Fisher 400 receiver- one 7868 tube red plating

[robnec]

Hello. I have pulled my Fisher 400 from storage for routine check up. Unfortunately one of the power tubes started red plating and gradually loosing bias. What I did to find out the cause.
1. All coupling caps were disconnected. All power tubes were holding -17 volts bias.
2. resistors 1k and 330k and 0.047 cap were replaced
3. Tube pins voltages are correct according to manual.
It looks as somewhere by the splitter this channel power tube drains to ground.
What else to check? Thank you. Robert.

 

[gadget73]

Swap it to another socket and see if the problem follows the tube or if the problem stays with the socket. If it follows the tube, its a bad tube.

 

[BrassTeacher]

Swap it to another socket and see if the problem follows the tube or if the problem stays with the socket. If it follows the tube, its a bad tube.

After doing this, make sure the socket is not toasted, or even discolored. You replaced the coupling caps with new ones, yes? Because if you didn't, that is why that tube is red-plating. Ask me how I know this...

 

[robnec]

Hello. Thank you for input. Problem stays with this socket only. Parts are new. Again with coupling caps disconnected all tubes hold is -17 volts bias. That is a reason I think problem is down stream with splitter tube. I may be wrong. Robert.

 

[oldflame]

I'd second Brass Teacher's suggestion about checking the socket for tight connections and no carbon-tracking, but wouldn't those problems affect voltage readings, just like a bad primary? Not being an expert, I'm curious about the cure, too.

 

[ramjet]

Hello. I have pulled my Fisher 400 from storage for routine check up. Unfortunately one of the power tubes started red plating and gradually loosing bias. What I did to find out the cause.
1. All coupling caps were disconnected. All power tubes were holding -17 volts bias.
2. resistors 1k and 330k and 0.047 cap were replaced
3. Tube pins voltages are correct according to manual.
It looks as somewhere by the splitter this channel power tube drains to ground.
What else to check? Thank you. Robert.View attachment 2878441

Make sure that pin 3 on this tube socket is indeed grounded.

 

[robnec]

Hello. I played with it a little. With coupling caps disconnected it still red plates slowly. I measured the primaries and luckily they are ok. Now everything points to broken socket. I bought one already for replacement. So expensive. I will get back when it arrives. Thank you all again. Robert.

 

[RS Steve]

Problem could be with V15 which causes V14 to compensate and run hot.

 

[robnec]

Hello. I was thinking about it too but moving tubes around did not prevented red plating. I will leave it for now and wait for a new socket. Robert.

 

[RS Steve]

Hello. I was thinking about it too but moving tubes around did not prevented red plating. I will leave it for now and wait for a new socket. Robert.

Like I said, it could be the V15 socket not working correctly and causing V14 to run hot trying to make up for V15. I would do an ohm check on each pin socket and compare to the next socket to see if their are any obvious differences.

 

[robnec]

Hello. The good news is that now my Fisher 400 is working. As with old stuff nothing was really broken but on the other hand nothing was working as should.
Firstly I install 10 ohms resisters on cathodes to know exactly what was going on. New coupling cap was also no good so another went in. I reduced bias voltage to -21 volts and replaced 330kohs with 220.
Now all four power tubes run at about 0.33volts with 414 volts on plates. For me it seems to be ok. What do you think? Robert.

 

[Tom Bavis]

Sounds good - 13.6W per, about 70% of max dissipation rating.

 

[Mr Boochie]

Hello. The good news is that now my Fisher 400 is working. As with old stuff nothing was really broken but on the other hand nothing was working as should.
Firstly I install 10 ohms resisters on cathodes to know exactly what was going on. New coupling cap was also no good so another went in. I reduced bias voltage to -21 volts and replaced 330kohs with 220.
Now all four power tubes run at about 0.33volts with 414 volts on plates. For me it seems to be ok. What do you think? Robert.

Did you make sure that the socket tangs tension are tight? Sometimes, the receptors get loose, and you temporarily lose bias voltage contact on the signal grid.

 

[robnec]

Hello. I have a new socket but after some cleaning the old ones held well so I had left them alone. Fisher played all day yesterday and behaved well. I think lowering the bias to -21 volts helped the most. Robert.

 

[Mr Boochie]

Hello. I have a new socket but after some cleaning the old ones held well so I had left them alone. Fisher played all day yesterday and behaved well. I think lowering the bias to -21 volts helped the most. Robert.

Cool beans!

Sound fine?

 

[robnec]

Hello. It sings as well as it looks. No complains on my part. Robert.

 

[egellings]

After doing this, make sure the socket is not toasted, or even discolored. You replaced the coupling caps with new ones, yes? Because if you didn't, that is why that tube is red-plating. Ask me how I know this...

Yes, and also check that the socket's pins are tightly gripping the tube's pins. If not, then re-tension the socket pins to get a tight grip on the tube's pins.

 

[robnec]

Hello. I did all things as I was told.
Looking back, I think that Fisher was driving power tubes too hard. Lowering the bias cured rest of the problems. Robert.

 

[Mr Boochie]

Hello. I did all things as I was told.
Looking back, I think that Fisher was driving power tubes too hard. Lowering the bias cured rest of the problems. Robert.

In that era that Fisher was made many manufacturers pushed their tubes. Kind of a "wattage war." Remember, replacement tubes were fairly common.

The parameters that some mfg ran precious 7591 and similar El Primo tubes at were pretty amazing.

6V6GT sometimes had as much plate voltage on them as 6L6GC of that era. Of course, not the idle current, but still...

 

[gadget73]

never honestly understood the logic though. You don't get more power output from running overly high idle current on an AB amp.

a bigger variable is modern 7868/7591 family tubes though. They need more negative grid voltage to idle at the same current level as old tubes, so if thats what OP is using, thats probably got far more to do with it.