PX25 amplifier - thoughts?

carbondated · 04:24 AM

I've found this rather rococo schematic for a PX25 tube (300B switchable) -- has anyone come across or made something like this before, or have any idea if it's worth building speculatively? It looks like it would work (though I don't know why the 6V6 cathode is tied to the filament taps). I plan to build it straight as a PX25 without the switching option.

Let me know what you guys think of the circuit and if it can work without boutique Japanese transformers.

http://dl.dropbox.com/u/316191/PX25%20copy.png

barrymagrec · 2nd May 2011, 10:27 AM

I don`t think the 6V6 cathode is tied to the filament taps, they`re all tied to ground, its just the insane way the circuit`s been drawn. (why can`t people put the HT line at the top?) Looks as if it would work, quite a lot of gain, no obvious need for very special transformers - hope you have the PX25s, they`re a lunatic price these days.

Barry

zigzagflux · 2nd May 2011, 11:10 AM

The resistors across the IT secondary are a little suspect. It appears they are attempting to use them as grid leaks, which is completely unnecessary when using an IT in the manner they do. An unloaded IT usually operates better than a heavily loaded one.

The DC rectification for the output tubes will be full of hash, hum pot or not. Design is similar to Cary amps, which have audible buzz of high frequency noise.

2nd May 2011, 04:00 AM	#1
carbondated diyAudio Member Join Date: Jan 2011	PX25 amplifier - thoughts? I've found this rather rococo schematic for a PX25 tube (300B switchable) -- has anyone come across or made something like this before, or have any idea if it's worth building speculatively? It looks like it would work (though I don't know why the 6V6 cathode is tied to the filament taps). I plan to build it straight as a PX25 without the switching option. Let me know what you guys think of the circuit and if it can work without boutique Japanese transformers. http://dl.dropbox.com/u/316191/PX25%20copy.png Last edited by carbondated; 2nd May 2011 at 04:24 AM. Reason: added information

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2nd May 2011, 10:27 AM	#2
barrymagrec diyAudio Member Join Date: Apr 2010	I don`t think the 6V6 cathode is tied to the filament taps, they`re all tied to ground, its just the insane way the circuit`s been drawn. (why can`t people put the HT line at the top?) Looks as if it would work, quite a lot of gain, no obvious need for very special transformers - hope you have the PX25s, they`re a lunatic price these days. Barry

2nd May 2011, 11:10 AM	#3
zigzagflux diyAudio Member Join Date: Oct 2006	The resistors across the IT secondary are a little suspect. It appears they are attempting to use them as grid leaks, which is completely unnecessary when using an IT in the manner they do. An unloaded IT usually operates better than a heavily loaded one. The DC rectification for the output tubes will be full of hash, hum pot or not. Design is similar to Cary amps, which have audible buzz of high frequency noise.

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