Gary,
By the letter of the law, you don't need to do anything as long as you do
not exceed 3000 Plate Volts. The resting plate current is only 750 Watts
dissipation until you place RF on the input.
The amp would run considerably cooler if you installed a single 5.1 V 50
Watt Zener Diode in the cathode B- line after all the metering, just before
the B- heads into the filament transformer.
If this was my amplifier I would be shooting for 170-180 ma of resting
current. If the 5.1 V cuts it back too far, try a 3.0 volt Zener or
possibly a string of big reverse biased diodes until you get the voltage you
are looking for. This is a pretty simple modification.
Rule of thumb as far as recommended plate current? Just crank them back
until it no longer smells hot, HAHAHAHa. I don't like to dissipate over 1/2
of the maximum plate dissipation in the tubes myself.
Full power operation will result in the tubes showing a BRIGHT ORANGE COLOR.
I always liked the 3-500 better myself and usually changed out 3-400's
whenever they would fit.
BOB DD
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
Behalf Of Gary Schafer
Sent: Friday, March 16, 2007 4:14 PM
To: 'John T. M. Lyles'; amps@contesting.com
Subject: Re: [Amps] 3-400Z zsac tspa
Hi John,
Thanks for the info. I finally found the data sheet online. It shows 100 ma
each at 3kv with zero bias and 73 ma at 2500 volts. I had gotten a few
replies with various amounts of current. Bob said that they would draw
around 250 ma at 3 kv and no bias. The amp I have shows right around 250 ma
at 3 kv so I guess it is in the ball park.
The amp is an old TMC unit that has auto presets for the tuning. Had it
laying around for some years and just now dragging it out and getting it
going.
The next question is how much should I run the ZSAC at with 3 kv on the
plates. At 250 ma that is dissipating 750 watts which seems awfully high.
At 75 ma each at 2500 volts is about 375 watts dissipation.
Is there a general rule of thumb for idle dissipation compared to plate
dissipation capability?
This amp was originally shipped with 8163's in it. I see in some of the
handbooks they show the 8163 as having an upper frequency limit of 30 MHz
and slightly higher plate current than the 3-400Z. The 3-400Z has an upper
frequency limit of 110 MHz.
On the eimac spec sheet that I found the sheet lists it as 8163/3-400Z which
would indicate that they are the same tube??
I can't clearly read the printing on the tubes in the amp so I am not sure
exactly what they are.
73
Gary K4FMX
> -----Original Message-----
> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
> Behalf Of John T. M. Lyles
> Sent: Friday, March 16, 2007 3:02 PM
> To: amps@contesting.com
> Subject: [Amps] 3-400Z zsac tspa
>
> Hi Gary
> You can look it up on the tube constant current curves in the
> datasheet. That should be available online somewhere.
>
> Depending on your Grid or Cathode Bias voltage, pick off the closed
> anode current estimate, above 3 kV anode voltage. Not sure if you are
> talking zero bias or with a slight bias of 10-20 volts there?
>
> john
> K5PRO
>
> >
> >Date: Thu, 15 Mar 2007 21:26:09 -0400
> >From: "Gary Schafer" <garyschafer@comcast.net>
> >Subject: [Amps] 3-400Z ZSAC
> >To: "'amps'" <amps@contesting.com>
> >Message-ID: <20070316012550.E593F319B34@dayton.contesting.com>
> >Content-Type: text/plain; charset="US-ASCII"
> >
> >Can anyone tell me what the ZSAC should be on a pair of 3-400Z tubes at
> 3000
> >volts?
> >
> >
> >
> >Thanks
> >
> >Gary K4FMX
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