Fisher Tubes - HOT HOT HOT

Hey all -

Newbie here. Came into a Fisher 500c recently and wow. The sound is kind of crazy. It's a huge upgrade from my Yamaha CR-420. I was a little cynical of all the fusing about vintage tubes, but man, it's real. It's kind of amazing, really.

Anyways, with all the pleasure that tubes have brought me, they've also brought some concerns. Mainly - is this thing a fire hazard??

The tubes almost IMMEDIATELY get scorching hot when I turn the thing on. After 10 minutes, the are too hot to touch. I've poked around on here and found that other people say the same thing, but I just wanted to double check that this is normal.

Another concern that I have is red plating - none of my tubes are turning red, but they're all emitting an orange glow for sure. The tops of the tubes have scorch marks. is THAT normal?

Sorry if these are dumb questions! Just want to make sure I don't burn my house down. Thanks in advance!

 

What tubes are they? Replace them and check the cathode bias...see if you can find out what the resistance is supposed to be from a schematic.....hope this helps....

 

The power tubes can get to 350 degrees F or so. The smaller tubes should be less - but may still be uncomfortable to touch.

Over time, the heat can cause visible blackening or other discolorations on the surface of some of the tubes.

 

Tubes run hot- no problem. The large power tubes must not have red or orange color glowing plates inside the glass.

 

BTW- I am not referring to the tiny small orange dot at top center of tube.

 

I recently bought a Gemtune X-1, went the very-affordable Chi-Fi route to entry-level tube goodness. I found exactly the same, which is that they sound wonderful, as in how does this old technology sound so sweet? But the power tubes (EL34) run HOT, as in blister your fingers, maybe melt the skin off of them kinda hot. Getting my hands near the tubes makes me wonder whether I'm going to enjoy using it in the summer time.

Tubes have been so enjoyable for me that I would love to find a reasonable way to power my main set-up, which includes turntable, television and streaming box powering a 2.1 speaker set-up through an AVR. The AVR doesn't have pre-outs and is otherwise so convenient for HDMI switching and room correction that I'm loathe to change it out. In my best world, I would use the AVR for TV and movies but power the sound for the turntable through a tube amp, integrating all of it for ease of use. Haven't gotten there yet, so when I want that full tube goodness, it's to the home office system!

 

Get the 500c checked out. Selenium rectifier should be replaced and all the caps if not most. If it’s sat for awhile, it’ll be a bomb waiting to blow.

 

Absolutely, these old Fishers need a complete rebuilding including the power supplies, rectifiers and coupling caps. Check resistors for going out of tolerance too.

 

The glow you are seeing is normal. That is the heater element of the tubes. The tubes also get really hot, the larger ones, due to very high voltage operation and current draw. It is normal for them to get so hot that you can't touch them. Don't place the amp on carpet and block air flow from underneath. Don't place it below curtains or other flammable material. And never leave the house with it on. Old amps like that can start to smoke and even ignite when some internal failures occur. it's old enough now that it is certain that some components will fail eventually, whether they go out with a whimper or a flash and a bang, remains a mystery for another day.
-Bill

 

whoa - back up. What??

Do you mean literally? That makes me nervous. Even if the unit is powered off?

I bought it from a seemingly reliable ebay seller (100% positive feedback) that had the unit serviced 2 years ago. Some of the guts look new. The tubes all say Fisher though, so I know those have not been replaced. I'm worried they're red plating - I'll post a pic tonight to see what ya'll think. Thanks again everyone!

 

If it’s powered off, it’ll be fine. However, have it checked out by a reputable tech. Some units I’ve seen have been “serviced” and it was loaded with crappy caps and cold solder joints. It’s worth the investment, especially with the old Fishers. Put it on a power strip and use the power strip to turn it off and on. The power switch is hard to find and the less wear and tear the better. There’s loads of tips and such on these boards as well as others. You’ve got a groovy little receiver, keep it healthy and it’ll outlast you.

 

The seller could very well be reliable and you may have a gem of a unit, but if it were mine I'd still have it thoroughly checked over by a tech experienced with these units specifically. "Serviced" can mean almost anything. You have a complicated piece of equipment built over half a century ago. If you have any doubt, just take the bottom cover off and have a look (but don't touch anything)!

 

Take the lid off and post a pic. Many forum members here will let you know if it’s safe.

 

Audiokarma has a Fisher forum. Dave G. knows everything there is to know about your receiver.

 

Thanks everyone. Here is a picture that I snapped this evening:




Looks like redplating to me. What do you guys think? Starting to feel bummed.

 

This is 100% true, but the OP appears to not be qualified to check it himself, even with Dave's guidance.

The smart thing is to have a knowledgeable tech go through it and evaluate its condition.

 

The Fisher 400/500/800 series have old electrolytic capacitors past their service life. If a capacitor shorts in the first section voltage doubler, it will blow the fuse without any damage to the receiver. If a capacitor fails down steam of the first section, it will likely burn out a series resistor. I never heard of a Fisher in this series short at the bias bridge rectifier, just safe bias to the power tubes will be protected. The power tube coupling capacitors imported from Germany are of high quality in the 500C thus will not short out the power tube bias as I never known one to fail. Other coupling capacitor are of less quality and may fail, but damage to other components is unlikely. So, the Fisher is not a time bomb, but should be rebuilt with new capacitors except through out the receiver. The exception are ceramic capacitors that never fail. However, the poor sounding ceramic type capacitors in the tone controls should be changed to higher quality parts.

 

One more thing,
Check the fuse and make sure it's the correct value.
You'd be surprised how many people blow a fuse and use what's on hand rather than the correct value.

A wrong fuse speaks volumes about the service history.

The Fisher 500C uses 7591A output tubes. The original "Made in The USA" tubes are more durable than current production Russian tubes and are very valuable. The 500C ran those output tubes to the limit in an effort to squeeze every ounce of performance from the unit. The unit was designed when tubes were cheap and it wasn't a big deal to replace them every year or two.

So if you decide to have a tech look it over, read what Dave has to say and has done before you drop it off at "any old place".
Dave has come up with a few mods to make the unit more reliable.

There are a lot of "Fisher Guys" on that forum that do the work and know what they're doing. Perhaps one lives near you.

 

This is super helpful guys. Thank you!

Can someone take a look at the photo and confirm that as redplating? I want I know for sure if I have to keep the thing off.

 

Google electron tube theory or electron tube operation and do a little reading. It'll set your mind at ease.

It's difficult to say looking at the pic as I don't know in what light the pic was taken.

 

You will need to take a pic at night with the lights out to really see red-plating.

jeff

 

Picture only shows one power tube plate and it is not red. A picture of all four tubes would be better, of course. Two tubes have the getters burned off indicating high use and approaching end of life. Other two are well worn as well. However there may be a few thousands hours left in the tubes. I have seen and measured tubes like that. I have restored over 100 Fishers.

 

As I understand it red-platting refers to when the large metal plates inside the glass look red. In that pic, at least the closest tube, the plate looks the correct gray/silver color.

 

Red plate tube.

 

I dumped my Mcintosh tube system specifically because it caused my listening room to heat up unbearably in the summer. I miss it a bit, but I don't miss that heat!