[AMRadio] Current on 6146s

Jim candela jcandela at prodigy.net
Sun Feb 11 20:41:14 EST 2007

>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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