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Powering Up Old Tube Amps/Receivers
Powering Up Old Tube Amps/Receivers
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Old 20th January 2007, 11:17 PM   #1
DreadPirate is offline DreadPirate  United States
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Default Powering Up Old Tube Amps/Receivers

I just picked up a Fisher 800-C, a Garrard turntable, and a pair of 16" alnico woofers (maybe fisher, all I remember is that they said "oakton" on them, will get them tomorrow) out of a console I bought with lunch money. The older gentleman purchased this new and said it was "top of the line" when he got it. It still has original Fisher tubes installed, absolutely mint. It has not been powered up in at least 20 years. I love this country.

I found this post on stevehoffman.tv and I'll repost here because it seems great advice. I have minimal experience in electronic troubleshooting, none with tubes. This is my second tube amp ever, the first doubled as a rat's nest at some point in time, so I've been reluctant to do much with it, this baby has project potential, I'm willing to dive in and do whatever is needed for complete restoration.

If anyone can contribute with hints and suggestions, I would greatly appreciate it. A photo will be provided shortly.

Is the "dim bulb test" a light dimmer? Will that be a good substitute for a variac, because I don't have one. I do have esr tester, a good meter, and will most likely acquire an o-scope shortly.

Posted by Hegeman's Ghost


If you take it to a local repair person, be sure he has tube amp experience. If you don't mind shipping the amp. there are a few qualified people who you could send it to for repairs. If you are interested in the latter, email me off line and I will provide you with contact information.

Following is a list of the steps that I follow when bringing up a vintage amp/radio/receiver that I have just purchased.

1. Open the unit and inspect the electronic components and wiring. Look for broken or loose wires, leaking capacitors, burnt or badly discolored resistors, etc.
2. Check the power cord. Make sure it is not frayed or broken.
3. Hook an ohm meter to the prongs on the plug(unpluged of course). A reading of 5 to 20 ohms is normal.
4. Check the fuse. Is it good and is it the correct amp rating for the unit. I'm guessing your unit would use a 3 amp fuse.
5. Clean all of the pots and switches with contact cleaner.
6. Test all of the tubes on a reliable tube tester.
7. Clean all of the tube tines and tube sockets with contact cleaner.
8. Clean all of the inputs, and outputs with contact cleaner.
9. Use a variac or dim bulb tester to power the amp up slowly. I start at 20 volts, then go to 40 volts, 60 volts, 100 volts, and finally 120 volts. Obviously, if you blow a fuse at any of the lower voltages, don't go higher until you find out what caused the problem.

In your case, you have already applied 120 volts and the unit took it without blowing a fuse. So it is pointless now to worry about bringing it up slowly.

Question, have you hooked the unit up to a turntable or cd player and tried to pass a signal?

After you do steps 1 - 8 above, you might want to try this and see what happens. You can't imagine how may problems are simply the result of dirty pots, switches and sockets in equipment that has been sitting idle for years.

By the way, if you don't have a tube tester, skip item 6 on the list. If they are new tubes, it's unlikely that both channels will have a defective tube.

Good luck, HG
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Old 21st January 2007, 01:19 AM   #2
Tom Bavis is offline Tom Bavis
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In restored condition, this is a $500 item... don't mess it up! You'll need a schematic - see my web page, near the bottom: http://www.audiophool.cjb.net/Audio.html

OK, a dim bulb tester is simple - you cut one wire of an extension cord and connect the cut ends to a light bulb socket, so that it's in series to limit the current. No matter HOW bad the short, you won't blow a fuse or circuit breaker, it will just light the bulb. a 60W bulb will allow no more than .5A, a 150W bulb will allow about 1.3A. A light dimmer will NOT work!

Using a bulb in series, you can get an idea of the state of the unit - voltages will be low, but should be proportional. The bias supply is a weak link in these units - it has a selenium rectifier that can't be trusted even if it measures good, and an electrolytic cap that has probably dried out. The reduced voltage will allow the electrolytic capacitors to "re-form" - after long storage, electrolytics have high leakage - they can overheat if brought directly to rated voltage. Since you have an ESR meter, check them all first, and replace any that are questionable.
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Old 21st January 2007, 01:46 AM   #3
anatech is online now anatech  Canada
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Powering Up Old Tube Amps/Receivers
Hi Tom,
A light dimmer will NOT work!
Agreed! One of these would be sure to destroy the power transformer!

Hi DreadPirate,
One of these in good shape is valuable. You do have Oaktron speakers, and they were pretty good units at the time. I used to sell the raw drivers.

No one can over emphasize the value of a good visual inspection first. It's also very important to take lots of pictures before you start any work.

Don't try to clean the dial glass. Let an expert do that. All the markings will come right off if you try. Even water will remove the paint.

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Old 21st January 2007, 03:23 AM   #4
DreadPirate is offline DreadPirate  United States
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Thanks for the comments so far. I know this is a special unit and intend on doing it right. It will most likely get parked on a shelf for awhile until I gather enough experience to proceed. In fact, the other tube receiver I have, a Pioneer SX-110, will likely be the first victim (I have the schematic), so you may see some posts on it over the next months (or years?). It has a mint faceplate but I found it set out on the curb destined for the dump the weekend after heavy rains and has some interior rust on the "motherboard." After removing a field mouse's carcass from the inside (how in the world did it get in there, I cannot say), I dried it out, and not knowing much about handling this type of equipment, plugged it in and powered her up. The FM reception was quite nice and it appeared to work right off the bat. That was about a year ago and I've gotten as far as removing the cover to wirebrush some rust. Next time I power it up, I'll go for the bulb method.

So these capacitors benefit from being powered up at low voltages? Should I leave the power on for a period of time to "reform" them? What am I looking for while I'm bulbing? I'll give it a go through with my Dick Smith ESR meter beforehand, but I think I remember there was a whole mess of caps, resistors, and such underneath. That area was actually very clean, the rust is on the topside where the tubes were. Some of the tubes were not original like on the Fisher, so the unit may have actually gotten some use within the last 10 years or so (that mousey was kind of dried out, though).
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