Fisher 500B Tube & Component Questions

[bigcaddy64]

Over the weekend I ended up buying a 500B for what I though was a fair price.

It looked dirty but I could see the real jewel lurking beneath the dirty exterior. Since it was a very busy weekend, I just now had some time to look it over and write things down.

My main questions are about the tubes and does it appear to have any service work/component repair done. I noticed a repair shop sticker on the back of a transformer but the shop is closed permanently. Nothing really jumps out as new on the underside but I’m not knowledgeable at all with Fisher items.

The second question was regarding the tubes. I looked at each one, noted the info/markings and reinstalled them one by one. Did Fisher use one particular brand during production periods or did the tubes get mixed as they cycled through them.

My unit has a mix of Sylvania/Zenith/RCA but the rest (60%) are all Fisher marked and are a mix of Mullard/Telefunken and possibly Amperex

Is the mixup of manufacturers with factory labels consistent with original equipment or does it indicate routine replacement with Fisher branded tubes over the years?

 

[larryderouin]

On the 500B Tele's or Amperex for the small signal tubes (12AX/AT7's are TELE's) Amperex or Mullard for the RF/IF Tubes. OUTPUT's are FISHER Branded Westinghouse 7591's. The rest are replacements. A few years later Sylvania tubes were used, with the Westie 7591's.

Inside the Turquoise coupling caps are replacements. Originals would have been .047 250V or 400V ERO-FOL Films. Other than that it looks factory "fresh".

 

[njcanuck]

Mixed tubes usually indicate routine maintenance replacement. Those blue capacitors underneath are certainly replacements. That’s all that catches my eye but the real experts will be along shortly.
There are many restoration threads here on AK. Looks like a good project.

Edit: I see Larry beat me to it! LOL

 

[Watthour]

I cannot comment on the OEM tube choices, but the layout of your 500B is substantially different from the 500B I worked on. I know there was at least one subcontractor building units for Fisher during that run, but I don't know which might be which.

At a minimum, the output transformer and output tube positions are transposed. The underside of the chassis also reflects those changes.

It also appears that there are some new(er) mylar caps in the output tube circuits, probably in the coupling from the driver stages. That would indicate that someone has been in there at some time since the 1960s.

 

[bigcaddy64]

Well that confirms my thinking that the unit was serviced within the last 10 years. I was thinking those big turquoise pills didn’t look like anything old I’ve seen before on tube equipment.

Is there a recommended modern/nos replacement tube for the 7591? Mine are exceptionally crispy at the base (it resembles the charred surface of burnt wood) and I doubt I have 4 of those tubes laying around.

I’ll have the tubes tested and give it a good cleaning (using AudioDon’s guide). I’ll need one brass cap for the knob but otherwise I think it will clean nicely, even with the scuffs/scratches.

 

[xmike]

the layout of your 500B is substantially different from the 500B I worked on. I know there was at least one subcontractor building units for Fisher during that run, but I don't know which might be which.

I have a 500b with the ouput tubes behind the opt transformers closest to the back. I think the ones with the tubes centered was the earlier version.

I would be interested to read about the different layouts... are you suggesting one or the other is less diserable?

 

[Watthour]

And far be it from my habits to comment on another guy's receiver, but it appears that there might still be some loose Siemens floating around in that unit. Before loading it up with fresh tubes, it might pay to check that out and rectify the problem (pun intended) since that alone can greatly affect the output tube biasing. Those poor 7591s run hot enough already, so there's little point in running them with 80+mA of bias.

From the lone photo of the underside, it appears that the screen grid bleeder resistor modification has not been added, which is another consideraton for updating the unit.

And then there are the cathode safety resistors.

A nickel says you also have a cap or two out of tolerance.

 

[Watthour]

I have a 500b with the ouput tubes behind the opt transformers closest to the back. I think the ones with the tubes centered was the earlier version.

I would be interested to read about the different layouts... are you suggesting one or the other is less diserable?

I'm not aware if one is more desireable than the other. Personally, I would prefer the one that worked well. Since the outputs run really hot in these units putting them behind the OP trannies might be advantageous, but only IF they would have put enough ventilation in the rear bulkhead wall. IMO, they didn't, so it might not matter a lot.

 

[bigcaddy64]

And far be it from my habits to comment on another guy's receiver, but it appears that there might still be some loose Siemens floating around in that unit. Before loading it up with fresh tubes, it might pay to check that out and rectify the problem (pun intended) since that alone can greatly affect the output tube biasing. Those poor 7591s run hot enough already, so there's little point in running them with 80+mA of bias.

From the lone photo of the underside, it appears that the screen grid bleeder resistor modification has not been added, which is another consideraton for updating the unit.

And then there are the cathode safety resistors.

A nickel says you also have a cap or two out of tolerance.

I’ll keep the nickel and agree. I bet quite a few of them have drifted a tad from the stated values. Since high stakes voltage isn’t my cup of tea, I’ll be sending it out to somebody that loves the organized chaos of tube driven products.

I’ve read all about the list of mods that make these play better, safer and lengthen the life of output tubes. I’ll see if it’s even going to turn in before I start ordering parts.

I very much appreciate the info and insight provide. This is the second piece of Fisher I’ve found in the past 2.5 months. The first was a 500S so I don’t think I’m doing too bad so far.

 

[ncwalz]

bigcaddy64: Your 500B is the earlier version with the output tubes in front of the transformers. Later 500Bs have the output tubes behind the transformers.

Early and later 800Bs also differ in the same way.

 

[larryderouin]

If those 7591's test out ok, and you've got some junk octal tubes laying around, swap the bases for good ones. It's done more than you think it would be. Just make absolutely sure the #1 lead is in the #1 pin (key and pin orientation!) before soldering.

 

[Tim D]

That big green resistor in the lower right corner of the second picture in post #1 looks heat stressed. When you replace it I'd recommend to bump the power handing up to the next higher rating. I.e. if it is 5W replace with a 7W or 10W value.

I've got a later version 500B in the queue with the output tubes on the back porch of the unit instead of inside like yours. Its been waiting and waiting and waiting for me to find the time. So much queue and so little time to work on them.

In terms of the tube manufacturers when new, I think the 500B was too early for the transition to Sylvania tubes, but Larry could confirm. The meaning would be that the original Fisher branded tubes wouldn't have been Sylvania manufactured.

Good luck. I'll be interested in seeing how you progress.

 

[gadget73]

Those output tubes may have a fair bit of miles on them. The getters look pretty well used up. Might still be OK, might not. Would be worth testing them.

otherwise agree that it looks basically as-built except the four light blue caps around the output tubes.

There are 3 7591 sources now, JJ, Electro-Harmonix, and Tung-Sol. Only the T-S tubes are physically the same size as the old ones. JJ are taller, EH are taller and fatter.

 

[bigcaddy64]

Those output tubes may have a fair bit of miles on them. The getters look pretty well used up. Might still be OK, might not. Would be worth testing them.

otherwise agree that it looks basically as-built except the four light blue caps around the output tubes.

There are 3 7591 sources now, JJ, Electro-Harmonix, and Tung-Sol. Only the T-S tubes are physically the same size as the old ones. JJ are taller, EH are taller and fatter.

Thanks for the info. It was a bit late last night but i was tempted to pull down my tube stash and see what i have stored. For somebody that doesn't use any tube gear in regular rotation, i have a large supply of good tubes that i can pull from. All the years i've spent looking at sales has netted me a good stock of them. Its possible i have the 7591's laying around as well as a variety of 12AX7's to play with.

Ill post some more pictures of the chassis underside this afternoon when i get home.

 

[larryderouin]

The "B" models are too early for factory installed Sylvia's. Late (post-'65) units got them due to production of tube units winding down, cost of Sylvania's vs Telefunkens, and other unknown own factors. In My 2 400s. The early is populated with Tele's, the late with Sylvania's. Can't hear any difference.

 

[xmike]

I know there was at least one subcontractor building units for Fisher during that run, but I don't know which might be which.

I would be interested to learn more about this, anyone know anything further?

 

[larryderouin]

I know there was at least one subcontractor building units for Fisher during that run, but I don't know which might be which.

1st I've heard of this!!! To my knowledge, FISHER realized in the early version, that the location of the tubes in front of the transformers was a heating nightmare, so tubes and trannies were swapped for later version. TMK FISHER kept production in house until about 1970-71 when Hitachi was OEM'ed for S.S. stuff.

 

[Watthour]

I believe a company named "Levco" fabbed their faces and chassis for a period of time, instead of them being done in-house. Whether anyone else completed any of their units is uncertain.

 

[bigcaddy64]

I got a little bit of time this evening to take some pictures of the chassis underside for you guys.

 

[bigcaddy64]

I also did a bit of cleaning on the knobs. I didn’t get a picture before but the liquid in my ultrasonic cleaner looked like beef stock once they cycle completed.

a little bit of polish (and one new brass cap) will have them looking good again.

The face is pretty sticky but I think it will also clean well. As for my tube supply, I have nothing except 12ax7 variants so I’ll ask what my brother has in his. They might test okay so I’ll leave it alone for now.