Zener Diode for dropping B plus voltage?

[smackoj]

hello amigos; I need to drop 30-40 volts from an old PT. I think a zener diode would be a good attempt prior to swapping out the PT for a lower output model. Is a 10 watt zener a safe starting point or does it need to be rated at more watts?

thanks, jack d
 

[sluckey]

Depends on how big the amp is. 40V and 10W implies 250mA. Half that for safety. So, if the B+ current will be 125mA or less, I'd use a 10 watter,,, with a heatsink.

[smackoj]

OK; so a pair of 6L6s could draw up to 144 ma and a 6SL7 an additional 30 ma....would these current numbers from the tube manual be pushed to the max figures which would be beyond normal playing range?   thanks

 

[DummyLoad]

it is doubtful that a 6SL7 would be drawing 30mA - at least not in a guitar amp. 2mA would be typical - maybe 4-5mA worst case.


--pete

[phsyconoodler]

I have used a 25 watt resistor bypassed by a reversed 450volts  capacitor with positive to ground as a voltage reduction placed in series with the pt center tap.
  Works on several amps i have tried. They are still in service after five years

[smackoj]

3-4 ma for a 6SL7. thanks, read the tube chart wrong. 

I'm not for sure if I understand noodler's suggestion. I'll re-read it a few more times and see if the light bulb goes on in the cranium.

jack   

[DummyLoad]

a good read.


http://www.aikenamps.com/index.php/what-is-back-biasing


--pete

[smackoj]

good link to the schemo.  thanks, understand it now.

 

[mresistor]

I used a string of four 1N5353B 16V, 5W ZENER DIODEs on the PT center tap to lower the B+ in an old organ PS I converted to a guitar amp. Lowered it around 48 volts if I remember right. The amp has two 6sl7s a 5R4 rectifier and two 6L6GCs in it and works fine.
 

[jjasilli]

a good read.

http://www.aikenamps.com/index.php/what-is-back-biasing

--pete

Very cool!

[smackoj]

I used a string of four 1N5353B 16V, 5W ZENER DIODEs on the PT center tap to lower the B+ in an old organ PS I converted to a guitar amp. Lowered it around 48 volts if I remember right. The amp has two 6sl7s a 5R4 rectifier and two 6L6GCs in it and works fine.

cool idea, thanks.   

[HotBluePlates]

I believe I have seen the ground artificially elevated using mosfets.

Psychonoodler already posted a back-bias arrangement that does exactly this with 1 resistor and 1 cap.

One problem I see, is the cathode to heater voltage difference.

Psychonoodler's approach causes no issues with heater-to-cathode voltage. Neither does using zener diodes.

[JPK]

a good read.


http://www.aikenamps.com/index.php/what-is-back-biasing


--pete

Thanks for posting this. I got lost in his sea of white pages....again. What incredibly valuable amplifier info that guy writes.

[kagliostro]

Using the zener on the CT you must pay attention to the voltage of the bias if the amp is a fixed bias and the bias comes from the same winding

K