Screen Protection circuit done - now what?

It's FINE! Don't worry about it unless you get a minature Mushroom cloud above each one should they blow. (jk!)
 
I hooked up a small box 4"-square fan that is turned on when the 500-C is turned on and I have it pointed at the power tubes. Keeping them cool that way should give them a little longer life.
 
Doubtful. I've never put a fan on FISHER gear, even with the 800c inside the Exec console. Now my Sansui 1000A is a different animal altogether. I havea 12" oscillator blowing across it on low. Not really needed with it outside of a cabinet, But if it soothes your OCD........
 
Thanks for the laugh. I read somewhere that blowing the heat away from the tubes keeps the heat buildup inside the tube somewhat minimized. Seems logical. If nothing else, I have another heat source on cold winter days. :D Not only tube dissipation but now also 500-C dissipation !
 
Blue is not necessarily a bad thing, most power-handling tubes, from table radios to big mother 6L6's, kt66/88's etc will show some. I've seen 3v4's in old battery-electric portables with it. Looks cool, especially when volume & bass is cranked & it pulsates!:rockon:
 
Pulsates? NO WAY ! Fisher really was way ahead of everyone - light shows inside their receivers !! :rockon:
 
I need help with a theory. If I want my power tubes to work less hard, that is put out less power so I can have my volume knob at 12 o'clock instead of where it is now, 9 o'clock, would I turn down the bias on each tube? Right now I have the "loudness" control on so I can get the good tone I like on the lower volume which is how I listen most of the time. My thinking is that if I can crank up the volume, I can turn off the loudness control and get closer to a more natural sound, closer to flat frequency (besides, I have an equalizer in my stereo system which is where I dial in compensations for room acoustics, etc.). I know this theory works on guitar amplifiers. It's hard to get good tone at low volumes on guitar amps. I assume the same principal is at work on a Fisher 500-C. Or is the tone directly tied to the power tube output such that I'll lose tone if I dial down the bias? I'm pretty sure tone is handled by a different stage in a tube amp and the power amp (tubes) section is just amplifying whatever the preamp tubes are doing. I mean this is just a math exercise to lower the watts, right?

(later that day) - I dialed the cathode bias down to 31mA. On one of the tubes I couldn't get it to go any lower. The pot was turning, but the voltage was not going down. So I set all 4 power tubes to 31mA. It seemed to me that the right channel was lower in volume than the left channel although it could well have been the radio station. Of course I didn't think to check other radio stations...DUH ! It also seemed that one fo the power tubes was significantly less bright than the others even though the cathode biases were close. Anyway, now I'm running a slightly lower wattage. By the way, I measured the voltage at tube V14 pin 4 where the Filament out from the SDS board goes and it measured -22.3 vdc. I thought it is supposed to be a little more negative but close enough. Comments?
 
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About the only way you'd got any results that would somewhat mimic what you are theorizing, would be to get the bias down into the weeds (about 10-15ma). The output on the Volume control wouldn't change but the distortion @ 9:00 would be so horrible that Guitar friends would jump out 2nd and 3rd story windows from fright. You've got enough bias spread for 31ma to about 40ma. Plenty good enough.
 
I can always count on you for a good laugh, Larry. Thanks. Yeah, it's probably just as well I can't get the cathode bias below 31mA.

There so many mods I want to try, but I can't map the schematic to the chassis to find the parts that I would swap out. I've been trying to find C35 and C36. Just can't. I thought I found the surrounding parts, but that meant that C35 and C36 are these 2 huge, orange capacitors that look like they've already been replaced and are heavy duty enough as is. I also want to try Dave's RIAA mod, but forget it. His schematic I think is an older 500-C than mine which makes mapping components even more challenging. Oh well. I love this thing.
 
RE: RIAA mod. FISHER used the SAME IDENTICAL 50187-3 PEC from literally the TA-600 in 1959 thru to the last 400-500c-800c-1800 tube units in 1969.

C35 and C36 are on either side of V14 (Pin2 and Pin7). As these are 1000pf 1000v ceramics, leave them be. 0.001uf/1KV would be film replacement if you are so inclined. Films are going to be huge at that value/voltage.
 
Thanks, Larry. Man, you're good. I'm talking about the "Fisher 500c EFB & IBAM mod11.pdf" schematic (can't upload - too large) that contains some of Dave's mods. It is VERY different from my 500-C schematic (I know my SDS board wouldn't be in there). Could he have actually redrawn the entire schematic???

I was just looking at the schematic for 1800pF and 2.2M that are part of Dave's RIAA mod and found them as C68 and R70. I can't find a 390pF though. Not in the parts list.
 
Gimme a link to the thread / schematic. I think LUIS (eduarsan1) drew in the mods like he did on the 400&EFB.
 
On the subject of BLUE GLOW IN TUBES

YES it PERTAINS to ALL vacuum tubes. Even Russkie and Chinie tubes

Reasons-for-blue-glow-in-tubes.jpg




 
Thanks for the article, Larry. I haven't seen that blue glow since I last reported it. I'm guessing it was because they were new and hadn't been biased yet (if I'm remembering correctly) so there was some extreme electron and maybe gas activity going on.

Here's a link to the thread with Dave's schematic:
http://audiokarma.org/forums/index.php?threads/500c-acquisition.668466/page-9

I'm not really concerned that it's so different from mine. I've been trying to find how it differs from mine as a clue as to where his mods are plus his version contains a lot of helpful, identifying text not on my schematic (such as wire insulation colors). But there are so many differences that I've abandoned using it that way and instead I'm saving links to threads where he discusses his mods. I have taken so many notes while reading AK threads so I can remember where to find good info. Yours and Dave's posts of course are full of gems and my notes must be 10 pages long by now. At this point the mods I'd like to tackle are the RIAA mod and maybe the bass improvement mod which you advised against (it's only 2 caps so I could undo if I don't like the outcome).

I don't recall if I mentioned that I made a version 2 of the IBAM board which includes posts with which I can measure the pin 5 cathode voltage so I don't have to open the unit to measure. But since the new Tung-Sol 7591A's seem to be fairly stable, I may just leave it as is.

In addition to those mods I want to restore those 2 AR-2Ax speakers. I have the parts. I just need to make a plan so I don't get fiberglass fibers all over the place. I'll probably use a large leaf bag to store while I replace parts (or maybe I'll just buy some new absorption material) and be careful as hell. I tend to like to keep components as close to original as possible unless the changes make significant improvements.
 
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I keep getting a Page doesn't exist from Photobucket. Which isn't surprising, the cheesy bastards have locked out just about anything and everything unless you cough up extortion $$$$. Even to get your own pic's downloaded.

You might want to get with Dave on the mods (and just happen to mention your's has the SDS board before it gets too involved.) and see what he thinks.

As for the AR2ax;s. Wear long sleeve shirt with closed cuffs, and long pants (no scaring the local wimmenfolks). Close up the collar on the shirt. Close fitting dust mask or keep upwind of the insulation. Nytrile Gloves, and bag the insulation straight into plastic trash bags. 1 bag per spkr. Then after you've redone the electronics, just put the insulation back inside, seal with plumbers putty between the drivers and baffle board. Get out of the clothes taking care to NOT touch the outer surfaces to bare skin. Bag the clothes and go in and take a long shower with plenty of soap. Cornstarch on the skin after drying off helps with itching. Fill the Utility sink 1/2 full of Hot Water and Laundry Detergent. Open the bag and ease out the clothes into the water and submerge them with a stick. Swish the clothes (not you!) around some then drain the sink. Put the clothes in the Washer by themselves and give them a HOT WASH/COLD RINSE with extra rinse cycle. More Detergent and add a large dose of DOWNY liquid to the machine. Make sure there is an extra rinse. If you have to run another spin cycle. Then take clothes out and hang up on line. DO NOT run thru Dryer. Dryer will tend to set the fiberglass in the cloth. Hanging up to dry stretches the fabric and releases more of the fiberglass fibers. If they feel itchy after drying run them thru washer again (same cycle). and Hang up to dry and let them get some ventilation for a day or two.
 
Holy cow, Larry, you don't miss a beat ! I've worked with fiberglass bats before so I'm familiar with the required protections. But the insulation particles in speakers have probably been loosened big time from moving the speakers and from the sound vibrations so there are a lot more loose pieces in there.
 
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I opened mine up 3-4 years ago and they were pretty solid. Very little if any remains in the bottom.

I was one of 3 Safety Engineer/Tech's in a Shipyard during the late 70's to the late 80's. Mainly worked with a Marine Chemist to determine "Safe for Entry- Safe for Hot Work" in Tank's / Compartments, voids, cofferdams, Fuel Tanks, Gasoline tanks (that was always fun), Oil Tankers, Inerting voids/tanks with CO2 Gas to 51% CO2 so hot work could be done on outside of voids/tanks, Asbestos certifications of Engine Rooms, etc., environmental monitoring, enforcement of OSHA Reg's for Marine work, etc. It was fun, and I got to climb thru all kinds of tanks on military Targets (if it ain't a SUB, it's a TARGET!), and did most all of the Bay Trials on the conversions we did for the Navy and Air Farce. They retired Observation Island about 4 years ago. 1st time I saw her was at the Cape for Polaris Trials in 1960-61 as a 5-6 year old. 20 years later we converted her to a phased array radar instrumentation ship. (Cobra Judy). Took out the missile tubes and put on a 40 ft turret with this huge Raytheon Radar on it.

This Montage was taken by the Shipyard photographer about 10 miles south of Annapolis in April 81 on Bay Trials. I'm one of the Dummies standing above the Bridge on the Port side. Had 7x50's glued to my eye's and a Sound powered Headset to the Bridge. 4 hours on 4 hours off. We spent over 20 hours out on Bay Trials. She was a good feeder too! In addition to Lookout I had the Medical Dept to run for the trials. No injuries, nobody got sick of or from the food.

Now you know the rest of the story.
USNS_OBSERVATION_ISLAND_(T-AGM-23)_Montage_bay trials 4-81.jpg
 
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