Fisher 500b Red Plate

[James2001]

Hi, I'm new here but have been following and gaining knowledge here for many years.

I have had a Fisher 500b for about 25 years, no wood cabinet. It is serial number 11233 and has the 7591 tubes inboard of the OPTs, and has the Z on the mpx. When I bought it, it had original Fisher 7591 tubes in it. About 15 years ago I decided to put in a matched quad of JJ's and it's worked that way in my garage every day perfectly since then until a few weeks ago. One of the tubes red plated. No matter where I put that tube it still red plated, so this time I decided on trying a matched quad of Tung Sol 7591A. When I put them in they all started red plating after about 5 minutes or so and I shut down. I tried all combinations, but they all still red plated. I never ran them more than about 5 minutes until I could see if they would red plate or not. So I just ordered a matched quad of JJ's again since they worked before. When I put the old JJ's back in, only the same tube red plated as before. With the Tung Sols in I measured -10.4V on each number 6 pin (same with JJs in).

The manual I downloaded says for serial numbers after 20001 and is for the 7591's outboard of the OPTs. It says I should be measuring -14.7V, but I wonder if the schematic in the manual for my earlier serial number also says -14.7V or maybe -10.4V?

Nothing looks burnt underneath, and it still sounded great when the JJ red plated. Does anyone know where I can get the manual for serial numbers before 20001, with the 7591's inboard of the OPTs? Also, is there a reason the Tung Sols would all red plate when the JJ's don't all red plate? Is there a fixed bias mod I can make to allow the Tung Sols to work? I've searched the site for this particular problem but can't find a thread.

Thanks Jim

 

[gadget73]

The Russian tubes need more negative voltage in order to bias correctly vs the Westinghouse this probably came with. Since there is no means of adjustment in stock form, basically the fix is you modify it for adjustable bias.

-10.4 also just sounds too low. That probably points towards issues with the selenium rectifier in the bias circuit if you made me throw a dart at something without further info.

Can you post some pics of the underside so we know what you're starting with here?

 

[James2001]

Thank you for replying and I will take a look at the selenium rectifier. Looks like that is SR1 coming off T3 taps from the schematic I have. I actually have the unit put back together and playing through the AR2ax's right now in the garage. Yesterday I went into storage and found a box of hundreds of tubes I remember having. I found an old JJ 7591 that tested good on my BK. After trying many combinations I found one where none of the tubes red plated. So while I wait for the new JJ quad to come in, I was going to use it this way. But later today I can remove the cover again and take a few shots and post them.

 

[James2001]

I took a few photos. I only have 20.8VDC coming off the selenium rectifier, and only -17VDC at the V arrow junction on the schematic. From a thread I just read from LarryDerouin I should be around -24VDC at that split, so I think you are right about the selenium rectifier being off. I am not sure what the T3 taps AC volts should be going into the selenium rectifier and it is just too tight to try and put a probe in there and measure it. I am off to the electronics store for a full wave bridge rectifier 400V 8amp, and a few other resistor values for R128 to try and dial that V arrow junction to about -24VDC.

 

[gadget73]

This thing looks as original as any I've seen, so just be aware that lots of parts in this are pretty solidly past normal service life. If its been used regularly that tends to make things survive better but I doubt you'd make this thing worse by replacing the set of can caps, the couple of electrolytic caps underneath, and probably at least those coupling caps on the output tubes. Also wouldn't be a terrible idea to add the 10 ohm cathode resistors, 100 ohm screen resistors, and some means of adjusting bias, even if its just a single trimmer that does all 4 tubes in one go.

 

[Tim D]

Hi, I'm new here but have been following and gaining knowledge here for many years.

I have had a Fisher 500b for about 25 years, no wood cabinet. It is serial number 11233 and has the 7591 tubes inboard of the OPTs, and has the Z on the mpx. When I bought it, it had original Fisher 7591 tubes in it. About 15 years ago I decided to put in a matched quad of JJ's and it's worked that way in my garage every day perfectly since then until a few weeks ago. One of the tubes red plated. No matter where I put that tube it still red plated, so this time I decided on trying a matched quad of Tung Sol 7591A. When I put them in they all started red plating after about 5 minutes or so and I shut down. I tried all combinations, but they all still red plated. I never ran them more than about 5 minutes until I could see if they would red plate or not. So I just ordered a matched quad of JJ's again since they worked before. When I put the old JJ's back in, only the same tube red plated as before. With the Tung Sols in I measured -10.4V on each number 6 pin (same with JJs in).

The manual I downloaded says for serial numbers after 20001 and is for the 7591's outboard of the OPTs. It says I should be measuring -14.7V, but I wonder if the schematic in the manual for my earlier serial number also says -14.7V or maybe -10.4V?

Nothing looks burnt underneath, and it still sounded great when the JJ red plated. Does anyone know where I can get the manual for serial numbers before 20001, with the 7591's inboard of the OPTs? Also, is there a reason the Tung Sols would all red plate when the JJ's don't all red plate? Is there a fixed bias mod I can make to allow the Tung Sols to work? I've searched the site for this particular problem but can't find a thread.

Thanks Jim

It's hard to find a Fisher 500B service manual for your serial number series, but there's one on the big auction site right now. If it was cheaper I woulda picked it up and scanned it in for the AK database, I can't comment on the differences in the circuits of the two different serial number series. I have a 500B that I just started restoring but it is for the later series.

Others have you covered on how to solve your redplating issue. Sorry for your troubles.

 

[James2001]

Tim, thanks for letting me know about that original manual for my early serial number, "I bought the Bloody Thing!!", haha.

Gadget73, thanks for letting me know about the selenium rectifier, that was it!!! Now I am back to -14.7VDC at pin 6 bias like the schematic says, for all 4x 7591's. It sounds better now too. The Tung Sols still have a faint red plate but nothing like before, but I don't plan to use them anymore. The JJ's don't have a hint of red plate anymore, no matter what combination I put them in. I even put the JJ red plate tube I marked as bad, back in, and no problem now! I've had a lot of old receivers and tube tuners, and this Fisher 500 sounds better to me even after all these years than any of those others, amazing solid rock FM AC/DC, Van Halen, as well as FM classical, and FM Jazz!! I'm a retired Mech Engr, but I'm fascinated with old US tube electronics from the 50's, 60's and 70's, and I will take your advice and upgrade the caps and resistors on this unit. I just hope the modern components don't change the wonderful characteristics of this old receiver.

I posted a few more photos. I went a bit overkill on the rectifier with 25A, but that's all the shop had today. Sometimes I just enjoy listening to this Fisher FM radio and AR2AXs tones in the garage, more than the fancy stereo inside the house.

Thanks for both your inputs!! God Bless!! Jim

 

[Tim D]

Glad you have it sorted.

That bridge should last forever given it's rated at 12.5A.

I'm glad that someone that needed that service manual now has it. Would it be possible to scan and upload it to the AK database for others with the early models?

 

[James2001]

Tim, yes I will scan and upload it to AK.

 

[James2001]

Tim, I received and scanned the Fisher 500b early serial numbers manual. The problem is the file is too large to attach at 3.8mb. I zipped it down to 3mb and it's still too large. The schematics look the same to me, and the only difference I see is the inboard location of the output tubes. I wanted to hear the new Tung Sols matched quad, so I brought the bias on pin 6 from -14.7VDC to -18.1VDC, by adding 13.5k (2x 27k in parallel) plus a 10k pot in series with the R135 10k resistor. The pot gives me a range of -17.3 to -18.1 at pin 6. I've been using the Tung Sols for a week now, with no hint of red plate. The power transformer seems to run less hot too. So far I like the Tung Sols better than the JJ's, a bit smoother and better detail to my ears.

 

[Tim D]

Tim, I received and scanned the Fisher 500b early serial numbers manual. The problem is the file is too large to attach at 3.8mb. I zipped it down to 3mb and it's still too large. The schematics look the same to me, and the only difference I see is the inboard location of the output tubes. I wanted to hear the new Tung Sols matched quad, so I brought the bias on pin 6 from -14.7VDC to -18.1VDC, by adding 13.5k (2x 27k in parallel) plus a 10k pot in series with the R135 10k resistor. The pot gives me a range of -17.3 to -18.1 at pin 6. I've been using the Tung Sols for a week now, with no hint of red plate. The power transformer seems to run less hot too. So far I like the Tung Sols better than the JJ's, a bit smoother and better detail to my ears.
View attachment 2282065

Oh. Thanks. You want to send it to the AK database. It can take the much larger files. I think you send them to ak.dbfs@gmail.com. It can take files of 20 meg.