ORIGINAL MESSAGE:
On Tue, 5 May 2009 15:36:41 +0200, "Peter Voelpel" <df3kv@t-online.de>
wrote:
>Remember, with a tetrode amp you need to tune with your tune cap for maximum
>screen current and tune your load cap for specified screen current.
REPLY:
Peter is correct, but he should say "....specified screen current at
full output". At less than full output, the screen current could be
anything, either positive or negative.
The procedure I use when tuning a tetrode up for the first time is
this:
1. Apply a small amount of drive.
2. Adjust tune and load caps for maximum output.
3. At low drive, screen current will likely be negative. Don't worry
about it at this point. Hopefully your screen current meter is a
zero-center type.
4. Gradually increase drive while retuning both caps for maximum
output.
5. As you approach full output, the screen current will move toward
positive values.
6. As you approach full power, adjust the tune cap for maximum
positive screen current. This means if the screen current is negative
at some point, adjust it for the least negative, i.e. most positive.
Continue adjusting the load cap for maximum output.
7. When you reach 1500 watts output, note the screen current.
Typically for most tetrodes at that power level it will be somewhere
around 25 to 50 mA. My GU-84b data sheet does not specify a value, so
this is a guess. Whatever it is, remember it and use that as a target
when tuning up in the future. Remember, adjust the tune cap for peak
screen current and the load cap to set the value of the peak. Both
settings should coincide with maximum output for a given drive level.
Once you know the screen current values for your amp, you can tune it
up correctly every time without even watching the power output,
believe it or not. It will be very repeatable.
Hope this makes sense. Email back if it doesn't.
73, Bill W6WRT
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