Elevated Heater & B+ Voltage

[John Caswell]

Hi all,
All valves have a specified maximum voltage heater to cathode voltage both +ve and -ve so try not to exceed this.
In an SRPP or cascode circuit where one valve sits upon another, some juggling of the "stand off" voltage may be required to make sure both sections of the valve do not exceed this voltage.
Simplest way is to stick a 500K pot across HT and ground with the wiper to the isolated heater supply and adjust for best results and voltages.
John

[Audiophile]

Thanks Andy!!! You are right I should indeed do more reading but I dont know If my brain can handle any more information! Here is a link to some measurements of the designers circuit. http://www.lampizator.eu/lampizator/...surements.html I will upload a pic of what I am using too, I think Ive gone on a bit of a wild goose chase with the power supply because now I remember the designer saying the circuit is immune to power supply quality, maybe I should of led with that. So it probably doesnt need anything more than what I originally posted, I was thinking maybe the extra Rs would drop some more voltage but I understand now that the end result is the same on that front with the addition of it been a better supply. No worries though as I will now only use one 470uf and 2x 100uf caps on my future builds, that will save some pennies and gave me space for the elevated supply components.

[Audiophile]

Originally Posted by John Caswell

Hi all,
All valves have a specified maximum voltage heater to cathode voltage both +ve and -ve so try not to exceed this.
In an SRPP or cascode circuit where one valve sits upon another, some juggling of the "stand off" voltage may be required to make sure both sections of the valve do not exceed this voltage.
Simplest way is to stick a 500K pot across HT and ground with the wiper to the isolated heater supply and adjust for best results and voltages.
John

Hi John! good to see your still helping out round here, you may remember repairing a couple of valve amps for me about 10 years ago, are you still doing that?
So what you are saying is the elevated heater circuit is good but not perfect and using a pot allows me to get the right? I obviously have a few questions...

Given the diagrams ive uploaded what is the correct voltage for the elevated heater? Or do I need to finish my build and give some real world voltage figures for the anode before this can be determined?
Or maybe im barking up the proverbial?
How does the pot fit into this scenario? taken as your more than likely aware from the valve wizzard, Im not disagreeing im just trying to understand.

"although the lower arm (R2) should not be excessively large or Rhk(max) may be grossly exceeded, so it is advisable not to make it greater than 100k"

I get that the wiper is a variable voltage divider so how relevant is it to the statement above?

Alan (Apollo Satellite)

[Audiophile]

So something like this? A lot more than a pair of resistors! https://uk.rs-online.com/web/p/potentiometers/0410249/

[Audiophile]

Originally Posted by bikerhifinut

That circuit should work, I'd be inclined to increase the potential dodvider resistors to something like 100k and 47k to reduce the current draw through which is really wasted.
Andy.

I got my resistors from RS today, I had another look at the valve wizzard site and he recommends 40-60v for the elevation, I think Im going to be over 80v, should I be looking at different resistor values to get it down to the recommended 50v? Johns Idea of a pot doing the job sounds fantastic but I just dont know how much power thats going to use or if it will exceed Rhk(max) as I posted from the valve wizzard site.

R2) should not be excessively large or Rhk(max) may be grossly exceeded, so it is advisable not to make it greater than 100k.

Would a 25K R2 draw or waste too much current?

[Audiophile]

For anyone who finds this via a search engine.....

So I found a free preview of Morgan Jones' valve Amplifiers book... I have attached some simple math from it bellow...... If you want to read 155 pages for free here it is.
https://play.google.com/books/reader..._GB&pg=GBS.PP1

This was more than enough for me to decide it would be a waste of nearly £40. So if like me you want to understand a bit more about what it means when they talk about elevated heaters then I suggest you read this. https://groupdiy.com/index.php?topic=50513.0
NOW I UNDERSTAND MORE. Of course this doesnt answer all my questions but hopefully it will explain the workings of Vhk(MAX). It looks like this guy is still helping folks out over there so may even be worth joining the forum.



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