD]

Looks like I'll be calling Gibson tomorrow.  I'm still leaning towards a current production tube and revamping the bias circuit.  Now EL34's seem like the best bet.

[DummyLoad]

Looks like I'll be calling Gibson tomorrow.  I'm still leaning towards a current production tube and revamping the bias circuit.  Now EL34's seem like the best bet.

rework bias ckt. - use 6550/KT88. 

[FYL]

Usually 8417's are repalced by EL34's.

Sylvania designed the 8417 as a kind of hard vacuum EL34 fitted with large plates, 7027A-style, and a high transconductance because of it's very tight construction - screens and plates are frightfully close. Hence the very high 23 mA/v Gm - vs. typically 12.5 mA/V for an EL34, 11 mA/V for a 6550A or a KT88 and 4.7 mA/V for a 6L6GC.

Nice and easy when it comes to drive them, but the structure makes it a nightmare when it comes to reliability: too much screen current leads to internal shorts and dismal failures. Ditto for grid resistors: the 8417 is spec'ed at 100K max in fixed bias, 220K may be OK but more is a sure recipe for disaster.
 

8417 are rated for 550 screen volts (though your specs say 500).

I've also got 660V plate, 500V screen design max values.

Datasheet: http://tubedata.milbert.com/sheets/168/8/8417.pdf

[PRR]

"it used 8417 output tubes.... Operating at 675 volts on the tube plates, this thing should do a very solid 100 watts or more!!"

It is *probably* using the data-sheet's 100W conditions. 560V (under load) plate and 300V screens.

The genuine EL34 will not pass the current nor stand the heat. (Some new-production EL34 might?)

The 6550/KT88 will; IMHO the 8417 is a hi-gain weak 6550 for consumer junk (tho my Bogen 8417 amps held up fine).

At these voltages, 8417 needs 15V bias and drive, 6550 needs 32V bias and drive.

The "great" thing about 8417 is that you can build a driver with one bottle. The Bogen 150W/ch managed a quad of 8417 with just a dual-triode (the one that is half AX7 and half AU7). The rare Dynaco quad-8417 120 Watt mono beast gave great performance with the pentode/triode. (Because Dyna ran the screens hotter, the bias+drive is more like 30V.)

OK. the 8417 amp is designed with lightweight drive. If you go 6550 (or EL34) you will need double the drive. Can the existing driver do that? (i.e. did the designer over-do things?) On the Dyna, I was dubious about finding 60 good Volts per grid. (Also the loss of NFB was a factor in a hi-fi amp.)

[DummyLoad]

it is likely that it will simply be re-tubed...i found some 8417s - new and used for a decent price. i'm picking them up for richard tomorrow.

--ISO

[RicharD]

It certainly seems like the best bet to stick with the "right" tube.  It keeps the amp authentic vintage (for whatever that's worth) and I don't have to redesign the bias supply and PI.

The million dollar questions are:  What would cause an output tube to crack?  What (if any) other collateral damage occurred?  I'm gonna put the amp through the usual paces, check power supplies, OPT, caps etc. before installing the rare replacement tubes.

[RicharD]

Benched the amp.  Power supply looks fine.  Installed new output tubes and both idle ar 25mA.  After about 1 minute, maybe 2, one tube starts increasing in current.  I shut it off at 30mA so as to not ruin the tube.  Swap tubes, problem stays with the socket.  Swapped the offending socket, both coupling caps, both screen resistors, and both neg bias series resistor.  The problem persists.  Standby on this amp disconnects the screens only.  As current increases, neg bias on the offending tube drops about 1.5 volts.  Switching into standby, the neg bias voltage stays reduced 1.5V.  With the output tubes pulled, neg bias is staying equal.  I'm thinking blown output transformer.  I haven't made it to the point of pushing signal through it, but I do hear hiss.  Doesn't really thud like I'd expect when coming off of standby.

Opinions?  Last ditch efforts before trying to find an appropriate OPT?  I haven't measured the turns ratio on this xfmr yet.  Still wanting to be in denial that it's shot.  I do know the guy was rockin it when it went down.  It's line circuit breaker was tripped.

-Richard

[PRR]

> I'm thinking blown output transformer.

Why?

Replace the OT with shorts, the DC conditions should not change. Nothing that happens at the plate (except no-Voltage) affects plate current much.

What you describe sounds like my last experience with 8417s. They have SO MUCH gain that they will go into radio oscillations for no good reason. Layout of plate and cathode leads is CRITICAL.

Alternatively, the too-tight electrode spacing and too-cheap assembly gets out of alignment as things get warm.

Those are incidental reasons I would favor re-biasing for 6550.

[RicharD]

I put the leads right back like they were.  I was think that 1/2 a primary coil was shorting as the transformer warmed up.  It does have a bias pot which should swing between -22 & -12.  Right now it's set for -17.5 which is where it's "painted" in place from the factory.  I'll swing it up to -22 and watch it.  Probably time to scope for ultrasonic oscillations.

I had a Hiwatt doing the same thang and Mark Huss told me to change the socket which fixed the problem.  I was a little disappointed when the same trick failed today.

Something's gone wonky.  The amp was fine for 36 years then cracked an output tube.  Now that same socket is problematic.

[DummyLoad]

come on by iffin' ya wanna tackle this thing again...

--ISO

[RicharD]

I'm completely at a loss.  We played pick-n- choose between 4 tubes and got a pair that for the most part would not run away.  If you turn the volume up less than 1/2 way and push signal through it, it stars arcing, pop pop pop pop.... about 4 times a second.

What's been replaced:
100k resistors between bias pot and grid stoppers
100R screen resistors
1 octal socket
grid coupling caps
tubes

That arcing really makes me think it's the OPT.  We did measure the turns ratio 33.9:1 which = 4600 ohms with a 4 ohm load.  That certainly looks right, but if it has internal insulation breakdown, a little 33.9V:1V test won't reveal this.  The B+ is 660V.  If I do replace the OPT, I'm thinking a DynaClone since it's a bass amp.  http://www.triodeelectronics.com/a4dyoutr43oh.html  I can't find an OPT with the same footprint.  I dislike everything about this amp other than it's "curb appeal".  SS front end, questionable output tubes, flimsy PCB, etc. 

[DummyLoad]

on a footnote... we probed with a scope and see no AF/RF oscillation. still wondering why it's motorboating/arcing at LF. checked NFB components and replaced the PI tube just because there's nothing else to swap in the final stage. 8417 tubes are idling @ 33mA / 27mA.

we're leaning towards a KT88/6550 retrofit after we confirm the PI is capable of driving 70Vp-p without too many mods. the PI supply is a tap off of the G2 supply and IIRC is ~~325V.

the bias ckt is a tap off of the low volt (-) rail for the SS ckts. and is good for -23V max - well need to revamp the bias ckt. and CC tap off the B+ since it's a v-doubler recto. we're looking at ~~ -36V target for bias - with a range of -40somethingV to -30V

mid-70's PCB junk amp. my gut feeling is to scrap the tube stuff and install 200W SS amp - it is a bass amp.