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Old 17th July 2007, 06:44 AM   #21
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I saw the 70V tap on the output. This is (if I recall correctly) for stringing a long series of small speakers for background music or PA.

It may make a good guitar or vocal amp. It is probably designed to not waste power on lo-freq.

Might want to find someone with a tube checker.

If there is any sort of puddle of black potting compound underneath the power transformer or output transformer, or if the wires are black with it, then it has overheated. The black potting compound melts and flows down the wires.

If it has a tube rectifier then it may be OK because they restrict maximum current, but a solid state rectifier will not limit current when the filter caps leak. I see solid state rectifiers on the schematic, so you might want to check the transformer leads. If it has gotten hot it could still be OK.

You need to check all of the resistors. Carbon composition is notorious for going as much as 100% out of tolerance.
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Old 17th July 2007, 03:08 PM   #22
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So in other words it is bad at producing low frequency or it is good at it? What do all those screw things in the back do anyways?

I have a question about buying capacitors. I need 5 .02 MFD 400 VDC. I have read that MFD means that it is in uF measurements. I have also read that the voltage can be higher than what is needed because it is a maximum. Do I need to replace these with the exact same type of caps (close to impossible) or can I use .02 uF 600 or 1000 volt ones? Also does it have to be the kind with the liquid in them, or could I used film type ones? Thanks again.
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Old 17th July 2007, 04:21 PM   #23
kevinkr is offline kevinkr  United States
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You can replace those .02uF caps with a modern film cap of 0.02uF - 0.022uF (current standard value) with any voltage rating equal or greater than the original part.

I would start with the power supply and work from there.
Before you go very far I would verify that this power transformer is infact ok.

I would also recommend you read and familiarize yourself with the high voltage safety thread here:
Safety Practices, General and Ultra-High Voltage

Something that would be useful in trouble shooting is what I call a ballast lamp - basically a lamp wired in series with the hot side of an extension cord. Has the nice property of limiting current and preventing serious component damage.

Check that power transformer by disconnecting and insulating the high voltage and bias secondary leads. Remove the tubes. Use a 100W lamp in the ballast lamp mentioned in the last paragraph. The bulb should be extremely dim and the filament voltage should be close to 6.3V.. Refer to the schematic for AC voltages for the bias and high voltage winding and measure them carefully. If the bulb glows brightly with no load on this power transformer that is a very good indication that it is toast and you will need to decide whether you are committed enough to spend the money for a replacement.

Once you have confirmed the power transformer is ok, then you should go ahead and replace the rectifiers and ALL electrolytic caps.
Paper types in the power stages should be next and last and least critical to the safety of the amplifier (should still be replaced) are the coupling caps in the small signal stages.

This PA amplifier should make a very nice vocal amp with a little work. Webster (IIRC) made some pretty decent PA amplifiers. PA amplifiers deliberately roll off the LF response to limit the amount of power required to do the job, that doesn't mean that it necessarily has a bad output transformer either. Vocals don't need a lot of power below 300Hz or so, so I doubt this is a real concern here.

As Chris suggested a good DVM would really help you out, the odd readings you get with this meter are probably due to resistances above the range of the meter. Fluke meters are durable, accurate, and unlike cheap meters are generally forgiving of mistakes as long as you don't exceed their maximum rated voltage.

Antique Electronics Supply at www.tubesandmore.com is a good place to get a lot of odds and ends needed to fix this thing. (I have no affiliation with them, but have been doing business with them since about 1990.)
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Old 17th July 2007, 07:55 PM   #24
anatech is offline anatech  Canada
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Hi Kevin,
Thanks. We are in agreement.

I personally think this will be a great vocal amp, with power to spare.

Hi Mark,
Quote:
I don't mean to rain on your parade, but you may be disappointed with the results.
The generalizations you are making will often be far off the mark. Old Bogen amps were pretty good with no shortage of bass. I had a CHA-75A that I fixed up for a bassist. He kept blasting the woofer VC out of the gap. He also commented that it was a killer amp. This one looks to be similar.

-Chris
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Old 17th July 2007, 09:14 PM   #25
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Quote:
The generalizations you are making will often be far off the mark
I thought he was going to use it for reproduction of recorded sound. I did use the word may. Sure, it is great for vocals, that is what a PA amp is for.

The yellow caps may be OK. They are Cornell-Dubilier WMF film/foil polyester. They are of modern technology.
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Old 17th July 2007, 11:12 PM   #26
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Wow that tubesandmore site is cool. I'm still trying to hunt down certain parts. I can't find a 200 MDF cap. The same site has 220uF ones. Is that too far off? They also have 100uF ones. Could I put two in parallel? What's with the 5% and 10% tolerances? I can't figure out what the old ones were.
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Old 17th July 2007, 11:22 PM   #27
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Quote:
Originally posted by DeadSpeaker
Wow that tubesandmore site is cool. I'm still trying to hunt down certain parts. I can't find a 200 MDF cap. The same site has 220uF ones. Is that too far off? They also have 100uF ones. Could I put two in parallel? What's with the 5% and 10% tolerances? I can't figure out what the old ones were.
I'm in agreement with Chris on this one, with a little TLC this should be quite a nice vocal amp, and should be suitable for some mods once working to fit it even better to the task..

To answer backwards, most likely the old electrolytic caps were +80% to -20% tolerance which just means that a nominal 200uF could range from about 360uF to 160uF in value, modern caps can be much better than this.

In the case of film caps with 5% - 10% tolerance should be fine for signal caps in this application, you can match the output tube coupling caps to a tighter tolerance than this if possible with better LF balance resulting to the output tube grids, not critical though. (A very little less distortion)

You can use a 220uF capacitor without a problem. I wouldn't bother with 2 x 100uF in parallel, it's a bother and one more component to go wrong later..

Do the transformer test I suggested before going too much further though, and do read the safety thread before the nitty gritty of troubleshooting the updated amplifier begins.

Glad you like the AES website, quite useful, lots of stuff for these sorts of repairs in one place.

If the amplifier has CDE WMF those are mylar or poly films and as long as they are not physically damaged should be fine without replacement.
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Old 17th July 2007, 11:57 PM   #28
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All of the yellow ones are cde wmf. I haven't gotten a good meter yet, but the one that I do have does not pick up a reading on any of them. I am getting a reading from the diodes so I don't think that they are blown (because I only get the reading one way). the majority of the resistors seem to not give readings. I don't know if this is do to my meter not being very good or if it is because they are actually shot. Now what you said about the transformer test is very confusing to me. Are you saying to take an average table lamp and connect the plug to the transformer? Do I take the transformer out of the chassis? Where is the power coming from? Thanks for the help
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Old 18th July 2007, 12:18 AM   #29
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Don't remove the transformer from the amplifier..

Go a local home center or hardware store, buy a lamp socket and extension cord. Identify the hot wire (usually the smooth insulation side of the extension cord) and cut it so you can wire the lamp socket in series with the extension cord hot. The amplifier plugs into the extension cord, cord plugs into isolation transformer if you have one or directly into the wall if you don't. 100W bulb in socket.

Disconnect the high voltage and bias windings from their respective rectifiers. Insulate..

Do tests as previously described. Be careful as high voltages will be present if the transformer is good or bad.

Did you read the safety thread yet?

Resistors that measure open on your meter might be good or bad, sounds like the meter doesn't have the range required. Meters generally read open on caps unless they are bad. (shorted) WMF will not need to be replaced.
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Old 18th July 2007, 12:47 AM   #30
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Wink Halt! and prepare for the job

Hi DeadSpeaker,

Please stop right now.

Given that your meter is very suspect, please put this project to the side until you get a good one.

Your current meter is probably lying to you. You can still use this one after you check the voltage readings against you new good one. Then just use it for measuring DC volts. It's always very handy to have more than one meter about.

Meantime, read the safety section and other threads where guys fixed a tube amp. I know you want to jump right in and do it, but I want you to be successful. This might be the start of a great hobby if you enjoy yourself.

So do not trust any resistance readings yet, especially on capacitors.

-Chris
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