Preamp Voltage Drop/Zener Diode?

[studioit]

Hi all, is this possible? I swear I've read about it somewhere but have searched and can find no mention of it anywhere. I want to reduce the plate voltage of the first gain stage only of a tweed deluxe style amp. Want to drop it about 20 volts at both 12ay plates - but I dont want to change the 12ax7 voltages as I want the headroom. Other options? A resistor?
Thanks,
Ian

[R.G.]

Since preamp tubes run at low currents, you can just put a 20V zener in series with the plate supply.

A 1/2W 20V zener can pass 0.5/20 = 25ma before it is at full dissipation. Any current under that it just lets pass through while dropping its 20V. You could do one in each plate supply.

[studioit]

Great, thats what I thought, but then I thought I had imagined it as I could only find references to putting it between the center tap and ground which I have done but that drops the whole supply. I just ordered several 5 w zeners in different voltages so I can experiment. What's strange, and part of the reason for this experiment is that I used different chassis mounted zeners on the center tap to figure out where I wanted the basic pwr supply voltages to be - because I started this amp with the P.T. from an old Superscope reel to reel which was higher than needed. I then bought a new P.T. with the voltges I thought I liked the sound of the amp with. When installed the pwr tube voltages were 5 volts lower than my last test with a diode but the 12ay7 plate voltages went up a bit (about 8-10 volts) with the new P.T. installed. Not sure why but everything else seems fine.??

When I install the diode in series with the preamp supply the Cathode goes to the negative side as measured with my meter?

Thanks
Ian

[R.G.]

A 5W 20V zener can pass 5W/20V = 250ma. That will certainly work for preamps.

When you install a zener, remember that it is installed cathode/band to the POSITIVE side. A zener works in reverse-breakdown mode. If you put the cathode to the negative side, the diode will only drop 0.6V or so, like a normal silicon diode.

But this brings up an interesting question. It's good to dink with this and get the thing set up just like you want, but are you sure that changes in plate voltage of 5-20V make an audible difference? On many amps I've seen that would not be a huge difference in tone.

[Don Moose]

RG makes an excellent point - if you're up for it, how about mounting a rotary switch strung with a set of 6 10V zeners in series, selecting any drop from 0 to 60V?

Just mount it solidly and insulate the crap out of it so you don't get bit when you change the setting.

[studioit]

Thanks all- Don, I think I'll just throw 1 30v in first and see if it's good,bad, or Ugly.

Ian

[Don Moose]

I am kind of a feature creep

[R.G.]

Just remember - if your new product is so loaded down with fripperies, intriguing but complex solutions to rare conditions, flashy colors based on demographics and colorists' opinions, and moulded plastic techoid buttons that you can hardly claw your way through the crap to get to the basic function, it really, really needs a few more bells and whistles added before the ship date.

[Earl]

Just like to add that depending on the type zener, there may not be enough current through it to 'make it' reference. Check the data sheet to see what the zener current requirements are.

In some cases, you may need to add a resistor to ground on the anode side to keep the diode in it's regulation region.