Zener Diode Question??

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trescat
Junior tube assistant
Posts: 10
(2/14/04 3:42 am)
  Zener Diode Question??
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 I know I have asked this question before, but as I still haven't got around to doing the mod, I have forgotten what the answer was, so here goes again.
I am wanting to install a zener diode to reduce B+ voltage by placing it betweeen center-tap and ground. Can you please let me know which way that I should connect this type diode from the centre tap, should it be with anode to ground or cathode to ground??. Also what is the best wattage capability on the zener in this situation.??? Thankyou
 
Regards
Trevor
 
Tiny Daddy
I will work on all amps
Posts: 485
(2/14/04 9:26 am)
  Re: Zener Diode Question??
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 A few questions: What is the B+ voltage now? May we assume you are measuring it with no load attached?
How many volts do you want to drop across the Zener?
What is the expected B+ current, or what output tubes will you be using?
 
Rotting27
Junior tube assistant
Posts: 110
(2/14/04 12:32 pm)
  Re:
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 Here's the easy answer: Determine the exact number of volts you want to drop. Get a zener rated at that voltage, plug it in between centertap to ground with the cathode to ground. A one watt device is all you need.

Remember, if you are using the plate supply to derive your bias supply, then the zener will make this voltage MORE negative.
 
tomtweed
Junior tube assistant
Posts: 12
(2/14/04 1:02 pm)
  Re: Re:
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 volts to be dropped x transformer current=min. watts required.
Example:
25v x 150ma=3.75 watts. A 5 watt device will work, but will still get pretty hot. You could use a 10 & a 15 in series for the drop and have a 10 watt rating, and run pretty cool. They only cost less than a buck per. If I am wrong on this, someone Please correct me, I have used this to drop 75 volts, and it seems to work pretty well.
No $
400 volts=poor people/poorways

TT
 
Tiny Daddy
I will work on all amps
Posts: 488
(2/14/04 6:53 pm)
  Re: Zener Diode Question??
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 Seems to me you could place the Zener(s) in the cathode connection to the output tubes, thereby achieving bias and voltage drop at the same time.
 
trescat
Junior tube assistant
Posts: 11
(2/14/04 7:49 pm)
  Re: Zener Diode Question??
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 Tiny Daddy
The B+ voltage at present is 365 VAC, and I think it is just a little too high for the Princeton 5F2 that it supplies, basically the same design as the Tweed Champ 5E3 with a tone control added, I were hoping to bring the voltages down closer to more traditional levels associated with the original tweed amps of the 50's, I am looking to drop at least 30 VAC, with the zener installed. Thanks for any guidance.

Regards
Trevor

 
Tiny Daddy
I will work on all amps
Posts: 490
(2/15/04 1:43 am)
  Use a resistor
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 Looking at a 5F2 schematic, it shows 19V across the 470 ohms 6V6 cathode resistor which is 40mA, plus 2mA for the 12AX7, all class A, for a constant current. Therefore to drop 30V at 42mA I suggest installing a 680 or 700 ohms resistor in series with the power supply choke. The power in the resistor would be 1.3 Watts so use a 3 watt or 5 watt resistor. The extra resistance will help with power supply filtering. No Zener needed.
 
 
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