• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

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Steve Cresswell

Steve Cresswell

Member
Joined 2006
mastodon said:
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.
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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.

mastodon said:
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.
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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
 
mastodon

mastodon

Member
Joined 2011
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!
 
hidnplayr

hidnplayr

Member
Joined 2008

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mhouston

mhouston

Member
Joined 2006
retro-thermionic.blogspot.com/
My finished 300B. Tubes are TJ Full Music 300BSE premium limited edition, Avat Australian made power tranni, 50 year treasure 6SN7, Audio Note Silver and Mylar inter-stage caps, Mundorf M Tube poly filter caps, Hammond 30W OPTs, Hammond 10H choke , seperate dual secondary tranni for the 300B heaters (rectified and regulated), Golden Draggon 5U4G and silver wire in the audio path. Parts cost AU$3.5K

Made for a fellow member of the Melbourne Audio Club.
 

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lowpoke

Member
Joined 2009
homebuilthifi.com
mhouston said:
My finished 300B. Tubes are TJ Full Music 300BSE premium limited edition, Avat Australian made power tranni, 50 year treasure 6SN7, Audio Note Silver and Mylar inter-stage caps, Mundorf M Tube poly filter caps, Hammond 30W OPTs, Hammond 10H choke , seperate dual secondary tranni for the 300B heaters (rectified and regulated), Golden Draggon 5U4G and silver wire in the audio path. Parts cost AU$3.5K

Made for a fellow member of the Melbourne Audio Club.
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:eek: Wow, that set up looks very cool indeed. I bet the new owner is pleased.
 
L

lordvader

Member
Joined 2002
Nice !!!

Where'd you get the chassis from ? I'm getting back into the DIY audio after a bit of a hiatus, and am thinking that wood is good :)

mhouston said:
My finished 300B. Tubes are TJ Full Music 300BSE premium limited edition, Avat Australian made power tranni, 50 year treasure 6SN7, Audio Note Silver and Mylar inter-stage caps, Mundorf M Tube poly filter caps, Hammond 30W OPTs, Hammond 10H choke , seperate dual secondary tranni for the 300B heaters (rectified and regulated), Golden Draggon 5U4G and silver wire in the audio path. Parts cost AU$3.5K

Made for a fellow member of the Melbourne Audio Club.
Click to expand...
 
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