Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: chocopower on April 20, 2010, 12:18:57 pm

Title: Elevated heaters in a 2 power tubes with split cathodes resistors
Post by: chocopower on April 20, 2010, 12:18:57 pm

I have a cathode biased amp with 2xEL34 in cathode biased mode and a shared 270 ohm resistor. Heaters center tap is hookup to cathodes to eliminate noise.

http://www.el34world.com/Forum/index.php?topic=8349.0 (http://www.el34world.com/Forum/index.php?topic=8349.0)

If i want to use one cathode resistor for each tube to use the well know atenuattion swicht you describe in this forum, can i continue conecting the heaters center tap to just one tube cathode?

thanks in advance.

Saudos!!

David

Title: Re: Elevated heaters in a 2 power tubes with split cathodes resistors
Post by: HotBluePlates on April 20, 2010, 01:18:56 pm

Just derive the voltage some other way.

The d.c. on the cathode probably wouldn't be perfect without a huge bypass cap, and you'd be wanting to alter that for the attentuation.

I'd just use 2 resistor to form a divider from a B+ node. While you could have a cap across the resistor to ground, you need to think about the voltage rating and the maximum voltage it could ever see. Or, you could realize that if you pull from a fairly clean node, any hum present will also be divided by the same factor at the d.c. voltage, which will make the hum insignificant. You could even run this divider from the filter cap feeding the first preamp tube and place your 100 ohm resistors down on that end of the chassis, connected to the preamp tube's socket. There no reason those artificial center-tap resistors have to be close to the power transformer.

Title: Re: Elevated heaters in a 2 power tubes with split cathodes resistors
Post by: chocopower on April 20, 2010, 02:33:09 pm

If i use the artificial center tap and voltage divider in the preamp tube, as you says, i suposse i should avoid the original center tap, isn´t?

I calculate this:

Voltage in D node: 285v
R1: 22K  -  3.05w
R2: 2K2  -  0.3w
Voltage out: 25.9v

I use the 22k resistor as initial value for the calculations, just because i have a 10w one right here.
Wich cap value (uf) do you recomend for R2 bypass?

Title: Re: Elevated heaters in a 2 power tubes with split cathodes resistors
Post by: HotBluePlates on April 20, 2010, 03:00:06 pm

I didn't know you had a center-tap. You can use that and not the artificial center-tap, but the cleaner supply node may be far away. Of course, you can always extend that wire with another wire and heat-shrink.

Judging from your numbers, 25-30v is the desired voltage, and you have 285v to work from. Let's say the numbers are 290v, and 29v desired.

You'd like a ratio of 1:10, and you don't want to draw too much current from the supply, because it's wasted. So how about 10k and 90k. 290v/100k = 2.9mA, 2.9mA * 10k = 29v, 29v * 2.9mA = 0.08w, so 1/2w is plenty. 2.9mA * 90k = 261v, 261v * 2.9mA = 0.76w, so use a 2-3w part. 90k is an odd value, so you'll likely want 100k, 10k instead and slightly lower voltage.

Or you could use 220k, 22k for about 26v out with 0.03w in the 22k resistor and 0.32w in the 220k resistor. Use 22k 1/2w and 220k 2w (for reliability)

Title: Re: Elevated heaters in a 2 power tubes with split cathodes resistors
Post by: chocopower on April 22, 2010, 08:54:42 am

Thanks.

I can mount the voltage divider in the socket who connect the power amp with the preamp and send there the heaters center tap.

I´m gonna use the 100k/10k divider.

Thanks a lot!