The AM Forum

what’s the safest way to couple a scope to
the plate of a PA tube to check for hf and vhf
parasitics during operation ? assume the
scope can see up to 400 Mc or thereabouts.

Thanks!

https://www.ebay.com/itm/High-Voltage-Probes-low-noise-for-scope-DMM-50Mhz-15KV-X1000-Tektronix-OEM-HVP15/162948647945?hash=item25f07f0c09:g:HlEAAOSwPoZaq00w

You can use a HV scope probe. New HV probes are usually expensive.  Be sure the rating is at least X2 the working voltage. Look on eBay for deals.

Another way is to make a HV divider using  (20) 100K resistors in series.   The top of the ladder goes to the HV... the bottom to a 1K resistor to ground. The 1:1 scope probe gets connected across the 1K to ground.
2000:1 ratio... which makes 3KV = 1.5V at the scope.   Or, 0.15V using a 10:1 scope probe.

Or, you can make whatever ratio you want depending on parts or probes available.

* From what I understand, a well-compensated commercial scope probe is superior to seeing high freq parasitics compared to a resistive ladder.

T

https://www.w8ji.com/testing_for_stability.htm

Tom W8JI, gives two methods for testing for parasitics; one without additional equipment, one with a spectrum analyzer.

Look at the "RF Sniffer" in the March 1951 CQ.

It will not display parasitics, but it may help you detect them.

Good luck!

thanks for the feedback...I can hear the parasitics (the 807 squeals when they're present), what I want to do is see them on a scope so I can measure their frequency(ies) and tailor the remedy.

I'll play with a voltage divider.

Now that I think about it, a simple pickup loop, 5 turns across the scope probe, might give you a decent signal when placed next to the tube plate or tank area.   When I suggested the  resistive divider or HV scope probe, I thought you were looking for a direct connection.


As long as the RF pickup loop doesn't receive a lot of spurious garbage, you will certainly see the parasitics when they appear. They may appear as fine fuzz drifting around on the edges of the main signal.


T

When you hear the 807 squealing, I really doubt that it are parasitic s. It may be just oscillations due to insufficient neutralizing of the driver or the 807. If it are, the frequency of the squealing may change tuning the circuits.

I agree with PA0NVD. It is self ocillating but not parasitics.

you’re possibly both right...the reason I called it parasitics is because
the grid current doesn’t vary as I turn the plate tuning through its
range (with plate/screen bolts off the 807) and, more significantly,
the squealing gets reduced when I slap a suppressor on the plate. This
is a mopa where the plate of the xtal oscillator is called on to double
or triple the xtal freq. Slight detuning of the oscillator plate tank eliminates
the squealing.