If it flashes over, you should hear it.
W6DR
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Sent: Monday, August 19, 2024 9:00 AM
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Subject: Amps Digest, Vol 260, Issue 4
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Today's Topics:
1. Re: I like to think there are no stupid questions (Mark - N5OT)
2. Re: I like to think there are no stupid questions (Tom Hellem)
3. Re: 3CX800A7 Grid Current (jim.thom jim.thom@telus.net)
----------------------------------------------------------------------
Message: 1
Date: Mon, 19 Aug 2024 05:47:50 -0500
From: Mark - N5OT <r-emails@n5ot.com>
To: Amps <amps@contesting.com>
Subject: Re: [Amps] I like to think there are no stupid questions
Message-ID: <7cada5b5-935b-474e-95bd-469bb67cacad@n5ot.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Thanks everyone for all the comments.
I figured the idea of whether the two tubes were the same or different
would enter into the conversation. I am guessing that the meter on an
amp with 2 tubes in this case will show the total grid current
regardless of whether one tube is drawing more than the other. The seat
of my pants says that if the two tubes started out new and matched (an
assumption, I know) and have the same amount of "experience" in this
single application, they they are likely still relatively balanced. I
realize I could be wrong.
Anyway, I have been using it to transmit 1500 watts over the weekend and
with it tuned like JC N4IS describes (I already knew all that but I'm
glad to have my opinion backed by a competent authority) the grid meter
LEDs are never above 70ma.
When an amp trips out, how does one know if it is the tube that flashed
over, or something else? I use a number of matching networks and there
is the occasional arc which shoots the SWR high and the amp trips out.
If it was a tube, how would I know?
73 - Mark N5OT
On 8/16/2024 12:25 PM, Mark - N5OT wrote:
> I am pretty sure of the answer to this question but I'd hate to find out
> I am wrong the hard way.
>
> I am running a pair of pretty flat 3CX800s - Alpha 86.
>
> Spec on the tube says maximum grid current is 60ma.
>
> So if my Alpha grid current meter says 70ma, that means it's actually
> 35ma through each tube, so I'm good?? Am I actually in the okay zone if
> I see 120ma on the grid current meter?
>
> Thanks in advance.
>
> 73 - Mark N5OT
>
> _______________________________________________
> Amps mailing list
> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
------------------------------
Message: 2
Date: Mon, 19 Aug 2024 07:17:31 -0600
From: Tom Hellem <tom.hellem@gmail.com>
To: Mark - N5OT <r-emails@n5ot.com>
Cc: Amps <amps@contesting.com>
Subject: Re: [Amps] I like to think there are no stupid questions
Message-ID: <21C9DE87-1B27-440F-A406-FE51515B1061@gmail.com>
Content-Type: text/plain; charset=utf-8
Mark-
My LC800 was arcing and tripping out.
Turned out to be the plate tuning cap.
Once they start arcing it?s tough to fix ?em. Tried filing and cleaning up
the carbon tracks, but eventually ended up replacing it with a vac variable.
That amp uses 3 3cx800?s.
I figured it was abused by the previous owner.
I realize this doesn?t exactly answer your question, but might save you some
grief in case you have the same thing going on.
Tom
K0SN
> On Aug 19, 2024, at 4:52?AM, Mark - N5OT <r-emails@n5ot.com> wrote:
>
> ?Thanks everyone for all the comments.
>
> I figured the idea of whether the two tubes were the same or different
would enter into the conversation. I am guessing that the meter on an amp
with 2 tubes in this case will show the total grid current regardless of
whether one tube is drawing more than the other. The seat of my pants says
that if the two tubes started out new and matched (an assumption, I know)
and have the same amount of "experience" in this single application, they
they are likely still relatively balanced. I realize I could be wrong.
>
> Anyway, I have been using it to transmit 1500 watts over the weekend and
with it tuned like JC N4IS describes (I already knew all that but I'm glad
to have my opinion backed by a competent authority) the grid meter LEDs are
never above 70ma.
>
> When an amp trips out, how does one know if it is the tube that flashed
over, or something else? I use a number of matching networks and there is
the occasional arc which shoots the SWR high and the amp trips out. If it
was a tube, how would I know?
>
> 73 - Mark N5OT
>
>
>> On 8/16/2024 12:25 PM, Mark - N5OT wrote:
>> I am pretty sure of the answer to this question but I'd hate to find out
I am wrong the hard way.
>> I am running a pair of pretty flat 3CX800s - Alpha 86.
>> Spec on the tube says maximum grid current is 60ma.
>> So if my Alpha grid current meter says 70ma, that means it's actually
35ma through each tube, so I'm good? Am I actually in the okay zone if I
see 120ma on the grid current meter?
>> Thanks in advance.
>> 73 - Mark N5OT
>> _______________________________________________
>> Amps mailing list
>> Amps@contesting.com
>> http://lists.contesting.com/mailman/listinfo/amps
>
> _______________________________________________
> Amps mailing list
> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
------------------------------
Message: 3
Date: Mon, 19 Aug 2024 06:46:46 -0700
From: "jim.thom jim.thom@telus.net" <jim.thom@telus.net>
To: amps@contesting.com
Subject: Re: [Amps] 3CX800A7 Grid Current
Message-ID:
<CAB4ZMguhmp0kvynwDOtQLmww6QVQVdsrUyM4xAtPeKxXrxekJQ@mail.gmail.com>
Content-Type: text/plain; charset="UTF-8"
Date: Sat, 17 Aug 2024 20:51:43 +0000
From: "n4is@comcast.net" <n4is@comcast.net>
To: "amps@contesting.com" <amps@contesting.com>, Dennis W0JX
<w0jx@yahoo.com>
Subject: Re: [Amps] 3CX800A7 Grid Current
<I switched to a pair of 3CPX800 20 years ago and they are full power
today. The <3CX800 <is sensitive to plate load. The way to tune is to
increase the load ( open the <capacitor) <and drop the grid current, peak
again the plate capacitor to max output and <repeat the <process, increase
load to reduce grid current and peak plate capacitor, you <should find a
<load good for max power with lower grid current, max 50 mA per tube, <my
tubes run at 20 <mA grid current each for full legal power.
<My two cents,
<73'
<JC
<N4iS
Smart move. The 3CPX-800A7 is the pulse rated version, and will handle a
lot more B+ vs the plane version. (3500 vdc vs 2250 vdc).
Years ago, Eimac made cavity amplifiers for FM broadcast, using a single
3CX-800A7 (750 w PO)....and also a 2 x tube version (1.5 kw PO).
Later on, Eimac issued a bulletin for end users of the above 2 x amps to
switch to the pulse rated version. Reason was.... the tubes were being
destroyed by anode to grid arcs....due to transients / spikes / surges /
hiccups etc...coming in off the street via the 230/240 vac 50/60 hz
incoming power.
Both versions of that tube only have floozy 4 watt grids in em. Any anode
to grid arc will take out the tiny grid in an instant, even with a hd 50
ohm glitch resistor and fast HV fuse in the B+.
Jim VE7RF
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