Strange behaviour of a VTTC (13MHz?)

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "=?iso-8859-1?Q?=22Antti_Koskip=E4=E4=22?= by way of Terry Fritz <twftesla-at-uswest-dot-net>" <antti.koskipaa-at-nic.fi>

Hi all, I've been off the list for almost a year, but I'm
back to the tesla business. Last weekend me and my friend
tried a first light on our new VTTC. This unit is built
around the QBL5/3500; a whopping tetrode with 3kW max plate
dissipation and 8kW peak output power.

The screen grid was not connected during the tests, as we wanted
to get the feedback right etc. We also had a scope nearby, connected
to a small pick-up coil, to detect oscillations.

13 MHz???

No matter what we did, the thing outputted a steady 13 MHz
signal. Like it was crystal-controlled. We tried everything.
Changed the tank caps, the tank coil, the feedback circuit,
removing the secondary, changing topload, changing pickup
coil! That 13MHz signal was present (although weakly) even when
the coil was switched off!

This can't be an error signal, as the signal grew enormously
as we turned up the anode voltage. The (Tektronics) scope
isn't broken, since it had just come from service, with no faults.
It was connected to a grounded outlet.

We eliminated all potential noise sources: turned off all lights,
mobile phones, power supplies... (except the scope)

Maybe tube coils are not for us after all?

-- 
- Antti