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Old 30th July 2008, 12:16 AM   #11
Sheldon is offline Sheldon  United States
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Default Re: Re: Losing the treble and transient response in this PX25 SE amp

Quote:
Originally posted by analog_sa
PX25 is a wonderful tube and quite easy to drive. Still, not by a pentode. The cathode bypass probably reduced the output impedance a bit but it's still not a good solution.
I'm not sure I understand. Even assuming an infinite plate resistance for the pentode, with a 22k load resistor and estimating around 160pF input capacitance for the power tube, that still nets a roll off at around 45kHz. Maybe not the highest, but if that's the correct value it shouldn't cause an obvious loss of HF.

Sheldon
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Old 3rd August 2008, 12:27 PM   #12
rtsang is offline rtsang  Hong Kong
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Default Modified as suggested by forum members here.

and made some measurements too. Seems like the amp is optimized to use the PX4. There is a switch in the cathode resistor to change to 1K for the PX4.

Changed the cathode bypass cap of the power triode to a 47u film cap. The original one is a Cornelier Dublier paper case electrolytic, not sure if that sounded good.

Just started running it in. Will see if the treble is back.

Here is the schematic and measurements.
Attached Files
File Type: pdf ef37a px25 se v1.1.pdf (26.5 KB, 65 views)
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Old 3rd August 2008, 02:22 PM   #13
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Default Re: Re: Re: Losing the treble and transient response in this PX25 SE amp

Quote:
Originally posted by Sheldon


I'm not sure I understand. Even assuming an infinite plate resistance for the pentode, with a 22k load resistor

Maybe with an imaginary 22k load but not with 220k

Triodes absolutely require a low impedance driver in order to sound good.
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Old 3rd August 2008, 02:26 PM   #14
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Default Re: Modified as suggested by forum members here.

Quote:
Originally posted by rtsang
Here is the schematic and measurements.

Looking at the circuit i would expect good old fashioned tube sound. If that's your thing you're gonna love it.
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Old 3rd August 2008, 03:43 PM   #15
Sheldon is offline Sheldon  United States
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Default Re: Re: Re: Re: Losing the treble and transient response in this PX25 SE amp

Quote:
Originally posted by analog_sa
Maybe with an imaginary 22k load but not with 220k
Yeah, that would make a difference.

Sheldon
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Old 3rd August 2008, 04:46 PM   #16
Sheldon is offline Sheldon  United States
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Default Re: Re: Re: Re: Losing the treble and transient response in this PX25 SE amp

Quote:
Originally posted by analog_sa
Triodes absolutely require a low impedance driver in order to sound good.
http://www.diyaudio.com/forums/attac...amp=1214861014

This absolutely wouldn't sound good then.

Sheldon
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Old 3rd August 2008, 05:00 PM   #17
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Default Re: Re: Re: Re: Re: Losing the treble and transient response in this PX25 SE amp

Quote:
Originally posted by Sheldon


http://www.diyaudio.com/forums/attac...amp=1214861014

This absolutely wouldn't sound good then.

Sheldon
Actually, it might
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Old 21st June 2013, 04:14 PM   #18
chacha is offline chacha  Hong Kong
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Sorry, but this curquit sacks from A to Z and can not work properly..... Don't waste your time to try to fix it. Don't burn your precious tubes through this thing...
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Old 21st June 2013, 10:28 PM   #19
kevinkr is offline kevinkr  United States
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Five year dead thread. The OP has probably long since moved on.
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Old 23rd June 2013, 10:01 AM   #20
rtsang is offline rtsang  Hong Kong
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Default OP is still around and amp is still around

Though the amp had drastically changed.
Now it is a EF37 pentode driving a PX-4
It can drive a PX-25 with a flip of the switch to change the cathode resistor from 1k to 500R.

Recently the amp is put to storage to protect from the 1 year old that loves to throw things around. I have not made a cage to protect the valves. Those vintage PX25 and PX4 valves are to precious.
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File Type: pdf EF37A PX4-PX25 SE v3.2.pdf (23.9 KB, 30 views)
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