100V+ drop from Power on to Warmup...Problem?

[Cygnus X1]

Cathode biased 2x6L6GB build, diode rectified.
Meter connected to B+1.
I'm seeing 480VDC at power up, then drop to 365VDC after the tubes warm up.
Which works, but I am concerned about that large of a drop.

It is connected to a 100W light bulb limiter and seeing a steady faint (probably 10W) glow after the amp is running.

Maybe I just never noticed it before, as this amp does not have a standby installed (yet).

Ran it for several hours and the PT does not run too hot, maybe 130F.

If this is a problem, then where should I start looking?

[Andy Le Blanc]

there's always a change in voltage as the tubes warm and conduct.

what is PT rated for, whats the current draw your asking of it,
and what kind of rectification, bridge or full wave, are you using?

[tubeswell]

I prefer less of a drop myself. If the drop is that big, maybe your tubes are drawing too much off the heater winding.

[ampgeek]

Is the light bulb filament choking it down?
Double check the voltage on the power input/PT primary. If it's much less than ~115VAC then that is likely the case.
A light glow of the bulb is normal in my experience with 30 to 50'ish watt builds.
Good luck,
Dave O.

[pdf64]

If the drop is that big, maybe your tubes are drawing too much off the heater winding.

Surely the heater winding would have the highest current draw at power up, when they're cold? Once they've warmed up, it should drop down to normal.
Check the current draw of the output tubes, ie bias.
The lamp limiter is just for the first time power up; once it's established that there are no major faults (that illuminate the lamp strongly) then it's job is done and the amp should then be connected directly to line. Pete.

[Cygnus X1]

Is the light bulb filament choking it down?
A light glow of the bulb is normal in my experience with 30 to 50'ish watt builds.
Good luck,
Dave O.

We have a winner!

Thanks all.
Connected normally there is actually very little drop at all.
File this one under "learning lessons".
Just never noticed it before.

[ampgeek]

My pleasure mate.
It's all about accumulating and using those learning experiences!
Cheers,
Dave O.