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disco 26th September 2012 07:44 PM

all dht se PX4
After an escapade with 6C41C -great fun below 200Hz- back to the roots with an all DHT setup:

The big lead cells need recharging once a week, running three hours a day.

disco 26th September 2012 07:50 PM

On high efficiency speakers (95dB probably) no hiss or humm. Notice the distance between the power tranny and the #26...

Voices and piano to die for [uhrr... in a positive way that is],
despite the bad room acoustics in the shack.

andyjevans 26th September 2012 08:30 PM

You really should try filament bias for KC1 and the 26 - it's all on the thread "26 pre amp". And if it were me I'd be using the 4P1L instead of the 26, again in filament bias.

What is the interstage and what ratio?


disco 26th September 2012 08:37 PM

Hi Andy,

You'd suggest to use the negative filament as the cathode point and loose the resistors instead?

Well, might try that ;)
The internal resistance of a real lead cell is quite low,
the change will have less impact than with a linear reg I presume.

That's a C-core 1:1. BW > 35KHz @ 10mA if I recall correctly.
What's the improvement of the 4P1L?

andyjevans 26th September 2012 09:01 PM

You'd have to have a higher voltage for filament bias, that's true, but you would gain a lot by leaving out the large cathode resistor and its bypass.

What make interstage, and do you know its inductance in Henries? I'm always interested in interstages for these kinds of circuits


disco 26th September 2012 09:21 PM

It's the standard AE interstage (scroll down to the bottom of the page). They had a more expensive type with amorphe metal core too. Developped by Pieter btw. It has an overall nice sound, dynamics are a little tame perhaps in comparison to Tamura 5000 series, but these are 100,- each :)

I'm reading Thomas' extract here.

disco 1st October 2012 02:56 PM

Did some optimizing (by ears). Will do the calculations next to see if improvements are possible. There always are, not?

Some overall feedback is applied to help the damping factor, it reduced some of the outputs' nastiness too. From a clean sinus wave I could not hear a difference in output level < 15mV. 25mV level difference was hard to notice, it depended on frequency.

I used cheap iron, better grade will be tested as well as a shunt regulator for the driver stage. Sounds already pretty good from what I'm hearing right now.

disco 2nd October 2012 09:12 PM

Was thinking today about the double voltage amplifier setup and figured that the KC1 could do the work alone, leaving the 26 for buffering service. Tinkering with the load line I settled for 57K, producing circa 12Vrms at the expense of distortion. That we don't want and besides the 34Vpp was not enough to drive the PX4 to full output. Are there options unexplored? Well, yes (there always are, not?) an interstage under the cathode of the 26.

Have you tried this already 26-gang?

It must be an impedance thing, because the AE -which is a fine 1:1 interstage- didn't keep up with the former reproduction quality. I had another shot with an 1:2 interstage and -bingo- that 5K to 20K did the trick. In went a Tamura B-6004.

316a 3rd October 2012 10:13 AM

Just an observation : you should remove the cathode connection from the resistor centre tap , remove the resistors (which do nothing apart from run down the batteries) and tie the cathode connection to one side of the filament

I found 3C24 was a super , single stage driver for Osram PX4 . It's a pain in the neck to light up and needs it's own (higher) HT supply but IMO is worth it and sounds superb with PX4/2A3 etc :)


disco 3rd October 2012 07:56 PM

Beautiful valve. Guess that amp has a strong industrial design to it.
The reason for my low supply voltage is in the 30's era bulbs I'm using. Those oxide coated battery tubes are very linear small signal tubes. KC1 has high mu at the expense of high Ri, so possibilities are limited -hence the buffer stage.

The setup I'm working on now:

Yes, there should be a decoupling cap at the anode of the 26 ;)

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