Why Bleeder Resistors

From ha–(at)–ortel.ca Wed Dec 18 12:45:14 CST 1996
From: henry–(at)–nr.ca (Henry Pasternack)
Newsgroups: rec.audio.tubes
Subject: Re: Tube amp design notes.
Date: 18 Dec 1996 16:25:18 GMT
Reply-To: ha–(at)–ortel.ca

Andy Moss (enz–(at)–bm.net) wrote:

: Henry, you wrote a few messages back about two 10K bleeder resistors in
: your tube amplifier. I had mentioned that I found that rather suspect
: and you, if I recall, stated it was a generally accepted way of doing
: things.

: I still don’t believe. I would like you to provide some kind of *proof*
: to this end.

: I think that you may have misunderstood a different design as I can see
: no positive benefit to such excessive “bleeding”. It reminds me of
: “leeching” as being acceptable in medicine years ago.

: In short, lay the science on me – not the conjecture…..

The output voltage of a choke-input filter is approximately 0.9 times
the RMS secondary voltage under load. Unloaded, there is no current flow
through the choke, which effectively “disappears”. The filter now looks
like a capacitor-input, and the output rises to 1.4 times VRMS. To keep
the supply under regulation, it is necessary to draw a minimum amount of
current from it. Working through the math (which is tedious because you
have to do it in the time domain), you end up with a standard equation
that states, Imin = VDC / L, where L is given in Henrys. For instance,
a 400VDC supply with a 10H choke-input filter requires 40mA of bleed
current. In practice, the choke’s inductance will rise at low currents,
allowing a lower bleed current. There are such things (although they
are not common these days) as “swinging chokes” — chokes designed
deliberately to have much higher inductance at low currents than at
full load — intended specifically for choke-input filter applications.

I don’t have a specific reference handy, but you certainly can find
a discussion of all of this in the RDH, the Radio Amateur’s Handbook,
the RCA Receiving Tube Manual and so on. It’s for real, trust me.

-Henry

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