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Old 2nd May 2011, 04:00 AM   #1
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Default 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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Old 2nd May 2011, 10:27 AM   #2
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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
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Old 2nd May 2011, 11:10 AM   #3
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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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Old 7th November 2012, 05:35 PM   #4
samking is offline samking  India
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Default Px25

Quote:
Originally Posted by carbondated View Post
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
Which is better px25 or 300b
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Old 7th November 2012, 05:50 PM   #5
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Quote:
Originally Posted by samking View Post
Which is better px25 or 300b
You should get more power out of a PX25 but they are likely to be more expensive than a 300b. Ihave always been told that the PX4 was the king of DHTs.
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Old 8th November 2012, 11:49 AM   #6
316a is offline 316a  England
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Quote:
Originally Posted by barrymagrec View Post
You should get more power out of a PX25 but they are likely to be more expensive than a 300b. Ihave always been told that the PX4 was the king of DHTs.
PX25 in SE 5 watts , 300B in SE 8 watts . PX25 Ra 1k mu 10 , 300B Ra 700R mu 4 . PX25A are the low mu version similar to 300B , I only have one of these and would love to find another to make a pair . PX4 is a lovely valve , got several pairs in my stash . These are more or less a 2A3 with 4V 1A filament

316a
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Old 8th November 2012, 12:02 PM   #7
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Ah, yes, the 300b is much higher anode dissipation than I remembered. PX25 is 26 watts though so I would expect 7 /8 watts SE used carefully. I also have some PX4s which I used to use in a Williamson amp as well as a pair of NOS px25s.
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Old 8th November 2012, 12:40 PM   #8
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Illiffe books Radio Valve Data 1966 gives 8.5 watts for a PX 25 single ended. Different age data sheets give different figures for many old valves, you would be ill advised to run them hard for this reason.
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Old 11th November 2012, 11:27 PM   #9
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Quote:
Originally Posted by 316a View Post
PX25 in SE 5 watts , 300B in SE 8 watts . PX25 Ra 1k mu 10 , 300B Ra 700R mu 4 . PX25A are the low mu version similar to 300B , I only have one of these and would love to find another to make a pair . PX4 is a lovely valve , got several pairs in my stash . These are more or less a 2A3 with 4V 1A filament

316a
If you have some GEC KT88's in NOS condition I'd consider a swap, I have a couple of spare PX25's that I will never use..
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