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Loaded question about grid leak, class C power tubes...




 
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Author Topic: Loaded question about grid leak, class C power tubes...  (Read 501 times)
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K8DI
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« on: November 11, 2021, 03:24:25 PM »

OK, this came up while fiddling around with the broadcast transmitter I am converting.  It has an oscillator, amplifier, buffer, and finals.  The buffer is a 6146 running class C. The finals are 4-400A's whose bias is grid leak -- no DC voltage source, the cathodes are grounded.  The screens are fed from the plate power with a big resistor. The plate supply is a separate supply from all the lower level stuff. Its transformer is disconnected -- no plate or screen voltage exists.  I've been working on some issues with the low level stuff, so the low voltage (about 800 volts of low...) is on, the filaments are on, and RF is being generated and amplified by the low level stages. The finals are in their sockets. Their grids are being driven with RF, but again, there's no plate voltage.  But there is plenty of grid current. RF cannot show on the DC grid current meter, and is bypassed to ground and around the meter anyway.

Is this grid current the normal grid current one sees?

Is it being rectified with the grid acting as plate?

Should I worry about grid dissipation while doing this testing/work on the buffer amp?

The current reading is about 20% over the specified grid current when the finals are actually operating.

Ed
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Ed, K8DI, warming the air with RF, and working on lighting the shack with thoriated tungsten and mercury vapor...
KD6VXI
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« Reply #1 on: November 11, 2021, 05:04:57 PM »

That power has to be dissipated somehow.

The 3cx3000 makes a wonderful dummy load for 100 watt slop bucket radios.

--Shane
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W4AMV
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« Reply #2 on: November 11, 2021, 05:19:09 PM »

Correct on the reading of grid current without plate or screen supply (0V).
It is the rectified RF voltage and the resulting grid current is higher
than shown when screen and plate supply are on.

The data sheet says Ig ~ 4 mA max and the drive 0.3 W.

I would look at the rms value of the RF excitation voltage and
the product of the grid current drive you are currently measuring and check
its value is no more than the required Pg drive, 0.3 W

You are lucky having more than adequate drive. I always come
out on the short end.

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W4AMV
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« Reply #3 on: November 11, 2021, 06:03:13 PM »

And there is another way to skin this cat...

A calculation on that dynamic grid R gives 27 k.
At 4 mA grid were looking at 440 mW. That
hangs together with the typical grid drive power... As you increase
this grid Ig, so will you increase the dissipation required by the grid.

This might be a better way to gauge the grid current permitted
under the conditions you are now testing.

oops I may have read your post wrong. Sorry!The 4-400A are the issue.
However, the same rules would apply.

For the 4-400A common cathode, get 13 mA for max I grid.
Does that hang with what you are seeing?
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K8DI
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« Reply #4 on: November 11, 2021, 08:25:17 PM »

The 4-400A grid is what Iím seeing the current on that I wasnít certain about. Iím seeing 23mA on two tubes, on a 20k grid resistor. Math says thatís right at half their 10 watt grid limit. So itís safeÖand my question is resolved. Tomorrow Iíll see if the thing will make actual power, now that Iíve finally sorted out the multiple oscillator amp and buffer circuit failuresÖ

Ed
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Ed, K8DI, warming the air with RF, and working on lighting the shack with thoriated tungsten and mercury vapor...
K8DI
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« Reply #5 on: November 15, 2021, 08:12:32 AM »

For those following along and future searchers, the 120% drive current without plate voltage worked out just fine with plate voltage. Transmitter made well over its 1,000 watt rating even with a saggy 240v supply, grid current was more than enough, and the RF section of this beast is restored to normal operation.

Ed
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Ed, K8DI, warming the air with RF, and working on lighting the shack with thoriated tungsten and mercury vapor...
W7TFO
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« Reply #6 on: November 19, 2021, 11:03:01 PM »

You are the final judge on output power, but keep this in mind: They never work or sound as good under power cutback.

73DG
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