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Old 13th October 2005, 05:21 AM   #21
rdf is offline rdf  Canada
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Oh great, playing on my techno-paranoia now! I double checked this one, the waveforms still appear with the probes disconnected from the circuit and shorted to the ground clips. What finally tipped me off was the signal trace increased in size as the scope sensitivity was raised until the transition from 100 mv/div to 50 mv/div, then is disappeared. Never considered the AM possibility though, makes sense. There are two or three 50 kwatt stations pointed my way less than 10 km away. Crumpling up the disconnected probe leads almost completely eradicates the waveform. It's also one of those digital realtime scopes, borrowed from work. I'm not sure it's completely honest when the horizontal timing is reaching 5 ns/div.

This RF stuff is so ridiculuously finicky. My EL84 Mullard 3-3 plays radio stations when a Wima or silver mica are used in the two feedback positions and is dead silent when they're replaced with PIO k40y-9's (.01 uF and 430 pf versions are dirt cheap on Ebay.) More art than comprehensible science.
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Old 12th July 2006, 10:12 AM   #22
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Default Hello

Hi

I just found this thread, and found the schematic, which
I have designed.
Well, I build about 8 Mono amps using this schematic.
I had never a problem with oscillation.
The amps are still running without a problem.
The idea for this amp I got from a old glass-audio article,
which used a Ecc81 for phase inverter, and driver.
I did a modification for ECC85 and build a prototype with el84.
With this prototype, I optimised the values for the
components, by using a signal generator and a oscilloskop.
I also tried a lot of ohter circuits, with 2 or 1 phaseinverter/driver
tubes.
But my goal was to find a simple circuit with good bandwith and the best rectangle signal playback.
If you need less gain, you can omit the 1,5 k resistor
between the cathodes of the Ecc85.Or you can play arround with
different output taps for the feedback, or the values of the feedback -resistor(24K)and cap as well, of course.
cheers

Armin
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Old 27th February 2014, 08:08 PM   #23
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Default Oscillating Amplifer

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Its been a while since I was her but now am back working on my amp. I'm hoping someone has an idea to help solve my problem. It's a standard push-pull tube amp, 10 watts/channel.(EF86 to a 12ax7 to 2 EL84s) with negative feedback.

I have 10ohm wirewound resistors across the 8 home output of the transformers and each channel will oscillate except the channel with the oscilliscope probe across the 8ohm output. Move the probe to the oscillating channel and it stops and the other channel oscillates.

I'm looking at it with a textronix 2215A a dual trace oscilliscope .I'm putting a 1kc squarewave on the input. It looks good with a small ring about 35kc as long as the oscilliscope is across the output. It oscillates with sine or square wave.

What is going on?
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Old 27th February 2014, 09:18 PM   #24
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I was going to try to help, but unfortuntely your link will not open on my PC. Perhaps another member has better luck.
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Old 27th February 2014, 10:59 PM   #25
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Since you are using NFB, try disconnecting it and see if the oscillation is still there. If the oscillation is no longer there, you may have positive feedback instead. Try reversing the B+/plate leads on the output transformers.

Quote:
Originally Posted by gwalters View Post
Click the image to open in full size.
Its been a while since I was her but now am back working on my amp. I'm hoping someone has an idea to help solve my problem. It's a standard push-pull tube amp, 10 watts/channel.(EF86 to a 12ax7 to 2 EL84s) with negative feedback.

I have 10ohm wirewound resistors across the 8 home output of the transformers and each channel will oscillate except the channel with the oscilliscope probe across the 8ohm output. Move the probe to the oscillating channel and it stops and the other channel oscillates.

I'm looking at it with a textronix 2215A a dual trace oscilliscope .I'm putting a 1kc squarewave on the input. It looks good with a small ring about 35kc as long as the oscilliscope is across the output. It oscillates with sine or square wave.

What is going on?
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Old 28th February 2014, 02:27 AM   #26
kevinkr is offline kevinkr  United States
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Probably not since the amp stops oscillating as soon as the scope probe is connected to the output.. Do disconnect the feedback and see if the oscillation stops.

Try to post a schematic with any changes you have made.
Do you have lag compensation on the plate of the EF86? Do you have got the right feedback resistance (not too low) for the output transformer tap you are using? Output zobel?
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Old 28th February 2014, 04:15 AM   #27
roren is offline roren  Sweden
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Have you connect the secondary 0 tap to ground? I just got the idea that if your
scope is grounded, then the secondary winding gets grounded through the scope.
Just my wild guess.
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Old 28th February 2014, 10:18 AM   #28
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Quote:
Originally Posted by roren View Post
Have you connect the secondary 0 tap to ground? I just got the idea that if your
scope is grounded, then the secondary winding gets grounded through the scope.
Just my wild guess.
Wow, this fixed the problem. My schematic shows the secondary 0 tap grounded but I missed wiring it. Been diddling with this for 2 weeks.

There is no zobel but that was next after getting this oscillating fixed. Suggestions where to start appreciated.

I didn't know how to reallly attach the schematic so I attched it as a PDF. Any more ideas for improvement would be appreciated. I revese engineered an amp I made circa 1968 whose schematic I could no longer find in the library. It has been midified to match modern transformers.
Attached Files
File Type: pdf Schematic.pdf (27.4 KB, 24 views)
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Old 28th February 2014, 06:51 PM   #29
roren is offline roren  Sweden
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Wow, I'm so happy that it helped you.
I have a special power cord for my scope where the ground lead is cut. It's not
allways good to have a mains earth (ground) present when taking measurment
in an amp, or whatever it might be that we have on our bench.
If we have feedback from the OP secondary, the winding needs to be connected
to signal ground.
Glad it helped.
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Old 28th February 2014, 07:05 PM   #30
roren is offline roren  Sweden
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Quote:
Originally Posted by gwalters
Been diddling with this for 2 weeks.
I've been "diddling" with my 'Scott 222-B' for three weeks trying to get rid of a
quite nasty hum. That have occur after I have it stored. I recapp'd it about two
years ago. And now the RIAA phono stage hums quite severe. I don't know what causing it yet
and I've been all over it with my scope trying to figure it out. Still haven't found it.
Before I use to brag over how silent it was
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