Losing the treble and transient response in this PX25 SE amp - diyAudio
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Old 28th July 2008, 10:14 PM   #1
rtsang is offline rtsang  Hong Kong
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Default Losing the treble and transient response in this PX25 SE amp

An old radio artisan in his 70s recently build this PX25 SE amp for me.
It was build with old parts. Parts salvaged from old radios, telephone transmission amplifiers, etc. So some of the values of the components were not known.

I had only traces the circuit but had not measured the voltages with the tubes in it.

I found the amp lacked top range and transient response, yet the mid range was good and the sound stage was deep and wide.

I was wondering what happened?

The driver pentode did not have a cathode bypass cap. Could it be the driver was slew rate limited due to the high output impedance?

I added a 100u bypass cap to the cathode of the pentode and the treble and transient response seemed to be improved? Or am I just limited the gain in low end so that the cathode cap acted as a sort of "treble booster"?

Any ideas?

I will make some measurements soon and post it here.

Raymond.
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Old 28th July 2008, 10:20 PM   #2
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Old radios and telephone equipment are not known for their fidelity. It could be the limits of the output transformer. The 22k resistor on the first tube seems low for a pentode.
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Old 28th July 2008, 10:44 PM   #3
rtsang is offline rtsang  Hong Kong
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Default The first plate resistor should be 220k

My fault.

The output transformers were not salvages from old parts, but a newly custom winded one. I do not have a oscilloscope, so I could not measure the square wave waveform at 10kHz.

Raymond.
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Old 29th July 2008, 01:53 AM   #4
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Rather than bypass 100uf PX25 cathode to ground, then back up
from ground to the top end of the 22K plate resistor. Just couple
those points directly to each other with the 8uF...
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Old 29th July 2008, 02:47 AM   #5
SHiFTY is offline SHiFTY  New Zealand
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That's a strange operating point for a EF37A, usually they run similar to EF86, with plate voltage of 80-100V, 100K load resistor; and smaller screen cap.
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Old 29th July 2008, 08:00 AM   #6
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Default Re: Losing the treble and transient response in this PX25 SE amp

Quote:
Originally posted by rtsang
The driver pentode did not have a cathode bypass cap. Could it be the driver was slew rate limited due to the high output impedance?

I added a 100u bypass cap to the cathode of the pentode and the treble and transient response seemed to be improved?

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.
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Old 29th July 2008, 08:48 AM   #7
rtsang is offline rtsang  Hong Kong
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Default I have quite good experience in driving DHTs with pentodes

I am not sure about PX25, but I have 2 amps driven by pentodes.

First one is the WE91 Clone by Thorsten, using WE310A driving a 300B, his trick was to use a positive feedback in the driver stage to boost the high frequencys to counter the roll off at 10k.

The second one is a Yamamoto 2A3 using WE717A to drive a 2A3.

I previous had a 6SN7 cascade driving a 300B but I do not like the sound much.

I had yet to listen to the sound of a high Gm triode driving an interstage and the DHT.

I am not sure how much better or worse if I add a cathode follower after the pentode to drive the DHT.

Any ideas?

Raymond
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Old 29th July 2008, 09:23 AM   #8
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Default Re: Losing the treble and transient response in this PX25 SE amp

Quote:
Originally posted by rtsang
...

I found the amp lacked top range ...
Take a 50K volume pot.

Kind regards,
Darius
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Old 29th July 2008, 09:47 AM   #9
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Default Re: I have quite good experience in driving DHTs with pentodes

Quote:
Originally posted by rtsang
I had yet to listen to the sound of a high Gm triode driving an interstage and the DHT.


That's what i use. It's a radically different sound and you might not like it. In any case a cathode follower would be my last choice.
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Old 29th July 2008, 10:20 AM   #10
rtsang is offline rtsang  Hong Kong
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Default Using a 50k pot

Dear Darius,

I also thought the 500k pot is way too high. Maybe that this is what the builder had in hand.

Educate me, is it a high resistence pot would cause too put high frequency roll off due to the interaction with Miller capacitance in the driver?

Regards,

Raymond.
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