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rsumperl 31st December 2009 09:54 AM

Plate chokes
Greetings everyone and a happy new year!
What are the advantages to using a plate choke vs a resistor in the plate circuit? Besides there being more voltage at the plate, are there any other advantages? How does one choose the size (# of henries) for the choke?


SY 31st December 2009 10:19 AM

For a given B+, a plate choke will nearly double the possible voltage swing. In some situations, that's important.

Additionally, the choke provides a higher load impedance at high frequencies than a resistor, but not as much as a good constant current source (CCS).

Inductance is a bit trickier to calculate, but as a rule of thumb, choose an impedance at 5-10 times the tube's effective plate resistance. Using the inductance formula (|Z| = 2pi*f*L), calculate the required inductance at the lowest frequency of interest. This will probably give a larger value than you need, but will guarantee that the plate loading doesn't move around much as you go down in frequency.

For me, I would never use a plate choke unless I absolutely needed the swing- they're large, expensive, and subject to all sorts of unpleasant non-ideal behavior. A CCS or gyrator in the plate circuit will outperform any choke in all areas except swing.

tvrgeek 31st December 2009 11:50 AM

Sorry to butt in, newbee to tubes. I thought the CCS was normally a replacement for the cathode resistor. Or does it not matter?

Can you point me to a sketch of using a gryator in the plate? I have been collecting schematics to understand the why's of tube implementation and have not run into this idea.

SY 31st December 2009 12:43 PM

A tubee newbee? Cool, than I get to flog Morgan Jones's book one more time- get yourself a copy of "Valve Amplifiers" post-haste. It's the introduction I wish I'd had when I got started with tubes back in the day when LBJ was in the White House. MJ covers these topics, and most everything else you'd want to know about tube design, in exquisite detail.

In any case... a CCS in the plate circuit acts as (essentially) an infinite plate load, which runs the tube at maximum gain (mu) and minimum distortion (horizontal load line). The choke is an approximation to this at frequencies where its impedance is >> plate resistance.

In a cathode circuit, the extremely high Z drops the gain of the stage to unity, which is fine for a cathode follower, but exactly the opposite of what you'd want for a gain stage.

If you do a forum search for "gyrator" and posts by "Wavebourn," you'll find some very clever circuits.

Zen Mod 31st December 2009 12:46 PM


btw. where is Kwayzee Russian these days ?

( I don't have anything to add to rest of your replies :clown: )

SY 31st December 2009 12:49 PM

Probably working too hard.

tvrgeek 31st December 2009 02:45 PM

"Double plus thanks"

Planning on Jones as soon as I finish fighting my way through Finch. My calc is a bit rusty. What is fun is I find myself going for my Dad's old Terman handbook from when he went to school. They did not teach tubes when I went through.

Of course, if we add solid state power supplies, CCS's, then maybe an integrated ss phase splitter, pretty soon we are ready for FET outputs and no longer have a tube amp!

tvrgeek 31st December 2009 02:54 PM

OK, OK, ordered both of his books. Looking aththe TOC, his building looks good as I have observed execution can be as important as the circuit. VTL comes to mind. Supurb results with ordinary topology.

SY 31st December 2009 03:08 PM

You'll not regret that purchase. MJ is buggy about getting the build right (when viewing my construction efforts, he generally rolls his eyes heavenward). The cover item on VA (the Crystal Palace amplifier) is a model of optimizing very conventional topologies and attention to the details of engineering.

BTW, where in Maryland are you? (UMBC, class of '77)

tvrgeek 31st December 2009 04:36 PM

Bowie. LHS class of 73.

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