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

[Paul Christensen]

Ian,

Great information and thanks for sharing it.

Paul, W9AC

----- Original Message ----- 
From: "Ian White GM3SEK" <gm3sek at ifwtech.co.uk>
To: <amps at contesting.com>
Sent: Tuesday, October 30, 2012 4:20 AM
Subject: Re: [Amps] The GU84B and GU78B tubes / some brief history


> 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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