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Old 1st April 2007, 08:24 PM   #11
SY is offline SY  United States
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It must be said that moving probes around in a powered circuit is far and away the most common way of letting out the smoke. Despite that, many of us do it anyway. So you are officially warned to follow Brian's advice.

That said, you might have a tube that's gone south. It does sometimes happen. Try something silly like replacing the ECC88s and going up a bit in value on the ECC88 stoppers.

Get a scope. It's the single most useful tool in the electronics shop, tied with a good DVM.
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Old 1st April 2007, 08:46 PM   #12
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Thanks!

UPDATE: I just shortened the leads for the feeldback loop and replaced the loop wire with some sheilded stuff. The oscillation is maybe slightly less noticeable, but no other changes.

I also should confess that I reduced the overall feedback to something other than Morgan Jones' recommendation and this may be causing the issue as well. I'll try and change back to his feedback scheme and see if that makes a difference.

There are 330Rs for gridstoppers on the E88CCs. Maybe 500R would make a difference? Would the increase in gridstopper cause a decrease in input sensitivity?

Also, are there any superdupercheap function generators out there? How about this one?

I know. Too cheap, Kofi.

Yours,
Too-Cheap Kofi
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Old 1st April 2007, 08:53 PM   #13
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Be daring, go up to 1k.
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Old 1st April 2007, 09:35 PM   #14
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Try 10k. If it works, you know you're in the right part of the circuit and you can try reducing it until you find out what value is really needed. A photograph of your earthing arrangements around the input stage would be useful...
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Old 1st April 2007, 09:38 PM   #15
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Quote:
Be daring, go up to 1k.
Well, all I had was 1.5K so I decided to blow your concept of Kofi AWAY and increase even that by another 50%.

Well, of course, its working flawlessly now. All oscillation is completely gone.

I guess I can play with these values to get the right number. Also, I just used some generic metal film and it sounds a little grainy right now. I'll keep listening and see what break-in time does for this, if anything. Carbon comp would be best for this position, I guess, so I'll grab some of those in various values and see what occurs.

Thanks a million for the advice. Again, you all never stop amazing me at how helpful you can be.

Thanks to all.

Now-- about my superdupercheap function generator....

Kofi....
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Old 2nd April 2007, 01:16 AM   #16
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Quote:
Originally posted by Kofi Annan


...
Now-- about my superdupercheap function generator....

Kofi....
If you don't have to have it right away then eBay may be the way to go. I bid on maybe 25 function generators before winning an HP (don't know the model and too lazy to go downstairs and look) for $30 and $10 shipping.

Got a SOAR 2 channel oscilloscope w/2 probes for $60 and about $12 shipping after bidding on maybe 30-40 scopes.

If you need it right away then waiting until the week that nobody else wants what you do probably isn't the way to go.
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Old 2nd April 2007, 01:53 AM   #17
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For a super cheap fuction generator you can use a test CD in a CD player.

For a FREE function generator download this program and run it in an old PC. It is not a good idea to connect your shiny new quad core super computer up to a circuit that contains hundreds of volts unless you are sure that you know what you are doing. I use a USB sound card on an old laptop.

http://www.dr-jordan-design.de/signalgen.htm
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Old 2nd April 2007, 09:14 AM   #18
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Hmmm. To me, a function generator means something that produces square, triangular, and approximately sine waves up to 10MHz with the guarantee that a 10kHz square wave will be near perfect when inspected on a good oscilloscope. Flatness of frequency response within the audio band is also guaranteed by design, making it ideal for measuring frequency response of an audio amplifier.

Any source relying on digits to synthesise a signal (such as a soundcard or a CD player) cannot guarantee a flat response. It doesn't mean they're unusable, but it does mean that they demand a much more knowledgable user to overcome their limitations. A 10MHz function generator will be a little more expensive, but very useful.
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Old 2nd April 2007, 06:34 PM   #19
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Quote:
Any source relying on digits to synthesise a signal (such as a soundcard or a CD player) cannot guarantee a flat response.
This is true. Most CD players have 44.1KHz DACs which can't play a 10KHz square wave. It is still a useful tool for troubleshooting sick or dead amplifiers.

The Win MLS software I mentioned has tools that are capable of testing frequency response out to the limits of your sound card. These are not freeware though. You can get a 192KHz sound card for under $150, mine was $99 on sale. Loopthrough response is within .5db to 50 KHz. Synthesized 10 KHz square waves look decent on a scope.

I agree that for serious testing a dedicated generator is the best option. I got a Wavetek at a hamfest for $20, deal.
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Old 2nd April 2007, 06:36 PM   #20
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Two of the channels on the $25 Chaintech AV710 have 24bit 192khz DACs.
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