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  #16  
Old 09-26-2009, 01:30 PM
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Jeffhs Jeffhs is offline
<----Zenith C845
 
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Quote:
Originally Posted by Kingfisher View Post
Mine has a bad hum, part of the circuit board near the output tubes looked fried. I haven't used it in several years. This must have been one of the last tube models Arvin made since it was bought new in '68. Won't be finding any 50HK6 tubes at Rat Shack. They stopped selling that kind of stuff loooong ago. Arvin was known for wierd tube numbers, 50HK6, 20EZ7, etc. It's often a dead giveaway as to who actually made some store branded radios.
Some Zenith MJ-1035s used 50EH5s in the output stages, while others had 50C5s in the same position. Mine (an MJ-1035-1 from 1965) has 50EH5s as the output tubes, and has a couple of tubes with 19-volt filaments as well, wired in a series-parallel arrangement with a filament transformer. The tubes are wired in two separate strings, with the transformer feeding both. I'll have to look at the schematic to see how that's done, as before this one I had never seen any radios with two series string wired in parallel with a transformer. I guess Zenith did it that way because of the two 50-volt output tubes, which could be a series filament string by itself--50 volts x2 equals 100 volts, close to the line voltage. However, in my MJ1035-1, one 50EH5 is in one string while the other is in the second string. I don't know offhand what the combined filament voltages of both strings would be (probably well over 200 volts, since the MJ1035 series had either eleven or twelve tubes). Zenith may have had no recourse but to use series-parallel wiring and a 6.3-volt filament transformer. The downside is that the B+ for the tubes is still derived from the AC line (in fact, the radio chassis is directly connected to one side of the line, as attested to by a warning on the back cover), so the filament transformer will not provide any kind of isolation.

I agree that tubes are difficult to find these days, with everything being solid-state, surface-mount components and such. However, it is possible to find old tubes (even real oldies such as 01As, et al.) if you look around. John Kendall's Vintage Electronics (www.vintageelectronics.com) in suburban Baltimore has many vintage tubes currently for sale at dirt-cheap prices; I've purchased tubes on that site for my vintage sets that I might not have been able to find elsewhere. I don't know if John Kendall has 01As or anything older than the late '40s, but he does have tubes from the '50s to the end of the tube era.

One warning: While the price of the tubes on vintageelectronics.com may be low, the shipping charges, depending on where you live in relation to the Baltimore-Washington, D.C. area, could be quite high. I live in northeastern Ohio near Cleveland and found a shipping charge of something like $8 for my last order of tubes purchased from the site; the tubes themselves cost only about $2.25. This is shipping for tubes sent to Ohio; of course, shipping to other areas, especially to the West Coast, will be higher. Note as well that single tubes, even miniatures, will be shipped in large boxes, to survive the rigors of the USPS's automated sorting system. The reason tubes are not shipped in mailing envelopes, even padded ones, is precisely because of the automated sorting machinery. I was advised some time ago by an AK member that tubes sent in mailers would be crushed and/or smashed on their way through the sorting equipment, resulting in the customer receiving an envelope full of smashed glass and crushed metal.
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  #17  
Old 09-26-2009, 03:08 PM
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Chad Hauris Chad Hauris is offline
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Location: West Texas
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Quote:
Originally Posted by Kingfisher View Post
Mine has a bad hum, part of the circuit board near the output tubes looked fried. I haven't used it in several years. This must have been one of the last tube models Arvin made since it was bought new in '68. Won't be finding any 50HK6 tubes at Rat Shack. They stopped selling that kind of stuff loooong ago. Arvin was known for wierd tube numbers, 50HK6, 20EZ7, etc. It's often a dead giveaway as to who actually made some store branded radios.
I remember the 20EZ7 tubes, luckily I didn't lose them! I think they are a 100 ma. heater version of a 12AX7. There are two filament strings in this radio, one with the IF, AM convertor and multiplex tubes which uses the 100ma filament tubes (18FY6, 18FX6, etc.)
and the other filament string which has the 150 ma. filament tubes (50HK6, 50C5 and 12DT8 plus a resistor).

I didn't learn till fairly recently that the 50HK6 has part of its filament as a fusible resistor for the silicon diode B+ supply (I know in other radios this is how the 50HK6 is used and presumably in this one too). I never could figure out as a kid why putting in 2 50C5's caused there to be no power.

Even in 1984 although Radio Shack did have a tube tester and tubes in stock they still had to special order the 50HK6 tube for me.
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  #18  
Old 09-26-2009, 11:21 PM
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Kingfisher Kingfisher is offline
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Quote:
Originally Posted by Chad Hauris View Post
I remember the 20EZ7 tubes, luckily I didn't lose them! I think they are a 100 ma. heater version of a 12AX7.
I kind of figured that out based on the shape of the plate. I looked up the 20EZ7 some time ago out of curiosity and found that it is indeed similar to the 12AX7. Makes me wonder why no one has bothered using them for home-made amps as a 12AX7 alternative. I bought a couple from AES and they were cheap as dirt.
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  #19  
Old 09-27-2009, 12:43 AM
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Several years ago, I had a big Motorola table AM/FM stereo radio with phono input. It was in a wooden cabinet with two fold out speakers and a built in center channel speaker. I think it used 6BM8's for output tubes. The dial glass on mine was cracked in several places and I ended up selling the radio on epay when I thought I needed money.
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  #20  
Old 09-27-2009, 10:42 PM
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sloober sloober is offline
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Not exactly tabletop, but I got this last week. This one is stereo, but needs a rebuild.
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My gear- Sansui 500-Fisher 800c-HK Ballad-Eico HF-60-Dynaco MkII-HK ST-7- BIC 960- Mission TT and a bunch of other crap

Last edited by sloober; 09-27-2009 at 10:46 PM.
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  #21  
Old 09-29-2009, 01:18 AM
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Sam Cogley Sam Cogley is offline
The Devil Inside
 
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Location: SW Missouri
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Quote:
Originally Posted by Kingfisher View Post
Looks can be deceiving. I still have yet to recap mine. There is a lot of caps to replace, plus I've had trouble trying to get the chassis out because of the dial string being connected to it. There seems to be a lot of high voltage (1000v) caps in this one. Bought the Sams Photofact for this model last year on e-pay because it was missing from my collection.
I need to track that one down.
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