Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: smackoj on January 06, 2015, 08:12:29 am
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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
:icon_biggrin:
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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.
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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
:icon_biggrin:
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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
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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
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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 :icon_biggrin:
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a good read.
http://www.aikenamps.com/index.php/what-is-back-biasing (http://www.aikenamps.com/index.php/what-is-back-biasing)
--pete
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good link to the schemo. thanks, understand it now.
:icon_biggrin:
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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.
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a good read.
http://www.aikenamps.com/index.php/what-is-back-biasing (http://www.aikenamps.com/index.php/what-is-back-biasing)
--pete
Very cool!
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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. :icon_biggrin:
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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.
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a good read.
http://www.aikenamps.com/index.php/what-is-back-biasing (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.
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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