Plitron PAT4006 - advice please

[gingertube]

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

 

[Enzo]

Parasitics come from a variety of places. Have you considered it might be a lead dress issue?

 

[cogsncogs]

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

 

[gingertube]

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

 

[jleaman]

Pictures ? Schematic ?

 

[gingertube]

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

 

[jleaman]

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 ?

 

[SY]

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.

 

[gingertube]

Pics for Jason

Jason,
Here are the bricks.

Cheers,
Ian

 

[jleaman]

May i ask what it costed you for each tranny ? as it worth it . ?


Un us dollars

 

[gingertube]

Costs

Jason,

I see that you are in Canada - a big bonus for building these amps.

Boards and Trannies are from Plitron and the Chassis are Hammond Engineering - all good Canadian Stuff. Get local prices from their Websites. For me (being down under) shipping and customs duty added 1/3 again to the price.

I,n about to mod them again - I've been in contact with Menno and he suggests that I rewire them to Ultralinear + shunt feedback along the lines of mods he has done to his UL40-S2 Amps.
Details of that mod here
www.mennovanderveen.nl

Cheers,
Ian