[balderon]

I would recommend applying the Amphead and Regal mods as part of your initial build. It's certainly easier to make a few component changes now rather than later.

 

[funch]

Quote:

Originally Posted by balderon /img/forum/go_quote.gif X2 dean0: The temperature of my DAX was very warm with a similar setup as funch. With the 90ma 270DAX transfo (w/EZ81/Regal mod) my B+ was dropping to < 238v@96ma. Changing to the 270FX (or 370FX) has worked out very well. The transfo is barely warm and the B+ is at spec with plenty of power to spare and without any significant heat buildup.

I've never had a problem with my B+, but that may be due to the dedicated
1A heater tranny that I'm using for the EZ81 PS tube.

 

[adamus]

Quote:

Originally Posted by balderon /img/forum/go_quote.gif I would recommend applying the Amphead and Regal mods as part of your initial build. It's certainly easier to make a few component changes now rather than later.

agreed on the regal mod, but wont the amphead mod run the tubes out of spec with respect to heater to cathode voltage of the 6922.

why not use a the heater CT to reference the voltage? e.g. second diagram

this would appear to give the positives of ampheads mods, without the negatives?

 

[adamus]

Whilst in the modding mood, could we replace the cathod resistor of the first gain stage with an LED, thus eliminating the bypass resistor? It probably wouldnt work great stock, but with regals mods, the leds should be fairly linear (and have less impact on the signal than the bypass cap?

 

[fishski13]

isn't Amphead's mod just a virtual CT for the heaters, tied to ground - no DC elevation on the cathode biased 6922 as you're suggesting? http://cavalliaudio.com/bijou/main.php?page=heatermods

 

[adamus]

yes, thats precisely what it is, but by doing that you a running the top triode with a difference of 120ish volts between heater and cathode.

 

[regal]

Quote:

Originally Posted by adamus /img/forum/go_quote.gif Whilst in the modding mood, could we replace the cathod resistor of the first gain stage with an LED, thus eliminating the bypass resistor? It probably wouldnt work great stock, but with regals mods, the leds should be fairly linear (and have less impact on the signal than the bypass cap?

I remember looking for a suitable LED to do this and couldn't find one with the right foward voltage to give the correct mA's the PS could handle. If you find one let me know I would love to get rid of those big electrolytic bypass capacitors.

 

[fishski13]

Quote:

Originally Posted by adamus /img/forum/go_quote.gif yes, thats precisely what it is, but by doing that you a running the top triode with a difference of 120ish volts between heater and cathode.

what's the heater to cathode voltage rating? http://www.jj-electronic.com/pdf/E88CC.pdf

i also don't understand where you are getting 120V. http://cavalliaudio.com/bijou/images...elVoltages.gif

btw, my amp is silent with Amphead's mod.

 

[TimJo]

Quote:

Originally Posted by fishski13 /img/forum/go_quote.gif what's the heater to cathode voltage rating?

For a 6922

Heater negative with respect to the cathode - 60 volts max.
Heater positive with respect to the cathode - 120 volts max.

 

[adamus]

Quote:

Originally Posted by fishski13 /img/forum/go_quote.gif what's the heater to cathode voltage rating? http://www.jj-electronic.com/pdf/E88CC.pdf i also don't understand where you are getting 120V. http://cavalliaudio.com/bijou/images...elVoltages.gif btw, my amp is silent with Amphead's mod.

120v was a guess, i see its 103.

Surely ampheads mod puts the heater cathode rating over the edge?

 

[fishski13]

thanks adamus and TimJo.

so without Amphead's mod, we have 44V DC elevation with a voltage divider = (65K/65K+348K)x348K. i understand this. this easily gets us under 60V max when the heater goes negative to the cathode.

i'm no EE, but i thought that the artificial CT negated the need for heater elevation. doesn't the 2 100ohm in parallel to ground create a path of least resistance for any current coupling between the heater and cathode?

 

[adamus]

the potential difference still exists.

 

[runeight]

Sorry gents, been on to other things.

Amphead's mod puts the heaters at 0VDC. The ECC99s don't care too much because I think they have a max HK voltage of 200V.

But the 6922 has a max of 130V. With the cathode of the phase splitter at ~100V I opted to reduce this by elevating the heater voltages. It is not absolutely necessary, just an engineering choice. 6922s running with +100V on their cathodes will probably run forever.

Just be careful, if you make mods, not to stray out of the HK maximum ranges.

 

[adamus]

Alex, excuse my lack of knowledge, but in my head all Ampheads mod does is reduce noise by 'balancing' the 6.3vac (3.15v around 0v). Why not then elevate it with DC to create the safety margin for the HK rating?

With regard to diode bias, could we not use a red led in series with a small signal diode, thus having around 2.4v grid. Its not that accurate but you could mess with led's. The internal resistance would be around 10ohms, which may still be better than the bypass cap?

 

[TimJo]

Quote:

Originally Posted by adamus /img/forum/go_quote.gif Why not then elevate it with DC to create the safety margin for the HK rating?

I see no reason why you couldn't do this, but then again I am still a rookie when it comes to tubes.

Essentially that's what Alex did in the original design (although only to one side of the heater winding) but it seems for some reason to have induced hum on amphead's build.

If you have a clean way to tap ~50 vdc from the PS without introducing noise into the cathode, I don't really see any downside. But I'll let Alex chime in to be sure I'm not missing something.