[JMS] Is anyone running a 90881 amp?

Fri, 5 Mar 2004 13:52:10 -0700

Cal,

The amount of bias you use depends upon the "class" of operation that =
you choose.  If you decide, for example, to drive the final with a 90800 =
or 90801 and to go on CW, you would get the best efficiency and most =
power out by going for the "Class C, telegraphy" ratings.  If you decide =
to plate modulate the final, go for the "Class C, telephony" ratings.  =
If you decide to use it as a linear amplifier for a low-level AM or SSB =
exciter, then bias it as a Class B amplifier. =20

You can derive a portion of the bias from a power supply, like a 45 volt =
battery for instance, and the rest of the bias from the voltage drop =
created by the grid current flow across a resistor in series with the =
grid-current-to-ground path.  If you are running in Class C, you would =
probably be best off to make the battery (or power supply) bias at least =
enough to cut off the plate current.  That minimizes dissipation in the =
tubes and also keeps them from generating broad-band noise.  I had a =
pair of push-pull 809's on the air once upon a time.  They are rated at =
-75 volts bias and 25 ma grid current per tube, 50 ma total.  We (my =
brother and I) ran the grid current through a pair of VR75's in parallel =
with an electrolytic filter capacitor connected across them.  The first =
time we hit the final with grid drive, which we did before we turned on =
the final plate voltage, the grid current would "light up" the VR tubes =
and there would be a 75 volt drop across them, thus setting the =
appropriate grid bias.  The electrolytic capacitor charged up to 75 =
volts very quickly, and when the grid drive was removed and the tubes =
extinguished, the capacitor held the necessary bias voltage on the tubes =
to keep them cut off.  The capacitor would eventually discharge, but =
that would take hours.  So we were basically set for the evening or for =
however long we wanted to keep the rig on.  You could pull a similar =
trick with your 812's by using a pair of VR105's or VR150's.  You might =
want to put a low value resistor, say 100 ohms, in series with each tube =
separately so that one of them would not "hog" all of the current.  That =
way you could use a capacitor to keep 105 or 150 volts of bias on your =
812's without having to bother with a bias battery or power supply.

Jim, W8KGI
  ----- Original Message -----=20
  From: [email protected]<mailto:[email protected]>=20
  To: =
[email protected]<mailto:james_millen_society@mailman.=
qth.net>=20
  Cc: =
[email protected]<mailto:james_millen_society@mailman.=
qth.net>=20
  Sent: Friday, March 05, 2004 12:46 PM
  Subject: Re: [JMS] Is anyone running a 90881 amp?

  Hi Jim:
  Thanks but that is not the information I am looking for. It looks like =
the amp is in p-p configuration so I may have to go with the Class B -45 =
VDC rating. It would be nice to hear from someone who has "been there =
done that" with a live 90881 but I think for now, they are "shelf =
queens". I hope to change that! Cal. N6KYR.=20

  Cal,

  The ARRL Handbook ratings for 812's in Class C are -175 volts bias for =
1500=20
  volts on the plate (telegraphy) and -125 volts bias for 1250 volts on =
the plate=20
  (telephony).  They are also rated at -45 volts bias with 1500 volts on =
the plate=20
  for Class B service.

  Jim, W8KGI

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