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Old 9th December 2004, 11:55 PM   #1
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Default Plitron PAT4006 - advice please

I've built up the bog standard Menno VDV amps.
12AU7 commom cathode amp, direct coupled concertina splitter/driver, matched quad "Winged C" Svetlana EL34s with the PAT4006 output transformer Ultralinear connected.

The O/P Tranny has response to greater than 250kHz and so it will be no surprise to many of you that I have a problem with "squegging" - bursts of HF on on edge of the signal.

I have managed to cure the problem in two different ways
1) fitted 1K 2W metal film stoppers to each screen connection (there were no resitors in the screen connections previously)
OR
2) 100pF cap from screen to grid 1 of each EL34.

Any advise on which is the better method gratefully received.

Thanks
Ian

P.S. I lean toward 1) simply because it will help protect the output tubes - rail is +485V
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Old 10th December 2004, 07:40 AM   #2
Enzo is offline Enzo  United States
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Parasitics come from a variety of places. Have you considered it might be a lead dress issue?
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Old 11th December 2004, 01:12 PM   #3
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Grid stoppers to the screen grids (100R - 1k) is almost always needed to stop parasitic oscillations and to limit dynamic screen grid current, with an added benefit; your EL34's will last much longer! Parasitic oscillations can manifest itself by arcing seen near the screen grid in the tube itself.
After that other methods of compensation can come into play if needed!

Wayne
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Old 16th December 2004, 03:26 AM   #4
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Default Final results

I settled on 1K resistors in series with each screen connection.

This worked well.

I have now converted the amp to triode mode - the 1K s did not need any modification to suit triode mode.

Cheers,
Ian
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Old 16th December 2004, 04:05 AM   #5
jleaman is offline jleaman  Belgium
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Pictures ? Schematic ?
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Old 16th December 2004, 04:27 AM   #6
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Default Schematic

Jason,
This is the bog standard simple amp.

Go to http://www.plitron.com/

Click on Toroidal Output Transformers
Standard Series (push-pull)
Publications
and then right down the bottom click on the box which says
Easy to build Push Pull Amplifier using .....

Its easy to build and performance (in Triode Mode) is very good.

Will borrow the digital camera tonight and take a pic.

Cheers,
Ian
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Old 16th December 2004, 04:43 AM   #7
jleaman is offline jleaman  Belgium
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I was Informed that toroidals are not good for tubes ? Am i wrong or right ? Im a fan of toroidals and looking to build 2 stereo mono block tube amps for bi-amping .. good idea to use toroidals or not ?
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Old 16th December 2004, 11:09 AM   #8
SY is offline SY  United States
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Toroids have advantages and disadvantages. The main advantages are HF extension, tight coupling between input and output, and low radiated fields. The main disadvantage is a cruel sensitivity to any imbalance in the output stage. And price, of course.
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Old 16th December 2004, 09:46 PM   #9
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Jason,
Here are the bricks.

Cheers,
Ian
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Old 16th December 2004, 10:05 PM   #10
jleaman is offline jleaman  Belgium
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May i ask what it costed you for each tranny ? as it worth it . ?


Un us dollars
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