I replaced the 2n3055 the plate is now 50 to 60 m with no drive. the
2n3055 corrected the 190m with no drive.
-----Original Message----- From: Ken Brown
Sent: Thursday, August 09, 2018 12:47 AM
To: Discussion of Ten-Tec Equipment
Subject: Re: [TenTec] centurion
There are about two possible reasons a good tube will draw excessive
current when it has no drive. 1) The bias is wrong. 2) There is an
oscillation. Even "Zero Bias" Triodes have circuitry that establishes the
bias (however low it may be). Usually there are two bias modes, one when
the amplifier is in standby, and one when it is keyed. The standby mode
bias should be high enough to keep the current so low that there is no
gain
possible and therefore no oscillation. The operating mode bias level is
such that the tube can amplify and so there could be an oscillation when
there is no drive.
If the excessive current only occurs when the amplifier is "keyed" ready
for drive, but not driven it could be that the bias is wrong, or if the
bias is correct there is an oscillation. If it happens even when it is
not
keyed, it almost has to be a failure to establish cutoff bias.
Bias is often established by a zener diode. Most grounded grid zero bias
triode circuits have the grids at ground potential both DC and RF. The
cathode is held a bit positive by the zener diode, making the grid
negative
relative to the cathode, and cutting off the plate current to a very low
level during standby. When keyed the zener is shorted, or some other
method
is used to bring the cathode DC potential closer to zero, allowing the
proper idling current for class AB linear amplification.
If the problem is incorrect bias, you need to look around the circuitry
that establishes the bias.
If the problem is an oscillation you need to look a the output network,
plate voltage DC blocking capacitor pi or pi-L network and see if
there is
a fault there which makes the tube not have a proper RF load impedance.
Or it could be something else.
DE N6KB
On Wed, Aug 8, 2018 at 5:43 PM, Mcelmer1 <mcelmer1@netecin.net> wrote:
Rodney,
With no drive and cherry red, Houston, we have a problem. Short
somewhere,
been there done that with the Centurion but mine was drawing about 50
- 75
ma.
Perhaps I a missing part of conversation. Is this 190ma with no drive??
Other friends of mine have been thru this with the 3-500s: no drive but
tube(s) drawing current.
Mike
W8TRN
Mike
W8TRN
----- Original Message ----- From: "Rodney" <w3krq@dejazzd.com>
To: "Discussion of Ten-Tec Equipment" <tentec@contesting.com>
Sent: Wednesday, August 08, 2018 19:28
Subject: Re: [TenTec] centurion
no it was 190 m with no drive and tubes got red
-----Original Message----- From: Bob McGraw K4TAX
Sent: Wednesday, August 08, 2018 8:53 PM
To: tentec@contesting.com
Subject: Re: [TenTec] centurion
Is this idle current with no signal? If so it is about correct.
73
Bob, K4TAX
On 8/8/2018 7:42 PM, Rodney wrote:
I was on my Centurion talking and looked at my plate current it was
resting at 190m but the amp was putting out 1200w so tubes I think
are ok.
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