Paraglow 5965 Experimenters...

[binayak]

Any interesting alternatives? Is a Mullard 12AT7WA/CV4024 an option?

Thanks,
Binayak

[RPMac]

I converted to Paraglow II with 6N1P. I have a pair of Mullard CV4024 that I would like to try, but lack the design knowledge to convert. There must be a reason Doc and PJ use the 12AT7 in the Paramount.

[Paul Joppa]

"...There must be a reason Doc and PJ use the 12AT7 in the Paramount."

Well, it's more a historical accident than a reason. But recent measurements show that the 12AT7 gives remarkably low distortion when used with a current source load, so I think we lucked out. I wanted to convert to a 5670 because from the curves it looked like it would give us much less distortion, but in the lab the good old AT7 beats it by a whisker. (This was with Ed's tubes, Mullards I think; we have not studied a variety of tubes yet.)

One big advantage of the 12AT7 over 6N1Ps, 5670s, and 5965s is that there are many more to choose from if you like to roll tubes.

[binayak]

Hi Paul,

Love my Paraglows too much to get the Paramount :-)

Actually, I have swapped the 5965s for some NOS 7062 to good effect but I was'nt sure I could try the 12AT7 variants, hence my post. In any case, the Paraglows and my Klipsch Fortes make enough music to keep things groovy!

Binayak

[RPMac]

I've been studying/trying to learn how to make the conversion from 6N1P to 12AT7.(Following Doc's Blues Master article) This is what I have come up with...

2A3 present operating points: Vp=435; Vg=165; Vk=205 [Vp-k=230; -40 bias]

12AT7 operating points from plate curves: Vp=165V, 4 mA, -2V grid bias
C4S input; 205V

1. Change heater from 6V to 12V

2. Calculated C4S current set resistor; 237 (book says 249)

3. Calculated C4S bias resistor; 102k5 (board has 124k now...can I leave this value in place?)

4.Calculated Cathode resistor; 500

Am I safe to proceed? What am I missing?
Advise please.

[Grainger49]

Binayak,

The Paraglows and Parabees precede me as a Bottlehead.  Can you post some pictures with a brief description of them in the Gallery section?

Thanks,

P.S. Have you heard Paramounts in your system?  Just asking.

[Paul Birkeland]

2A3 present operating points: Vp=435; Vg=165; Vk=205 [Vp-k=230; -40 bias]

12AT7 operating points from plate curves: Vp=165V, 4 mA, -2V grid bias
C4S input; 205V

1. Change heater from 6V to 12V

2. Calculated C4S current set resistor; 237 (book says 249)

3. Calculated C4S bias resistor; 102k5 (board has 124k now...can I leave this value in place?)

4.Calculated Cathode resistor; 500

Am I safe to proceed? What am I missing?
Advise please.

Just wire the heaters in parallel to make them work at 6v.  Your operating points look good, I would go ahead and give it a try.  I would not be shocked to see things off by 5-10 volts.  On the LED bias set resistor, you can probably leave the 124k, or use a 100k if you have some handy.  On the cathode resistor, you want 2v and you will be running roughly 3.7ma with the 237 ohm resistor, so adjust accordingly (in other words, buy a pair of 500 and 550 ohm resistors and try them both). 

[RPMac]

Thanks, Paul, that's what I was looking for.
In the Paraglow II upgrade, the other half of the tube is used as a shunt reg. How would the tube change effect that part of the circuit? Will I just need to adjust the VR to get 205V at the C4S input?