Flashing standby light when turning on (Vox AC30/6TB)

[AleS]

So my Vox AC30/6TB has started behaving a bit weirdly upon turning on. It has a GZ34 rectified, which has meant I've never really used the standby switch upon turning the amp on as it's a slow starter anyway.

The way it would normally be, the ON lamp would turn on as soon as the amp was on, then the Standby lamp would slowly light up, after which I'd get sound from the amp, as expected as per the schematic attached.

In the past couple of days, the Standby lamp has started flashing a few times before settling down and there's a low but audible hum to go with each flash, which isn't unlike what it would normally do when warming up. It all settles and goes quite after a few seconds. There are bypass/protections diodes in place so I don't fear anything bad happening in the interim... or should I?

I've taken a quick video: https://drive.google.com/file/d/1th64t4 ... sp=sharing

That's what happens about 10 seconds after switching on. The flashing is the problem, the constant flicker (that can also be seen on the ON lamp, which is always on) is down the camera shutter angle, so nothing wrong with that.

I've looked for similar cases, but many amps seem to power the lamp via the heaters, meaning the symptoms and causes are bound to be different to my amp which powers the lamp via B+ and hopefully isn't an indication of any other bad tube aside from the GZ34. It's pretty new (about 3 or 4 months old?) but I have heard of bad ones which will display signs of bad behaviour pretty soon on and, while a bummer, that would be an ideal scenario as it's a simple swap - in my head that's very much the prime suspect and the way the lamp is wired I find it hard to look past that.

Any word of wisdom would be welcome!

Thanks in advance!

[Stevem]

It sounds to me like it’s flashing at the refresh rate of the DC that’s charging up the first power supply filter cap or maybe the power supply screen filter, as I kinda can’t see any of the other later / downstream power supply nodes drawing enough current to make that flash effect.

So it may just be a filter breaking down.

[pdf64]

How about if a different, known good GZ34 is fitted?

[R.G.]

Does the flashing still happen with
> the output tubes pulled out?
> only the PI tube(s) pulled out?
> only the tubes before the PI pulled out?
> all tubes other than the rectifier pulled out?

It's a quick test, and should tell you whether it's a tube other than the rectifier tube that has an issue.

Did you happen to read my article on faking a GZ34 with diodes, zeners, and resistors? It's at
https://www.dropbox.com/scl/fi/weeftd0q ... ucztp&dl=0
and shows a circuit that is remarkably like the 5AR4/GZ34, right down to the harmonics of the rectified ripple. That might get you out of worrying whether a new GZ34 is bad too.

[nuke]

I'll bet on the standby switch, or even the neon lamp itself, but switch is my first instinct.

[Suckseeder]

Not unusual,this series of AC30s always had flickering StdBy lights