Wavebourn Pyramid - VII driver stage

[Wavebourn]

Cheers to everyone who've asked how do I drive GU-50 tubes in my amps so they sound so nice.

Here is the schemo attached. Differences from Pyramid -V: there was 6N6P driver, 120V for screen grid of 6F12P was regulated. Now, screen grid is used for feedback by DC to stabilize working point of tubes when they drift or were replaced; drivers are made on 6P15P, because they are more linear than triodes driving low load resistance (parallel feedback by voltage across GU-50 tubes).

The schemo is attached.

Enjoy!

Anatoliy

 

[kenpeter]

Why is screen grid resistor R11 (360R) different than R16 (1K)?

Would there be an advantage to servo/gyrate R2, since this
node follows through to set the G1 voltage for both 6P15P?

 

[Wavebourn]

Why is screen grid resistor R11 (360R) different than R16 (1K)?

Stupid drawing software puts 1K default value. It is 360 Ohm. Scott Wurcer yesterday stepped on the same rakes confusing everyone who tried to follow him.

Would there be an advantage to servo/gyrate R2, since this
node follows through to set the G1 voltage for both 6P15P?

The whole thing is servoed already, through G2 from the "shortened long tail".

V1/1 is a pentode.

 

[Wavebourn]

Too late to edit...
If you look closer, R8 and a 82 uF 400V capacitor to ground shortens the tail increasing amplification factor of the last driver stage. Also, a voltage from the capacitor is used to drive a screen grid of the 1'st tube to stabilize a working point.

 

[Wavebourn]

Here is a filament PS (attached)

 

[kenpeter]

Extremely sneaky servo. I never even saw it coming.

 

[Wavebourn]

Extremely sneaky servo. I never even saw it coming.

Thanks!

Though, it does not help too much against tubes' geometry errors, but helps a lot against their aging.

All 6F12P tubes I possess have wide like barn's gates variations. This servo helps to keep them in the tight regime. I tried all 16 tubes I have, got from 88 to 134V variations on cathodes of 6P15P. Some of them did not work in Pyramid -V, but in Pyramid-VII they are good!

 

[Wavebourn]

Oops...
Please don't repeat as is; I've found that despite of nice sounding it oscillates on 200 KHz and is ringing on 05. Hz. Working on it.

 

[Wavebourn]

Done.
Among other things had to increase sensitivity to 300 mV on full power. In the next version input tube will be a different one, 6F12P is an overkill.

Frequency response was flat from 4 Hz to 120 KHz ON FULL POWER. Viva Edcor!
However, after harnessing of oscillations it is now flat from about 10 Hz to 20 KHz, with a sharp peak on 30 KHz (this time Edcor is guilty).

I'll post the schemo later.

 

[Jeb-D.]

Try using an RC snubber(s) on the transformer primary, or tweaking the compensation circuitry.

 

[Wavebourn]

Try using an RC snubber(s) on the transformer primary, or tweaking the compensation circuitry.

Thank you for the wisdom!


I fried already 5x100K 2W in parallel carbon comp resistors looking at 20 KHz squares on full power. I can live with slight sharp 30 KHz ringing, but Zobel can't live without appropriate resistors.


It is better to ask Edcor for a bit different transformers for the next version. I don't need a full power below 20 Hz.

 

[Wavebourn]

Here is it now:

http://wavebourn.com/forum/download.php?id=119&f=7