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Old 27th December 2010, 05:57 PM   #1
Stabby is offline Stabby  Belgium
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Default Revising an old tube preamp section: where to get the knowledge?

I have no experience with electronics other than replacing the opamps of a CD-player. Currently I'm restoring a 50-year old electric clavichord. It's mainly replacing mechanical parts, but now I am pretty sure the pre-amp section needs a revision too.

To start off, I know tubes are very dangerous to work with. Is there a detailed tutorial somewhere on how to decharge them?

The pre-amp section involves two tubes: an ECC83 and ECH81. I have no knowledge at all on amplifiers, let alone tube amplifiers. So it looks like I'll need to start building up my knowledge on these too.

Is there a good book to acquire all the necessary knowledge to start revising the amplifier section?

Thanks in advance. I'll put up some pictures soon.
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Old 27th December 2010, 06:08 PM   #2
LAJ is offline LAJ  United States
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I would replace the caps, check the values of the resistors and test the tubes. That shoud pretty well fix it.
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Old 27th December 2010, 06:25 PM   #3
Stabby is offline Stabby  Belgium
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Pics:

http://i.imgur.com/IdDv3.jpg
http://imgur.com/rH3KW.jpg


Quote:
Originally Posted by LAJ View Post
I would replace the caps, check the values of the resistors and test the tubes. That shoud pretty well fix it.
Can I use any capacitor with the same value to replace them? As you can see in the picture they are a lot different (bigger) from today's capacitors.

Also, any comprehensive tutorials on how to test the tubes? I should probably discharge them first?

Thanks

Last edited by Stabby; 27th December 2010 at 06:28 PM.
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Old 27th December 2010, 07:04 PM   #4
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My guess is that that amp will probably work without much revision.
A light-bulb current limiter is a good idea when first powering up diy or older equipment.
http://www.vintage-radio.com/projects/lamp-limiter.html
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Originally Posted by Stabby View Post
Also, any comprehensive tutorials on how to test the tubes? I should probably discharge them first?
Tubes don't retain a charge, and are safe to handle once removed from the socket.
It is the capacitors that can retain a lethal charge. The large uF, high-voltage caps in the power supply deserve respect. Make up an insulated lead with an alligator clip at one end (connect to chassis ground) and a (several-watt rated) 50k (value not critical 20k-100k) resistor at the other. Enclose everything except the last few mm of the resistor lead in heat-shrink or tape. Tape the resistor to a pencil-size stick of wood or plastic. Touch it to the + end of each cap for a minute. Check the voltage with your voltmeter to confirm it is safe.

If you can find some older (1940s-1950s) electronics books, you will have lots of info on tube circuits. For newer books, Morgan Jones is a respected author; many other resources on the web.

John

Last edited by VictoriaGuy; 27th December 2010 at 07:09 PM.
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Old 28th December 2010, 03:24 AM   #5
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Exclamation Selenium Rectifier

Looking at the second picture of the underneath wiring, I see a bridge rectifier right alongside of the power transformer. If this is a selenium type, and I suspect it is, it is cause for concern. They do get weak and the voltage output drops. But more important, they produce toxic gas that's unhealthy to breath in. (Smells like rotten eggs) It's difficult to tell from the picture if it's used for the high voltage or the filament supply. But it's something that will need attention either now or in the very near future, assuming it is selenium.
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Old 28th December 2010, 04:43 AM   #6
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When you shut you amp off do not put it on standby before or after switching it off. Wait a little while and the caps will discharge themselves at least a good amount if not completely. Then follow VictoriaGuys directions to make sure they are completely discharged.

If you decide to work on the amp while it is plugged in and turned on (don't if you don't have to) keep one hand behind your back and use only tools with insulated handles.
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Old 28th December 2010, 03:12 PM   #7
Stabby is offline Stabby  Belgium
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Quote:
Originally Posted by HollowState View Post
Looking at the second picture of the underneath wiring, I see a bridge rectifier right alongside of the power transformer. If this is a selenium type, and I suspect it is, it is cause for concern. They do get weak and the voltage output drops. But more important, they produce toxic gas that's unhealthy to breath in. (Smells like rotten eggs) It's difficult to tell from the picture if it's used for the high voltage or the filament supply. But it's something that will need attention either now or in the very near future, assuming it is selenium.
I've written down the information on the rectifier:

B250 075
KC068 22/8

How do I know if this is Selenium type? And how do I interpret the numbers? (what do B250, 075, KC068 stand for?). I should probably replace it, but I have no idea what specifications to look for.

On top of the amp (first picture) there are some parts that look like batteries. These are connected to the bridge rectifier. It says "350/395V -20/+70".

Last edited by Stabby; 28th December 2010 at 03:34 PM.
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Old 28th December 2010, 03:23 PM   #8
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Originally Posted by Stabby View Post
On top of the amp (first picture) there are some parts that look like batteries. These are connected to the bridge rectifier. It says "350/395V -20/+70".
I think those are the filter capacitors for the power supply. (Those are the ones to check with voltmeter before touching- after unplugging the mains.)
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Old 28th December 2010, 03:37 PM   #9
Stabby is offline Stabby  Belgium
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Originally Posted by VictoriaGuy View Post
I think those are the filter capacitors for the power supply. (Those are the ones to check with voltmeter before touching- after unplugging the mains.)
Thank you for all the valuable advice. I will check those caps first.

What do you think the problem could be? When I connect the keyboard to a power amplifier, the sound coming out remains very quiet, even with the highest volume and I keep hearing quite some ground noise. I've checked the ground connection and it's fine.
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Old 28th December 2010, 09:16 PM   #10
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What do you mean by "ground noise"?
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