[AMRadio] Current on 6146s


Mark W1EOF w1eof at hamnutz.com
Sun Feb 11 22:48:32 EST 2007


Isn't 15ma *WAY* too much grid current for a pair of 6146s? In my Viking 
II I used to run about 6-8 if I recall correctly. They can be easily 
destroyed pretty quickly by too much grid current.

73,

Mark W1EOF


Jim candela wrote:
>
> >From John E. Coleman (ARS WA5BXO):
>
>
> I have found it to be very important to make measurements while in
> operation.  Except that RF will mess up the meter readings.  This is why I
> said to ground the grid directly and then kick the rig to XMIT and make the
> measurements then.  Checking the plate voltage on the plate while the tube
> is cut off is not conclusive as there may be a high resistance between the
> plate point and the power supply point (all the way back in the power supply
> circuit) and same could be true of the screen.  This would cause the plate
> voltage to appear normal until you draw current then it may drop real low at
> the plate point but still be good at the power supply.  RF at the plate will
> cause the meter to read wrong and grounding the grid will kill the bias and
> RF.  If the rig were operating normal then grounding the grid would cause
> excessive plate current so this is a good check any way.
>
> 1. While in XMIT mode, when you ground the grid, the plate current should go
> above 200 ma. If it does, then troubleshoot the grid circuit.
>
> 2. If it does not, then leave the ground on the grid and stay in XMIT mode.
> Then measure the plate and screen voltages.  One of them is surely wrong.
> The only other possibility would be the filaments are not lit up good.
>
>
> Rule for the day:
> You can't have a 100 volt drop across a good 5ft #12 wire with out a fire.
> John
>
>
>
> Reply by Jim, WD5JKJO:
>
>  John, all good points for sure. I take a different approach that seems to
> work here. I have two HV probes, a 40KV probe at 1000X, and a 6 KV probe at
> 1000X . The resistors in these probes combined with the coax cable
> capacitance make a very effective low pass filter. This filters out the RF
> leaving just the DC. I touch the probe directly to the RF hot plate of a
> tube to read the plate voltage with my trusty DVM. I recently tried this on
> my 8877 amplifier, and it worked well even though the amplifier was running
> full boar into a dummy load (900w carrier or 2100w PEP SSB).
>
> These probes are not that expensive. Those of us that need to measure above
> 600 volts with a DVM should use a HV probe. It is safer for both the HAM,
> and the DVM. Here is one on Ebay:
>
>  Item number: 290080656981
>
>
> Regards,
> Jim
> JKO
>
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