|[AMRadio] Current on 6146s|
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.
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.
> 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
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