[HBR] about resistors

[Shoppa, Tim]

For ordinary class A amplifiers in ordinary applications, looking in the tube handbook will get you 90% of the way there.

e.g. for 6SN7 or 12AU7 triode go to http://tdsl.duncanamps.com/show.php?des=6SN7GT

Pick a typical RCA datasheet e.g. http://www.tubezone.net/pdf/6sn7.pdf

For grid see the entry called "Grid Circuit Resistance = max 2.2 Megohms". You can use (probably want to use) a lower resistance if the previous stage can drive it.

For cathode resistance, note the recommended class A 250V B+ parameters of plate current circa 9mA and recommended grid voltage -8V. Neither of those specs have the word "cathode" in it but there's easy math: the 9mA of plate current is exactly the same as 9mA of cathode current. To drop 8 V (the negative of the grid voltage) across the cathode resistor you want 8V/0.009A = 890 ohms of resistance in the cathode current. Really anything vaguely like 1K ohms will do fine.

For a RF or small signal AF stage, the decoupling resistor (maybe this is what you mean "voltage drop resisistor") should be small compared to the plate resistance but large enough to effectively decouple. Large enough depends on the decoupling capacitor too. Stages near the "output" of a chain get lower decoupling resistors (as low as 100 ohms) and stages near the "input" of the chain get larger decoupling resistors (say 10K). The scale is set roughly by the plate current. If the decoupling requirements aren't too hairy, a 1K or a few hundred ohms works great. Note that typical plate current of 10mA through a 1K ohm resistor is a drop of only 10V, quite reasonable in a circuit with a few hundred V B+.

For screen tubes which don't have some babysitting system like voltage regulator tubes for the screens, data sheet will have recommendations that hardly can be improved on.

In a resistance coupled audio amplifier, it's very hard to go wrong just by copying the recommendations of the manufacturer. e.g. RCA charts at http://www.tubezone.net/pdf/rcachart.pdf

Realistically very few of the resistances in a typical class A tube circuit bias system are all that critical. Double the cathode resistance and usually the plate current shifts only drops 40% or less, because the grid bias changes to help counteract. Change the decoupling resistor by a factor of two and the plate voltage just drops a little bit. etc.

Now when it comes to mixers... oscillators... etc.... things get more complicated but not all that much. Most of those start out as class A stages which are manipulated or driven to saturation to get desired nonlinearities.

Tim N3QE


________________________________________
From: hbr-bounces at mailman.qth.net [hbr-bounces at mailman.qth.net] On Behalf Of Chris Howard w0ep [w0ep at w0ep.us]
Sent: Sunday, May 06, 2012 2:29 PM
To: hbr at mailman.qth.net
Subject: [HBR] about resistors

That brings up a good point in my mind:

being more-or-less new to tube circuit building,
how does a person figure out what size resistors
go in what position?

For examples:
1) a signal-series grid resistor?
2) a grid-leak resistor?
3) a cathode resistor?
4) a voltage drop resistor on B+ to a plate or screen?

Do you generally use a rule-of-thumb, do you calculate
it out, or maybe you look for indications on the schematic
or in a similar circuit?






On 5/6/2012 6:54 AM, Bry Carling wrote:
> Hi guys,
>
> A lot of people asked me for some more of these resistors and I
> found some.
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