Massive voltage drop


2008-09-23 9:52 pm
Hi all

When this tube set powers on and on standby, it looks ok. All the voltage from the HV and also smoothing cap is ok.

But when it gets switched to transmit, the sc voltage drops to drastically to less than ten volts, and also the HV from the transformer drops to that level, so nothing happens in the set.

Could this be due to over current drawing somewhere in the set? I also measured the current in the smoothing cap, but there is no change in the current.

The set was powered on using the variac, so the input voltage was considerably lower (about 50- 60%) than usual mains, and also a lamp current limiter was inline.
This was to avoid any possible damage to the set caused by over voltage or over current under the full mains voltage.

I was also wondering if these current limiting devices are acting to reduce the current. But why does the voltage drop to less than ten volts at sc and hv? Any ideas?
Any shorts in the circuit? Or bad component? The set is a vintage tube transmitter.

Last edited:


2011-04-29 8:37 pm
Probably the bulb limiter has enough current to have a near normal voltage drop, which leaves
only a small voltage after it. That is, the circuit draws more current than the bulb limiter needs.
The bulb current limiter is strictly for initial testing to avoid blow-ups, not for normal operation.

Make sure that your Variac output is fused properly, or you can destroy the Variac from
excessive current draw. It doesn't matter where the voltage dial is set, the maximum wiper
current must still be limited by an output fuse sized at the Variac's rated maximum current, or less.
Last edited: