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#1 Zenner diodes vs Tube regs/ noise/ capacitor values

Posted: Wed Sep 07, 2022 1:00 pm
by Paul Barker
So Im attempting to build the McDonald ACF2.

My questing isnt about the signal section its about my power rails, for which Im using a SV572-160 per rail to shunt reg. Normally I use VR tubes, (to control the grid, as per Stephies method), but I dont have enough stock of the right VR’s and cant really order expensive parts at the minute. So for testing at least I’ll control the Svetlana grids with zenners. My question is about noise reduction of zenners. Is it a 0.1 cap? Or any ideas?

#2 Re: Zenner diodes vs Tube regs/ noise/ capacitor values

Posted: Wed Sep 07, 2022 1:25 pm
by Nick
The low impedance of a zener means that a simple cap will do little. Its more normal to use a RC circuit to add impedance to allow the cap to function and to provide a voltage reference. But if you are using it to control an A2 grid you need the low impedance, so in that situation I would use a zener with a RC filter to supply a source follower mosfet to drive the grid. But that does assume the noise level is going to be enough to worry about it. The PSRR of the ACF2 will be high anyway.

You also need to watch the dissipation of the zener it can get larger than you expect with high voltage ones. And unlike valve parts they will let you know if they are run outside their SOA by quickly failing.

#3 Re: Zenner diodes vs Tube regs/ noise/ capacitor values

Posted: Wed Sep 07, 2022 9:05 pm
by Paul Barker
Ok, I’ll check wattages aswell. Largest zenner is 100v 4 of those and adjust on test with 9v ones should get the shunt valve in right range.

#4 Re: Zenner diodes vs Tube regs/ noise/ capacitor values

Posted: Thu Sep 08, 2022 3:55 am
by Paul Barker
Ok so grid current shouldnt go over 20mA ive ordered already 3.25 watt for the 100v zeners so 2/3.5s of rating , give them long legs for cooling ought not to go pop. The 9.1v ones would coap with 111 mA so they’re not an issue.

#5 Re: Zenner diodes vs Tube regs/ noise/ capacitor values

Posted: Thu Sep 08, 2022 9:10 am
by Nick
I would be tempted to us 50v ones to give you more headroom, but YMMV

#6 Re: Zenner diodes vs Tube regs/ noise/ capacitor values

Posted: Fri Sep 09, 2022 7:35 am
by Paul Barker
Yes, Im a bit twitchy about the tolerence. Though if I were using vr’s it wouldnt matter at all if it went to 40mA grid current. Thats my problem, Im used to doing it with VR’s. Apart from not having the right vr’s the reason Im doing this simpler, is the other variables are unknown until everything is ready to work and gets brought together. The whole thing begins with two 55vct transformers on 400v primaries, which Im using backwards together on the 110 side in series. Theyre capable of 600mA, theyre backwards, and Im adjusting voltage on test with the first cap in semi choke input. But many factors influence. My particular over mains voltage, transformers backwards are underwound now on the new primary, whereas its usual to overwind the primary to make up for losses. Finally the voltage has to sit. right whether Im using the acf2 without driving grid current and also whatever grid current is demanded to 100mA (an unlikely occurance).

So its put it together and pop goes a zenner or its put it together and get something like what I calculated then adjust on test.

If I begin by puting a fan on the zenners until Ive tuned in right first cap it’ll give me time to get adjustments right. Maybe also bring it up on variac first.

I never do things easy. But it mostly works off the bat.

#7 Re: Zenner diodes vs Tube regs/ noise/ capacitor values

Posted: Fri Sep 09, 2022 8:54 am
by Nick
I would ask a different question. What voltage range does the ACF2 need? Where is the negative rail coming from.

#8 Re: Zenner diodes vs Tube regs/ noise/ capacitor values

Posted: Fri Sep 09, 2022 9:29 pm
by Paul Barker
One of the outputs is negative supply.

B+ 400 to 450 depending on results.
B- 400 per McDonald

#9 Re: Zenner diodes vs Tube regs/ noise/ capacitor values

Posted: Sat Sep 10, 2022 9:57 am
by Paul Barker
Image

Since the two 400v transformers aren’t 800v centre tapped, Ive taken the easy path for now. As Nick said psrr is good according to McDonald.

#10 Re: Zenner diodes vs Tube regs/ noise/ capacitor values

Posted: Sat Sep 10, 2022 8:47 pm
by jack
There's a lot of good info in the Statistical Regulator thread. I still have some boards etc