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Cathode feedback and DHTs

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jane

jane

Member
Joined 2004
veiset.net/
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.
An externally hosted image should be here but it was not working when we last tested it.


Jan E Veiset
 
EC8010

EC8010

Ex-Moderator
Joined 2003
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

jane

Member
Joined 2004
veiset.net/
EC8010 said:
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.
Click to expand...

Good point!
What went wrong with the 2A3s?

(@mobyd: I'm using MS Visio)

Jan
 
jane

jane

Member
Joined 2004
veiset.net/
Giaime said:
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? :xeye:
Click to expand...

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. ;)

An externally hosted image should be here but it was not working when we last tested it.


Jan E Veiset
 
rdf

rdf

Member
Joined 2004
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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