Determing Battery Voltage for cathode bias

[jeffdavison]

How does one determine the battery voltage rating needed when wanting to use a battery for cathode biasing in a tube? Particularly a 6SN7 ?

I have seen recomendations of 1.5v, 3v and 9v. all without any expainations of how that voltage number was arrived at.

I'd like to get a handle on how it is determined as a trusted source said 9V nimh for the 6SN7 with about 6 to 7 ma. current
Thanks

jd

[DougL]

How does one determine the battery voltage rating needed when wanting to use a battery for cathode biasing in a tube?

The ultimate way to accomplish finding the correct cathode voltage is to determine it by the load line of that tube and power supply voltage you are using.

I use Tube CAD to determine bias points.
http://glass-ware.stores.yahoo.net/adsoffromgla.html

The recommendation your friend gave you is valid for only one load, and one voltage. Without knowing the additional variables, I couldn't even determine if the advice is valid.

HTH

Doug

[jeffdavison]

Not really asking if advice had been good or bad....

Question was posed on How it is determined.

Software refference is not waht I need either, just the factors on how the number is factored.

What needs to be determined to arrive at whatever conclusion. example... you mention load line, but how is the load line chart used? What is needed to use the published curves?

No looking for a fish to eat today, but rather how to use the fishing pole so I can feed myself.



JD

[SY]

Start with what you want to do. Gain, required swing, load to be driven. We'll take it step by step from there.

[dsavitsk]

On Line Tube Learning for newbies....

http://www.tpub.com/content/neets/14178/

[Richard Ellis]

Just to get you started....there is more lines to be draw on this chart and many more calculations .....the limitations are not exact but this is a start.
_________________________________________Rick..... ..

[jeffdavison]

Richard,

Thanks... that chart gives me just about the info I'm looking for. Goes a long way to complete my understandg.

Dsavitsk....INFORMATION OVERLOAD!!!!!! I really don't need to know the different species of all the wood that I can design a fishing pole from.the different types of rods and reels, of hooks and lines and bait....it has a section or two on cathode biasing, but for the life of me I find nowhere refference to batteries being used in place of resistors . I mean finding R to limit current flow through a tube via the bias resistor and plate R for voltage to the anode is one thing, but can't determine from that encyclopedia to determing the voltage of a battery.... . but thanks anyways......


JD

[SY]

Once you determine operating point (and to do that, you need to break down and actually define requirements), then the battery voltage can be read right off the load line.

See, before someone can recommend the right way to cast or the right way to select a rod, it must be established what sort of fish are being sought.

[kevinkr]

Something else needs to be said about battery derived cathode bias! DO NOT USE ALKALINE BATTERIES!!- Unless of course you really want to see how quickly it can explode or best case vent corrosive goo all over the inside of your project.

Battery bias is not all that brilliant with the 6SN7 which is very well suited to conventional cathode bias. The only reasonable intersections from a linearity standpoint are going to be around 9V with something like 220V on the plates - may not be convenient, and not very tunable to put it mildly.

Incidentally with 10mA of cathode current flowing for good battery life you should operate the battery in trickle charge mode at about 1/10 to 1/20th of what is implied by the mAH rating of the battery.

8.2V nicad batteries actually don't sound that great, and NiMh types are even worse IMO. (Many cells in series, and they aren't as linear as you would believe, not to mention some odd noise mechanisms as well.)

In this application because of the bias voltage required you could use a 9V alkaline as a grid bias battery with a small cap and grid bias resistor.. This actually works quite well and sounds excellent if a good capacitor is used. Resist the temptation to use a battery in series with the grid circuit here as it just doesn't sound as good. (The signal has to go through 6 cells in series.) This works somewhat better with types like the 5842 where a single cell may be all that is required.

I've been doing battery bias of various sorts for about 10yrs with varying results. YMMV