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Re: [Amps] The GU84B and GU78B tubes / some brief history

To: amps@contesting.com
Subject: Re: [Amps] The GU84B and GU78B tubes / some brief history
From: Ian White GM3SEK <gm3sek@ifwtech.co.uk>
Reply-to: Ian White GM3SEK <gm3sek@ifwtech.co.uk>
Date: Tue, 30 Oct 2012 08:20:16 +0000
List-post: <amps@contesting.com">mailto:amps@contesting.com>
Paul Christensen wrote:


If you could list the top few important parameters to low distortion tetrode sign, what are they? I think most of us know the importance of well-regulated screen voltage. Other top factors?


First of all, do everything that you would for a triode: give it a reasonably stable heater voltage, and an anode voltage that remains reasonably stable at maximum current demand.

Both tetrodes and triodes require a stable G1 bias voltage (relative to the cathode) but this is done in different ways, each with its own distinctive set of advantages and pitfalls. GG triodes create negative grid bias by taking the cathode positive with respect to ground, using the familiar zener diode or "electronic zener" to create a constant voltage drop. This is very simple because no external bias supply is required, but the challenge is to maintain a constant voltage drop when the cathode current is varying strongly.

Tetrodes do it differently, but they always require a separate negative bias supply for G1, so that adds some complication. Voltage stabilization is much easier because very little current is involved, but it's important that the G1 bias voltage remains constant if the tube is driven into small amounts of either positive or even negative grid current. If the grid voltage varies when current flows, that will produce a sharp kink in the transfer characteristic, possibly leading to severe high-order IMD.

Another feature of the G1 supply is that most tetrodes have a very small grid dissipation, so they require some kind of protection against excessive grid current. In the normal setup for class AB1, my boards will generate an ALC or warning signal at about 100uA of grid current, and will trip the whole amplifier at about 3mA (which is well below the threshold of physical damage to the grid).

As for the screen supply, the obvious point is that tetrodes do require another separate power supply, with good voltage regulation for both positive and negative screen currents. Protection and fault recovery adds yet another layer of complication. Screen supplies are covered at much greater length in my QEX article: <http://www.ifwtech.co.uk/g3sek/boards/tetrode/tetrode-3.htm>

And then you have to think how to switch between RX and TX. If full QSK is not required, I strongly favor switching the screen to ground (in the typical DC-grounded cathode setup). This has the advantages of definitively cutting off the anode current in the RX condition, leaving the screen grid safely grounded and the screen supply safely disconnected. Protection in the RX condition is very important because, when you think about it, this is how an amateur amplifier actually spends most of its operating lifetime!

The problem is how to switch these relatively high screen voltages RELIABLY at full QSK rates. MOSFET switches have been tried, but are destroyed by the first HV arc that comes along... so that means mechanical relays, which have their own different set of problems including a limited mechanical lifetime. The best scheme I've been able to come up with is to use electronic grid-block keying for QSK while *also* switching the screen relay with a short hang-time to avoid it switching on individual dots and dashes. This gives the advantages of both switching methods but requires some careful sequencing.

To summarize, tetrodes require much more support circuitry than triodes do... at least, they do if you're intending to get everything right.


Well, that was the short(er) version, and before the second cup of morning coffee I'm sure there are other things I left out...


--

73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek
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