• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Revising an old tube preamp section: where to get the knowledge?

Stabby

Member
2010-09-07 1:56 am
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.
 

Stabby

Member
2010-09-07 1:56 am
Pics:

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


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 :)
 
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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
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
 
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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.
 
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.
 

Stabby

Member
2010-09-07 1:56 am
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".
 
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Stabby

Member
2010-09-07 1:56 am
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.
 
Apart from a lot of great intro books available, there are a few good general websites with safety and introductory faultfinding etc information for valves amps - I can recommend reading through the safety and faultfinding and other general areas of:
Classic ANZ Valve Guitar Amplifiers

I agree with Hollowstate that the rectifier is an old Selenium type, which like the electrolytics, are best replaced by a modern equivalent. For aesthetics, and historical character, it can be worth retaining some parts (leaving them in place but electrically isolated) - eg. the selenium bridge rectifier.

The biggest first restoration step to take is to search around for a circuit schematic and any other details of the equipment, or if that fails, then attempt to prepare a circuit schematic - which may be a steep learning curve, but very very worthwhile if you are the least bit interested in restoring the equipment yourself. That will help in asking questions of others, and in comparing what you have with other equipment, and in restoring and faultfinding.

Ciao, Tim
 

Stabby

Member
2010-09-07 1:56 am
I just tested the big electrolytics (350/385V) with a multimeter. I got 206V from one and 256V from the other. Could that be caused by the bridge rectifier?

I wouldn't know what to replace those electrolytics with. It says "16 MF", but I have no idea what's meant by that.
 
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Whoa....

Hold on.

There are two types of caps in this unit: signal path caps and filter caps.

The large value caps ur talking about now are filter caps. The values on the can (package) of the caps are the size of the cap, in ufd (microfarad or also MFD) and voltage rating. Your voltage found should be LESS than the voltage rating of the caps. Seems fine so far.

There is NO NEED to change ANYTHING AT ALL until you tell us A) does the thing work now? If not WHAT does it do? and B) have you changed the tubes?

RE-SEAT the tubes in their sockets - sockets often are intermittent due to oxidation on the pins of the tubes AND the socket itself!!

Of all the things that die the tubes are high on the list. Caps second, and far behind.

You will want to probably change out the rectifier. It is a "bridge rectifier" (look it up, google if you need to). The package for the one in there is very different than a modern one. So you will have to make it fit and work in the space. You want a 1000piv bridge, and something from 1amp up... 10amps or better seems nice. Cheap too, they are not expensive. BUT if that one works, there is no need to change it immediately.

How to know if it works? Measure the voltage IN (remove one leg that goes to one of the terminals marked " ~ " - there are two so marked ) and then use an AC meter (not ur fingers) to go between the two wires, one still on the ~, the other floating. See what the voltage is. Now you can go find the formula that tells you what the DC voltage OUT of a bridge with a "capacitor input filter" is expected to be. You should find THAT voltage on the ouput side of the bridge, or on the first filter cap.

IF you do not find that voltage or nearly so, then you likely have a fault either in the cap or the bridge or both. A shorted or leaky cap will show excess AC on the line that should have DC, so you can measure both AC volts (high voltage) and DC volts (or "ripple"). DC ripple should be <10% of the AC volts or LOWER. Actually 10% is highish. BUT GOOD ENOUGH TO MAKE AN AMP HAVE SOUND!!

Now you are starting to troubleshoot. You do this stage by stage - check voltages on all the pins of the tubes, write them down, compare to the suggested voltages found in an online tube manual. They should be close. Pay special attention to the grid voltage and plate voltages.

Of course you didn't say if you tried to make sound with this unit, and if you did what if anything it did or what you heard... so it's hard to know where to point you.

Again, CHANGE NOTHING until you have some facts. Ok, you can change the tubes. That's ok. But nothing else yet.

_-_-bear

PS. for example, you turn it on, and it makes no music, BUT you turn the volume control and you hear "scratchy" or "static" sound... etc...
 
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Stabby

Member
2010-09-07 1:56 am
Thanks for the help!

So I should cut off one of the ~ connections in the bridge to know if it works? Also I'm sorry but I'm not sure what to do when you say "and then use an AC meter (not ur fingers) to go between the two wires, one still on the ~, the other floating."

Anyway, I know it's a B250 bridge rectifier with 250V output. Doesn't that already tell me what output to expect? I measured the output and it's 256V.

And now I'm confused about the 10% DC-AC thing. The output of the rectifier is 250V, so the AC input should be 2500V?
 
I'm sorry to rain on your parade, but if you have difficulty recognising common components and making simple measurements then I suspect this is not the time to attempt this job. Do some reading and looking first. Learn how to read a circuit digram. Come back here to ask your questions, but my advice is not to do anything for a while. There is a danger you could harm yourself or the item.
 
I'm sorry to rain on your parade, but if you have difficulty recognising common components and making simple measurements then I suspect this is not the time to attempt this job. Do some reading and looking first. Learn how to read a circuit digram. Come back here to ask your questions, but my advice is not to do anything for a while. There is a danger you could harm yourself or the item.

This is good advice.

Why not read our sticky thread on safety if you haven't already done so.
You'll find it here.

www.diyaudio.com/forums/tubes-valves/30172-safety-practices-general-ultra-high-voltage.html

For a good overview of how these valve amps work try here

How to design valve guitar amplifiers
 

Knarf

Member
2004-08-09 11:39 am
Denmark
Of all the things that die the tubes are high on the list. Caps second, and far behind.

This has not been my experience for vintage non-HiFi consumer electronics, like radios and electric organs. The tubes are frequently somewhat conservatively rated in the output stages, and the schematics are designed with an eye toward keeping the equipment working even as the tubes age. It is not common that I have to replace a tube, unless the equipment has been abused. The abuse could be in the form of switching the equipment on before checking the signal capacitors for leaks.

On the other hand some brands of vintage European consumer electronics used *very* poor quality signal capacitors, employing paper and wax insulation. You have about a 100% chance of finding bad ones in a piece of equipment, which uses them. Basically, when you see one of these capacitors, you don't want to leave them in. I mean literally. You look at it, and realizes it is cracked and has swollen a bit.

This piece of equipment seems to be using Polystyrene capacitors for signals and tone controls, and those do happen to be extremely reliable. In this particular case it is less likely that there are any bad signal caps.

Checking for poor contacts in sockets and elsewhere I fully agree on.