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Re: [Amps] 3CX3000A7 on 144 MHz

To: <>
Subject: Re: [Amps] 3CX3000A7 on 144 MHz
From: "Jim Thomson" <>
Date: Fri, 7 Feb 2020 11:17:05 -0800
List-post: <>
## That sorta works. 2 x 5 ohm resistors results in .5 amp draw from the fil xfr. 2.5 watts total, 1.25 watts per resistor. 800 ma plate current + 300ma of grid current divides..with 550 ma flowing into each half. That’s another 1.51 watts per resistor. It's also 2.75 volts of additional bias... per resistor, 5.5 volts in total of yo-yoing bias..... in addition to normal 5-7 volts of regulated bias.

## he shoulda just removed equal turns from each end. New hammond 5v @ 30A ..... or 7.5 V @ 21A fil xfmrs were dirt cheap back then.

## Plan D .......esp if the fil xfr does not have a CT, is to wire a 5V- 12Volt @ 3-5A CT xfmr across the oem xfr.... then use the CT of small xfmr as the artificial CT. And also tape up the Un-used 120 v leads on the small xfr.

##  2 x 2 ohm  resistors  would  work  real good.   (5-10  watt  rated)

## I used to know why the 60hz hum appeared with an un-balanced CT, but have forgotten the tech explanation. Does anybody remember ???

Jim,   VE7RF

-----Original Message----- From:
Sent: Friday, February 7, 2020 9:43 AM
To: 'Jim Thomson'
Subject: RE: [Amps] 3CX3000A7 on 144 MHz

He should have used an artificial center tap.
Two low ohm resistors in series across the heater winding, between them
connect the ground, works very well.


-----Original Message-----
From: Amps [] On Behalf Of Jim Thomson
Sent: Freitag, 7. Februar 2020 18:37
Subject: [Amps] 3CX3000A7 on 144 MHz

Date: Fri, 7 Feb 2020 11:35:34 -0500
From: MU 4CX250B <>
To: Steve Thompson <>
Subject: Re: [Amps] 3CX3000A7 on 144 MHz

<     Seems to me this problem may  not be so troublesome. The dc
<resistance of the hot filament is 7.5V/51A=0.15 ohms. If the B- return
<is on one side of the filament, and if we assume the plate current is
<1ADC, then the maximum DC voltage drop across the filament would be
<only .15V
 <  Furthermore,  the filament emission is distributed across the
<entire length of the filament and not just at one end, so the average
<resistance is actually less than .15 ohm. What this means is that the
<variation in operating bias from one end of the filament to the other
<by using a switch mode power supply is at most a tiny fraction of a
<volt. For practical purposes, I think it could be neglected, which
<means a user would see no difference in performance, compared to
<using a center-tapped filament transformer.
<Jim w8zr

##  electrician buddy  across  town  re-wound a 24 volt  xfmr....and  used
new 10  ga  wire.
5.6  volts....  loaded  with  30A.....for a hb  2 x 3-500Z  amp.....complete
with  CT.

##  worked  good,  but loaded  V  at  the  cathodes  was  way  too  high.

##  he  removed  turns  from  ONE  end  only,  instead of  equal  amount  of
turns  from  both  ends !
That  effectively put  the  CT  off  to  one  side a bit.   We  could all
hear  the  60hz  hum  across  town  on  75m  ssb.
Go  figure.

Jim  VE7RF
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