Hi all
Long term lurker, first time poster. Typically, and probably annoyingly, my first post is a request for help....
I've built half a dozen valve amps before, all based on the AC15 with a few different tweaks, and all made on tagboard. This time around, just as a different approach, I'm making them on Merlin Valve Wizard's PCB set. These:
The Valve Wizard
I'm aiming for a Vox Cambridge-esque design, which is essentially a Princeton preamp and AC15 PI and power stage.
The problem I've got relates to the input voltage.....kind of. With no tubes in, everything checks out in terms of voltages and continuities. When I plug the EL34 pair in, the volts measured at the input of the PT (via variac and light bulb limiter) falls from 230V to about 185V, and then starts to drop away at around 1V/sec. Heater volts drop from 6.3V to around 4.5V.
If I take the EL34s out, no problem. If I disconnect the heater supply at the transformer, problem goes away. o I thought I might have a short somehwere in the heater connections, but that doesn't appear to be the case. I thought that maybe it's a duff valve, but both EL34s work fine in another amp.
So, without going into loads of specifics, I was wondering whether someone might be able to say "yeah.....that's symptomatic of yadda yadda yadda". Any ideas?
Thanks for help.
Long term lurker, first time poster. Typically, and probably annoyingly, my first post is a request for help....
I've built half a dozen valve amps before, all based on the AC15 with a few different tweaks, and all made on tagboard. This time around, just as a different approach, I'm making them on Merlin Valve Wizard's PCB set. These:
The Valve Wizard
I'm aiming for a Vox Cambridge-esque design, which is essentially a Princeton preamp and AC15 PI and power stage.
The problem I've got relates to the input voltage.....kind of. With no tubes in, everything checks out in terms of voltages and continuities. When I plug the EL34 pair in, the volts measured at the input of the PT (via variac and light bulb limiter) falls from 230V to about 185V, and then starts to drop away at around 1V/sec. Heater volts drop from 6.3V to around 4.5V.
If I take the EL34s out, no problem. If I disconnect the heater supply at the transformer, problem goes away. o I thought I might have a short somehwere in the heater connections, but that doesn't appear to be the case. I thought that maybe it's a duff valve, but both EL34s work fine in another amp.
So, without going into loads of specifics, I was wondering whether someone might be able to say "yeah.....that's symptomatic of yadda yadda yadda". Any ideas?
Thanks for help.
Check there is the correct negative voltage on the grids of the output valves or the cathodes of the output valves are sitting at or around 30volts when warmed up.
How big a bulb??
A pair of EL34 probably needs a 100 Watt light bulb. Maybe 150W for near-normal result.
If you use a 25W, and the amp wants to suck 90 Watts, of course the lamp will light and the amp won't.
And it needs saying today: INCANDESCENT!
None of these curly-tail or LED novelties!
"Halogen" IS incandescent and should be fine.
While 60W-100W incandescents are now out of stock many places, 150W and 200W bulbs are generally legal and perfectly kosher for big light short time. Garage, boat-house, etc. Not at the grocery store, but a hardware or building supply.
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