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Old 18th May 2011, 04:19 PM   #3461
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Quote:
Originally Posted by mastodon View Post
What OPT's are you using for this amp? And did you try adding NFB to this design and eventually left it out because it didn't help any, or did you just leave it out to begin with and didn't add it because it wasn't necessary? I'm inquiring because I'm drilled by the experts around here that 'single ended pentode requires NFB'. I might have misunderstood that though, so I'm interested in your view on this topic.
Hi the OPT is a 1K25 200mA transformer I had made by Majestic in the UK
The amp does use feedback.

The 220K resistor from the plate of the PL509 is part of a feedback network originally discussed in the 1930s by Mr Schade. The resistor feeds a proportion of the plate signal back through the coupling cap to the grid of the PL509. If this was left out there would be no bass and very little speaker damping factor, so those who say that SE pentodes need some form of feedback are absolutely right. The rule of thumb for getting a ballpark value is 5X the plate resistor of the driver stage, so my 47K driver stage resistor X 5 = around 235K. I settled on 220K.

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Btw, did you experience any issues with oscillations at this plate voltage? For the guitar amp project I mentioned above, I tried running 4 PL519's in parallel PP at about 500V with fixed bias (somewhere around -45V), but all I managed to generate was RF The concept worked just fine at 170V plate voltage though, although I obviously didn't get the output power I was looking for at the time.
Re oscillation
Bruce Rozenblit uses a 700V B+ in his push pull EL509 amplifier and is emphatic about the need for a high value 20K grid stopper to prevent oscillation of the output tube. So that's what I used.

Steve
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Old 18th May 2011, 06:53 PM   #3462
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Heres my Borbely EB894/211 Clone
http://img804.imageshack.us/img804/6012/dsir0113.jpg
http://img861.imageshack.us/img861/1114/dsir0112.jpg

And inside

http://img338.imageshack.us/img338/5536/imag0075doc.jpg
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Old 19th May 2011, 03:26 PM   #3463
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Steve, that's excellent, thanks for your reply! I completely overlooked the Schade network... Regarding the oscillation, I think I used something like 1k for a grid stopper back in the day; I would need to dig up the schematics. But I'm sure it was something like 1k or even smaller, so definitely not the 20k you're using. Thanks again!
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Old 19th May 2011, 04:11 PM   #3464
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I've never seen such tiny commercial power toroids in my life. What are their ratings? Where do you get them?
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Old 21st May 2011, 08:50 PM   #3465
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My latest creation: PL84 push pull
280V B+ and 250V on screens (as discussed here: Pl84 tubes)

more pics here: hidnplayr Gallery - My Photo Gallery
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File Type: jpg 18052011606.jpg (115.5 KB, 1409 views)
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Old 22nd May 2011, 03:52 AM   #3466
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Originally Posted by hidnplayr View Post
My latest creation: PL84 push pull
280V B+ and 250V on screens (as discussed here: Pl84 tubes)

more pics here: hidnplayr Gallery - My Photo Gallery
Them's nice big output irons for these tubes. What sourced those?
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Old 22nd May 2011, 08:20 AM   #3467
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Originally Posted by rcavictim View Post
Them's nice big output irons for these tubes. What sourced those?
They come from a vintage EL34 amp.
I really dont like the sound (and price) of an EL34 tube, but this PL84 is amazing..
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Old 23rd May 2011, 06:04 PM   #3468
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Default Single ended 6w

Here is my 6w single ended amplifier with my complex pre-amp.

Click the image to open in full size.

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Last edited by huseying; 23rd May 2011 at 06:18 PM.
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Old 23rd May 2011, 08:51 PM   #3469
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What are trhe tubes.
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Old 23rd May 2011, 09:01 PM   #3470
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looks like 6П14П and 6Н23П
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