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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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In Reply to: Yes, I understand that it's important. But posted by rontokyo on December 9, 2006 at 05:44:43:
No, do not make that assumption.If you have the ability to set the bias on each tube separately then you can compensate for tubes that pass more CATHODE (not plate - I'll explain why in a minute) current. However, if you can't make individual adjustments it's quite possible you could have one tube working really hard and the other tube loafing. At a given bias setting the currents can differ significantly. So for non-adjustable bias, or less than one bias adjustment per tube then current matching is the most important. Even if the Gm matches great - if one tube is nearly cut off and the other is red plating you have a problem!
In an application with individual bias control for each tube my rules change. As long as the currents aren't HUGELY different then Gm is my primary match criteria.
I would recommend that if possible you measure cathode current rather than plate current. In tetrodes, pentodes, and beam power tubes the screen current also flows through the cathode; and screen current varies from tube to tube, no question. A common way of measuring current through a tube is either to measure a voltage across a small value resistor (1 ohm or 10 ohms is common) in the tube cathode (and use Ohm's Law to convert it from volts to amps) - the Citation amps and MANY others are set up that way. In a few amps an ammeter gets connected in series between the cathode and ground to directly measure current.
So when you adjust bias on those amps you set it by measuring the CATHODE current. It's important to realize that the cathode current is the sum of the plate current and the screen current combined. Differences in screen current will impact the cathode current and will therefore need to be taken into account. Measuring just the plate current ignores the impact of the screen current and therefore could cause biasing errors.
If my goal in matching tubes is to provide conditions that mimic the real world operating comnditions as closely as possible, then it is incumbent upon me to measure CATHODE, not PLATE current. Why? Because when I ship you the tubes you will install them in your amp - and it is highly likely you will be measuring cathode current to set bias.
A "plate current matched quad" of tubes is not likely to be cathode current matched - due to variations from tube to tube in screen current!
So anyway, which is more important - plate current or Gm? The answer is... it depends! But the safest match criteria is cathode current first, Gm second.
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Follow Ups
- Re: Yes, I understand that it's important. But - Jim McShane 06:39:21 12/09/06 (3)
- Doesn't it depend on how the tubes are going to be used? - Ray Moth 13:42:40 12/09/06 (2)
- Re: Doesn't it depend on how the tubes are going to be used? - Jim McShane 15:51:11 12/09/06 (0)
- Clarification - Ray Moth 13:51:36 12/09/06 (0)
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