Cathode feedback and DHTs

jane · 15th February 2007, 06:24 AM

Are there any considerations I should be aware of using DHTs and cathode feedback? The filament transformer will be non-decoupled in the signal path -- any chances for capacitive or inductive coupling between the push and pull side or strange frequency response?

At the schematic a filament transformer, with two center-tapped windings, is connected to x-ct-x and y-ct-y.

Jan E Veiset

mobyd · 15th February 2007, 08:14 AM

Jan, what software do you use to make those nice schematics ?
Mike

EC8010 · 15th February 2007, 09:28 AM

When I tried the same trick with 2A3 (didn't work very well), I used a transformer kit to make the 2A3 heater windings. The significance is that it was a split bobbin construction with the primary in one half and the other half left empty for the secondary. Thus, there was very low capacitance between the mains and the 2.5V 2A3 heater windings. You might want to consider the same idea here.

jane · 15th February 2007, 11:05 AM

Quote:

Originally posted by EC8010
When I tried the same trick with 2A3 (didn't work very well), I used a transformer kit to make the 2A3 heater windings. The significance is that it was a split bobbin construction with the primary in one half and the other half left empty for the secondary. Thus, there was very low capacitance between the mains and the 2.5V 2A3 heater windings. You might want to consider the same idea here.

Good point!
What went wrong with the 2A3s?

(@mobyd: I'm using MS Visio)

Jan

EC8010 · 15th February 2007, 11:19 AM

I think it was a poor output transformer. The moment I saw it I had my doubts - the winding window wasn't full. Anyway, trying to add cathode feedback (same proportion as yours) ruined the square wave response even though it reduced distortion.

Giaime · 15th February 2007, 01:27 PM

Not so much gain available with DHT triodes, to use cathode feedback!

Most of them are already hard to drive, and increasing drive signal increases drive stage distortion. So, what's worse?

I've tried with 300B, and they become unity-gain devices

Linear, but useless.

jane · 15th February 2007, 02:43 PM

Quote:

Originally posted by Giaime
Not so much gain available with DHT triodes, to use cathode feedback!

Most of them are already hard to drive, and increasing drive signal increases drive stage distortion. So, what's worse?

PX25 is quite easy to drive, in this case ~70Vpeak at full power output with cathode feedback. The EL84 driver stage is tested and found very linear: 70V output with less than 0.1% THD, so I don’t think the main problem will be lack of clean voltage swing from the driver.

Jan E Veiset

Giaime · 15th February 2007, 03:46 PM

How much power will you expect?

jane · 15th February 2007, 04:43 PM

About 2x15W

Jan.

rdf · 15th February 2007, 05:23 PM

Hi Jan, a somewhat off-topic suggestion. I've been playing around with bias points for the triode-mode 6CW5/EL86, subbing for an EL84 in SE. At very near the operating conditions you're using the 3rd harmonic distortion component of NEC-branded version drops dramatically, multiple tens of dBs. I recall the plate current was more high teens than low twenties, otherwise plate load and B+ were near identical. You may be able to significantly better that 0.1% with an EL86 and get lower source impedance to boot. To my ear it also has a much more direct sound than the EL84.

15th February 2007, 06:24 AM	#1
jane diyAudio Member Join Date: Jul 2004 Location: Norway	Cathode feedback and DHTs Are there any considerations I should be aware of using DHTs and cathode feedback? The filament transformer will be non-decoupled in the signal path -- any chances for capacitive or inductive coupling between the push and pull side or strange frequency response? At the schematic a filament transformer, with two center-tapped windings, is connected to x-ct-x and y-ct-y. Jan E Veiset __________________ Life is hard - Then you die.

15th February 2007, 09:28 AM	#3
EC8010 diyAudio Moderator Join Date: Jan 2003 Location: Near London. UK	When I tried the same trick with 2A3 (didn't work very well), I used a transformer kit to make the 2A3 heater windings. The significance is that it was a split bobbin construction with the primary in one half and the other half left empty for the secondary. Thus, there was very low capacitance between the mains and the 2.5V 2A3 heater windings. You might want to consider the same idea here. __________________ The loudspeaker: The only commercial Hi-Fi item where a disproportionate part of the budget isn't spent on the box. And the one where it would make a difference...

15th February 2007, 11:19 AM	#5
EC8010 diyAudio Moderator Join Date: Jan 2003 Location: Near London. UK	I think it was a poor output transformer. The moment I saw it I had my doubts - the winding window wasn't full. Anyway, trying to add cathode feedback (same proportion as yours) ruined the square wave response even though it reduced distortion. __________________ The loudspeaker: The only commercial Hi-Fi item where a disproportionate part of the budget isn't spent on the box. And the one where it would make a difference...

15th February 2007, 04:43 PM	#9
jane diyAudio Member Join Date: Jul 2004 Location: Norway	About 2x15W Jan. __________________ Life is hard - Then you die.

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15th February 2007, 08:14 AM	#2
mobyd diyAudio Member Join Date: Sep 2004 Location: Auckland	Jan, what software do you use to make those nice schematics ? Mike

15th February 2007, 01:27 PM	#6
Giaime diyAudio Member Join Date: Oct 2004	Not so much gain available with DHT triodes, to use cathode feedback! Most of them are already hard to drive, and increasing drive signal increases drive stage distortion. So, what's worse? I've tried with 300B, and they become unity-gain devices Linear, but useless.

15th February 2007, 03:46 PM	#8
Giaime diyAudio Member Join Date: Oct 2004	How much power will you expect?

15th February 2007, 05:23 PM	#10
rdf diyAudio Member Join Date: Jun 2004 Location: big smoke	Hi Jan, a somewhat off-topic suggestion. I've been playing around with bias points for the triode-mode 6CW5/EL86, subbing for an EL84 in SE. At very near the operating conditions you're using the 3rd harmonic distortion component of NEC-branded version drops dramatically, multiple tens of dBs. I recall the plate current was more high teens than low twenties, otherwise plate load and B+ were near identical. You may be able to significantly better that 0.1% with an EL86 and get lower source impedance to boot. To my ear it also has a much more direct sound than the EL84.

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