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Fisher 500c rebuild
#16
The New Sensor import tubes are just rebranded Russian made tubes, although they are new production, they are all made in Saratov in the same plant that made Reflektor tubes in the Soviet era. So the way to save money is to find the number of the equivalent Soviet type, such as a 6P3 for example, and buy a surplus mil spec example from Russia, Ukraine, or a former Warsaw pact country, I've never bothered since I have lots of good used North American tubes for personal repair work, but may if I decide to build a new amp or rebuild a friend's amp that has bad tubes.
Regards
Arran
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#17
Very interesting. Thanks for the information Ron and Arran. It sure is a shame we don't have any plants here in the USA. I checked prices on those tubes, both the 12AX7 and 7591a and I have to say I had sticker shock! I have come across a few pairs of the 7591a tubes for a reasonable price, but most are nothing short of stupid. I kind of expected that, tho. But, I sure didn't expect our own NOS USA branded 12AX7a tubes to be quite that pricy. I guess I understand why so many of those Fishers are sold knobless and tubeless.  Icon_problem If I recall, for awhile, there weren't any new production 7591 tubes. I think I recall one new production brand (been some years ago) that you had to change a resistor or something to make it work. At least they didn't rob the tubes from the IF strip. Missing the 6HA5. Well, still doing reading on AK. Some very interesting forums on this type of project. Trying to decide what kit to get, or if I should get the separate parts. Still got to get the electrolytics. And still thinking of Hayseed, but they are a little pricy. May be simpler tho.
If I could find the place called "Somewhere", I could find "Anything" Icon_confused 

Tim P.

Matthew 6:34 "So don't worry about tomorrow, for tomorrow will bring its own worries. Today's trouble is enough for today."
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#18
Regarding the electrolytic cans, your 500-C is certainly worth the investment in Hayseed Hamfest replacements. Pricey, yes, but the set will look much better with those than if you opened up and restuffed the original cans. Plus, there's something to be said for safety. Having come close to seriously cutting a finger or thumb in opening and restuffing those electrolytic cans...yes...I would bite the bullet and invest in the Hayseed caps.
--
Ron Ramirez
Ferdinand, IN
Я этого не понимаю
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#19
another thread for you to study if you wish...EFB (Enhanced Fixed Bias)

http://audiokarma.org/forums/index.php?t...rs.539265/
--
Ron Ramirez
Ferdinand, IN
Я этого не понимаю
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#20
I would stick with rebuilding this thing to stock first, unless it has some obvious design flaw, and after it's working then start looking into mods.
Regards
Arran
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#21
Disagree. Fisher ran their output tubes way too hot, so a stock restore combined with today's higher line voltages is a recipe for short 7591 tube life. IBAM or IBBA plus EFB is the way to go.

When I rebuilt my TA-600, I incorporated the IBAM mod as part of the restoration and the set was not turned on until after I had done the IBAM mod.
--
Ron Ramirez
Ferdinand, IN
Я этого не понимаю
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#22
I have been doing a lot of reading and continue to. I am surprised at the amount of info on this. Everything but the knobs lol Icon_rolleyes ! From my experience with this, those outputs do run pretty hot. I killed a set of tubes back in the day when new old production tubes were still available, and not terribly expensive. I had a friend from ITT tech that brought a set of output tubes from his Fisher to class, and it stymied me. Those tubes glass envelopes were melted and the bakellite bases were nearly completey burned off! This was back in the mid 1980's, so at that time, our 500s were 'only' about 20 or so years old! His was a 500B. So, I know what these units are capable of.

I am somewhat in agreement to power up and test in stock form before any mods are made. This has merit, and I often do this when practical. Then, start making adjustments. I do know, however, that their stock bias setup is less than perfect, then add 50 or so years to all that - yeah. I must have the bias supply working properly before I hit those tubes with full power. I believe the bias rectifier is a problem child, so I will at least replace that before I test it. I really don't want to destroy a set of known good 7591a tubes. I am leaning toward building the IBBA, which not only sets each tubes bias and current draw but also balances each pair. I'll probably go with the Hayseed E-caps for simplicity.
If I could find the place called "Somewhere", I could find "Anything" Icon_confused 

Tim P.

Matthew 6:34 "So don't worry about tomorrow, for tomorrow will bring its own worries. Today's trouble is enough for today."
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#23
Tim, I'm afraid that I don't have the experience with the Fishers such as yours, like those guys at AK have. I don't own a 500-C or 800-C. I do have a 400, my first Fisher tube receiver which I bought at Kutztown two years ago. It is similar in some ways to your 500-C but quite different in others. And I still have a 500-B and a rare 500-S waiting for me.

The output circuitry of the 500-C is quite different from the TA-600, so I can't offer much help other than to suggest reading up on the 500-C at AK.

When I get my 400 and 500-B restored, I might be able to offer up a modicum of advice. But I'm still learning at this point. A Fisher tube stereo receiver is quite different from a Philco 89. At least the output tube circuit in the 89 wasn't designed to run the 42 tube at (or near) its maximum limit. I think it's great that these guys have come up with ways to increase output tube life in those Fishers, and I will certainly incorporate their ideas into my 400, 500-B and 500-S when I restore them...just as I did with my TA-600 with its IBAM circuit.

Nevertheless...I'm still watching this thread with interest. As old Red Green said so often, "Remember, I'm pullin' for ya. We're all in this together." Icon_smile
--
Ron Ramirez
Ferdinand, IN
Я этого не понимаю
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#24
All in all I can't understand the appeal that these 500Cs, and many of these other tubed Fisher tuner-amplifiers seem to have, it sounds like the amplifier circuitry of these 500Cs is of a very poor design, it sort of reminds me of that Croften-White amp circuit that was tried back in the early 30s where you had to have both tubes in the circuit in perfect condition or bad things would happen. I've never really understood why anyone would build and market an amp with tubes driven to the maximum operating limits, other then just for bragging rights in marketing, something goes slightly wrong and it's meltdown time.
Regards
Arran
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#25
If you are opening electrolytic caps with anything besides a Dremel tool (thin cut), I am going to report you to - - - - whoever sews fingers back on. (I stab myself enough just stripping wire) (where is that tube of liquid Band-Aid (super glue)?) (It is such an annoyance to get blood on your project!) - - Yeah, yeah, tell me none of this is true. Icon_twisted

I only use US made tubes in these amps (Stars and Stripes - Forever, playing in background) because they are so d**ned good - - or because that is all I have. Icon_rolleyes
"I just might turn into smoke, but I feel fine"
http://www.russoldradios.com/
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#26
(02-11-2018, 03:29 AM)Arran Wrote: All in all I can't understand the appeal that these 500Cs, and many of these other tubed Fisher tuner-amplifiers seem to have, it sounds like the amplifier circuitry of these 500Cs is of a very poor design, it sort of reminds me of that Croften-White amp circuit that was tried back in the early 30s where you had to have both tubes in the circuit in perfect condition or bad things would happen. I've never really understood why anyone would build and market an amp with tubes driven to the maximum operating limits, other then just for bragging rights in marketing, something goes slightly wrong and it's meltdown time.
Regards
Arran

I have to agree - but only in a specific context.

I have Dynacos, Sherwoods, Scotts and so on. I use them daily. My setup(s) all are bi-amped using large (very large) solid state amps to drive woofers (sub woofers if you prefer). The largest of which is a DH500 driving both channels into a dual voice coil 15" woofer (4 ohms/channel, better than 1kw) and a Fisher X200 (7868) doing the rest (which can be instantly switched in and out with a Sansui 9090DB for SS - hollow-state comparisons).

There are 4 amps set up as described above (the only valid HiFi comparison being a Scott at the moment). Anyway, I need to drive the base amp, preferably with a variable output so that it tracks. The input varies by pre-amp output, causing the center/tape out to follow, except the Dynaco amps have no center/tape out.

What I have found is that the other brands (less Dynaco since it does not provide this kind of output) Will do a good job with freqs below 100hz using center or tape out. The Fisher is a little muddy - sometimes, but only on the bass.
"I just might turn into smoke, but I feel fine"
http://www.russoldradios.com/
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#27
Russ;
  I was thinking more about the quality of some of the parts they used, such as the IF and discriminator transformers, they appear to be the same makes and styles as those used in millions of AC/DC sets and run of the mill radio-phono combo units from the late 1950s and early 60s, the ones with the crappy silver mica sandwich capacitors in the bottom. This is what the Miller/Automatic K-tran style ones have for sure, the cone topped ones might have something similar, but again nothing high end about them even if they used ZPO ceramics or discrete silver micas inside. The switches and pots, again run of the mill stuff, probably made by CTC, Centralab, or one of the others, I don't see any Allen Bradley stuff in there, the band/function switch is your standard phenolic wafer construction. All of these components probably work fine , they certainly do in many other articles of consumer electronics, but Fisher was supposed to be premium priced, and so are many of the asking prices for these units today. I would expect that something that was premium priced should have used industrial grade or mil spec grade components all through, the only thing that I can see that is above average are the tube sockets perhaps, and the Phillips mustard caps, and that it should be ready to play with a minimum amount of work, such as replacing the electrolytics, not a rewire of the power output stages.
Regards
Arran
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#28
OK Arran. You don't care for Fisher. We get it.

I've just been bitten by an FM IF with silver mica disease. Model FM-100-C. It's up and running but after repeated alignments the signal strrength sucks. Tubes are good. I'll deal with it.

But this is Tim's thread, apologies for going off topic...
--
Ron Ramirez
Ferdinand, IN
Я этого не понимаю
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#29
I pulled the trigger on some Hayseed caps. The more I thought about it, the less I liked the idea of having a bunch of caps stuffed under the chassis. To restuff may be an option, but not on this unit. FYI, the Dynacos have bias problems on their ST70's at least, so it's not just Fisher. The mods available for those amps can drive you nuts. I like the Dynas too.
If I could find the place called "Somewhere", I could find "Anything" Icon_confused 

Tim P.

Matthew 6:34 "So don't worry about tomorrow, for tomorrow will bring its own worries. Today's trouble is enough for today."
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