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Losing the treble and transient response in this PX25 SE amp

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rtsang

Member
Joined 2005
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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  • ef37a px25 se.pdf
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analog_sa

analog_sa

Member
Joined 2002
rtsang said:
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?
Click to expand...


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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rtsang

Member
Joined 2005
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
 
R

rtsang

Member
Joined 2005
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.
 
S

Sheldon

Member
Joined 2003
Re: Re: Losing the treble and transient response in this PX25 SE amp

analog_sa said:
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.
Click to expand...

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
 
R

rtsang

Member
Joined 2005
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.
 

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  • ef37a px25 se v1.1.pdf
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R

rtsang

Member
Joined 2005
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.
 

Attachments

  • EF37A PX4-PX25 SE v3.2.pdf
    23.9 KB · Views: 93
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