500C owners...Where would you place your best tubes?

thevinoman

Nothin' but the Blues...
Just acquired a very nice 500C. What I'm wondering is, where would you experienced owners place your "best" tubes in the unit? I'm not a great schematic reader, so I thought I'd reach out here. I have lots of very good Mullard, Tele, RCA blackplate, and also some great 5751's. What are the most important tube slots for great sound? I know the 5751's are lower gain, but also less microphonic, so looking for advice there too. I know not to place them in the phono spot. Any help is appreciated. Thanks very much.
 
Best answer I could mention is make sure ALL the 500C's are up to snuff in the first place! My 2 cents... Al
 
Best Matched 12ax7's in the Voltage and Tone Amp positions. These positions work both channels, so they should be matched. Next best or equal should be in Phono (NO 5751's in any position on FISHER's). Next best tubes after these would be in the Driver/Phase Inverters. I would use matched section 12ax7's in all 3 positions in the MPX. V102 (3rd tube) is the MPX AMP tube so shoul be section matched and high in gm.

If you listen to FM, get 6au6's that have as high a gm as possible compared to the datasheet gm's. The Early 500c's used a 6gk5 and 6AQ8 as the front end tubes. These two get rode hard and put away wet. Especially the 6AQ8. Make sure it's a VERY HEALTHY Tube. Gm as close to or over what the Datasheet spec's. The 6gk5 should also be changed. In both versions V3 is a 6HR6 which is very expensive when you can find one used, and almost extortion NOS. A good sub for it is the 6AH6. And it's comparably cheap. A 6AU6 can also be used, but it doesn't have the gm and gain of a 6HR6/6HS6/6AH6. GE Tubes in the RF/IF strip are good choices as are Sylvania Yellow lettering tubes. V7 is the Stereo Beacon Amp. It gets lightly used BUT can also be weak. If it tests stronger than the 6GK5 in early models you can swap them and put up with diminished response from the stereo beacon until you can get one.

OUTPUT TUBES. 7591's to AVOID like the plague with a stock 500c are JJ7591's. They are the 71/75 Pinto's of the 7591's. BAD JU-JU. If the tubes have a fair amount of life left and you have 10 ohm cathode resistors installed, check the mv on the cathodes. Then shift the decimal 1 place to get ma. There should be no more than 3-5ma between them. If so they are unmatched and this should be corrected with a IBAM or IBBA board to adjust them so they are all working together at the same current level. 100ohm screen resistors should also be installed to decrease the possibility of tube arcing. If you do go with New Stock, you have the EH7591 big bottle or the Tung-Sol standard size bottle. Both will work well in the 500c with resistor changes noted above along with grid return resistor change from 330K to 220K or 200K. The Coupling caps will need to be changed to .068uf to keep the R/C timing constant approx the same. All of these changes will increase safety and reliability of the 500c. If you do nothing but change out tubes, you're not going to change anything as far as the sound. It's full of tired caps, and some resistors. It would be better to overhaul it with new replacement components, and then take a baseline sound with it as is, AND ONLY THEN start rolling tubes.

Larry
 
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Larry, first of all, I love your signature. I originally enlisted as a Navy Hospital Corpman, but they told me I was too smart, and stuck me in subs.....Should have known better.....

Anyway, thanks for all the info. Would you mind detailing the specific tube socket numbers you referred to above? That would really help me out.
 
You too? They wanted to make me an STS but I had a Guaranteed seat for HM"A" school, and told this Lcdr in Detailing at RTC that I'd take HM"A" School or a Honorable Discharge for the Gov't failing to make good on a written promise. Summer of 73 I got HM "A" School, then FMF School, 1 yr @ NAV DISP T.I. (The crotch of West coast Medical). Put in for FBM & HM Prog. Got Sub Scol in Oct 74, Casimer Pulaski SSBN-633 (the Crazy Polack) Blue April 75 out of Charleston. Did 4 patrols, Qualled and was suppose to go back to Sub Med School as a HM2 when I re-enlisted (I extended a year on a 3 yr enlistment to get FBM/HM). Detailer screwed with me and changed orders to a DD out of Pearl off the Pulaski. I told him flat out I wasn't gonna ride a TARGET, pulled my Re-Enlistment, had 59 days on the books, took 29, sold the rest back, and got out a HM3 with 3. The 70's were bad for jr HM's. They weren't too good for any HM. Rates were frozen, no-one was making Chief as all the chiefs that were in were not retiring and pushing for 30 years plus the rate was overloaded. So there were a lot of 20+ year HM1's.

Thinking back on it I should have taken the STS rate as they were getting VRB of 8 to 10 and HM's were a ZERO. Take basic rate, multiply by # of years re-upping and then multiply the result by the VRB. I saw 4 yr ETC's reupping for 6 years and walk away with almost 50K. What a racket! And most of these chief's couldn't fix a crystal radio. Book Smart, Yeah! But no leadership abilities, no street smarts. Friend of mine who went in as a ETSN(he was an Eagle Scout and had a GCT / ARI that was off the scale plus a Bachelors in Electronics) made ET3 out of Boot, 2nd out of BE& E, ET1 out of ETN "C" School, and ETC out of Prototype, All meritorious as 1st in class. Slick sleeve ETC with less than 4. It took my dad to make ETC 16 years in the Mid 60's. Like I said a Racket.


BACK ON TOPIC! Inside the MANUAL is a TUBE LAYOUT, with tube #'s and a description of what they do. It would do better than me. It's not an S5W operations manual, last I saw one it was something on the size of 4ft tall, and had changes made monthly that had to be incorporated into the manual pretty much immediately depending on how much of an a**hole the ENG was. But usually by the end of patrol. Gawd help you if there was an ORSE and the manuals weren't up to date! Uncle Hymie was such an anti-social prick, but the jobs got done and usually right the 1st time. If it weren't for him and his gang of thugs, we'd have ended up looking like the Red Navy with all their irradiated boats/crews.

The Main visual differences between early and late versions are the RF Tubes (V1-V2 Early) and (V1-V17-V18) Late version Pertinent pages below. Both the Early (10001-19999) and 2 late manuals (30001-39999 & 30001-49999) are on this page for download at the
http://www.fisherconsoles.com/service%20owner%20manual%20list.html This page is linked at the bottom of the front page of the site page................................ FISHERCONSOLES SITE. www.fisherconsoles.com (bookmark).

EARLY VERSION Serial #'s 10001-19999
Early 500c layout.jpg

LATE VERSION Serial #'s(30001-49999)
Late 500c layout.jpg
 
What rate did you end up as??

I volunteer on a Diesel Electric boat in Baltimore Harbor. Been doing this since 1998. We've done a hell of a lot of work on her in the last 19 years. I went thru qual's on the boat, It was easier than the Forward 1/2 of a Lafayette class, John Madison Variant. We didn't give the later boats a separate class designation at the time, it was Laf class, xxxx variant.

We've gotten up and running or replaced at least, Water systems(both potable and non potable), LP air systems, Sanitary tanks, replacing equipment that was stripped off her when she was struck from register. We're working (albeit slowly) on getting one of the F-M 10 Cyl Rock Crusher's operational. Bunch of 60 to 80 year olds acting like a bunch of 20 year olds. Dad was voted in as COB, and was active until about a year ago when he was diagnosed with pre-alzheimers. As the only non technical rate onboard (all ET's EM's, TM's and a couple of EN's and MOMM's) I've become the de facto 1st Lt. and Medical Dept. Commercial Epoxy 2 part paints make it a lot easier along with a lot of superstructure replacement metal. Deck is totally replaced with a South American wood that is tougher than TEAK. When the boat came into Baltimore in '73, some candy-a**ed, moron in the Baltimore city admin, who couldn't find his a** in a lit room with a 10 man working party, had an epiphany. He'd been reading a Little Golden Book of WWII Airplanes (this was at his limit of understanding and comprehension apparently) and saw the sharks teeth on a P-40 Warhawk. So TORSK (Ex-SS-423) got some G.D. goofy looking teeth on the bow just above the tube shutters. We Fought the Museum that runs the boat for the City and finally won. We've got kill flags painted on the tube doors, But the museum won't let us put kill flags on the sail. Got a couple of P-250's and 500ft of 1-1/2: line with all purpose nozzles to take care of the pigeons under the superstructure and in the sail. Once a year she get's Douched completely topside. Fun, Wet, and STINKS!!! Last part of th topside restoration is the bridge, putting new support steel in, then new decking, all phones, TBT, Emergency switches, and have gotten the sail bubble off the old Sailish (SSR-572). Luckily it fit perfectly, as both boats were built @ Portsmouth, and Torsk got her step sail in 1952 there. We've got about 8 fish onboard including MKXIV's, MK XXVII, MK 37 and a Mk45 Astor. One of the MKXIV's was on Torsk during active service in the 60's.

BACK ON TOPIC!

I would look up the serial on yours and match it to the manual. If your's is a 20001-29999 and matches the tube layout on the 30001-39999, get that one. If it matches the earlier 10001-19999 get both the Early and later (39999) as it may have later changes. Usually Fisher didn't change the manuals unless there was a bunch of changes and then it was done at the start of a 10K run.

If you get stuck ask questions. No question too dumb, no answer too sarcastic!:D:naughty: So get crackin on the manuals, Non Qual. No Movies, No liberty, and no sleep for the next 6 months. Watch, then Dept. Duties, study Quals, repeat and Rinse.


If you have a 20001-29999 version use both manuals, but use the one that is closest to yours. Fisher sometimes made undocumented changes in between manuals and then the manual was updated at the series change (usually every 10K units).

Ask Questions if you get stuck. No question too dumb, no answer too sarcastic!.
Remember ;
Answers............................................$1.00
Answers which require thought........$2.00
CORRECT ANSWERS...................$4.00
Dumb Looks.....................................FREE!

Get crackin Non Qual. No Movies, No Liberty, No Sleep. Free time (if any) is spent studying and doing hand over hand tracing. Systems need drawings, etc. You know the drill. Oh yeah, one more thing. In addition to all of the above you get to MESS CRANK for the next 3 month's. No rest for the weary or NQP's. While your at it send a plate of sticky buns and a black and bitter (Mid-Watch steamin all night stuff with 3 year old TM socks, Hydraulic fluid, and unavoidable remnants of the San Tanks after the COB blew the San Tank inboard, that's metamorphing into asphalt in the bottom of the urn, Real coffe you can stand a Fork in upside down, and would dissolve the tines off in 3 minutes), in a REAL Mug. (no Handle) down to the MED Locker. Only QM's, Chiefs, and O-Gangers use handles on a coffee mug/cup. :D :rflmao::p
 
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Don't put the 5751's in the phono preamp. I tried that once on a Sherwood and couldn't figure out why the phono stage sounded lousy. I'd completely forgotten about putting those tubes in there. Swapping them back for a 12ax7 like it called for fixed it right up. The Fisher phono circuit isn't grossly different than the Sherwood, so I'd expect a similar sort of thing.
 
Wow, thank you so much for all of the information. That's one of things I love about this place, is people helping others. I will reply with more a bit later in the next couple of days, but just wanted to tell you that I arrived at Sub School New London mid December 1974, straight from RTC San Diego. Spent the next 2-3 weeks while the school was closed standing firewatches all over the base. Yes, spent Christmas and New Year's there. A homesick boy I was for that brief time.....
 
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