light bulb glowing when Standby is swichted to ON

[Bieworm]

Hope you guys can shed some light on the problem with this build...

I built a TH30 with clean channel only. I have made some minor changes to the schematic:
- Omitted the overdrive channel
- added a stand by switch between the B+ and 1st filter cap.
- 500mA fuse on the - side of the FWB rectifier.
- used normal switching cliff jacks instead of the stereo jacks on the schematic
- matchless style Post PI master volume

The amp powers on fine. Nice 3.05V on the heaters (with the lightbulb between mains and IEC of the amp)
But when I switch the standby switch to on I can hear the power transformer crackling. With power tubes in it starts to glow when switched standby to on. When I disconnect the B+ wire to the first filter cap  I can measure 260VDC at the rectifier.

I've been looking for shorts after the rectifier, but I can't find any...

One thing I noticed is that I put the plates of the LTP PI on the B node instead of the C node. Could that cause this issue? I would think it only raises the voltage on the plates of the PI, but with no tubes in this shouldn't cause this, or am I overlooking something? In this case the PI shares the same node where G2's are connected...

I used a 36W power transformer and OT. The PT has HT 300V-0-300V, too much.. I know, but I'm going to knock it down with Zener diodes from the SBY switch to the 1st filter cap.

I sealed off the CT form the heater winding and the HT winding. Added a 100R hum balancing pot on the heaters and a FWB rectifier on the HT.

The question really is: is it a problem when the PI plates share the same node as the G2's ? I mean, a problem like I'm facing at the moment?
I will change that ofcourse, but next thing to do is probably lift the B+ dropping resistors 1 by 1 to track the source of the problem...

Thanks for helping 

[sluckey]

Your PT has 300-0-300 and you connected it to a FWB. That's feeding 600VAC to the FWB. When you flip the STBY switch there will be 848VDC at the first filter cap! That's your problem.

You can use that PT but you need to use a conventional rectifier with the HT CT connected to ground. 300-0-300 will produce 424VDC unloaded. That's most likely gonna be too high for 4xEL84s. If the PT has a small current capacity those EL84s "may" present a high enough load to drop the B+ down to a safe zone. I would not bet on it though.

[Bieworm]

Your PT has 300-0-300 and you connected it to a FWB. That's feeding 600VAC to the FWB. When you flip the STBY switch there will be 848VDC at the first filter cap! That's your problem.

You can use that PT but you need to use a conventional rectifier with the HT CT connected to ground. 300-0-300 will produce 424VDC unloaded. That's most likely gonna be too high for 4xEL84s. If the PT has a small current capacity those EL84s "may" present a high enough load to drop the B+ down to a safe zone. I would not bet on it though.

Thanks Steve!! I should have known!
Anyway.. I have used this PT before on a 4x EL84 36W Marshall build (with a conventional rectifier, DOH!! stupidly followed the TH30 schematic on that part). It's 600VCT @ 200mA, with 5A of heater current.
I intend to put some 5W zener diodes in a string between the rectifier and the 1st filter cap. That should do the trick somehow? It's always worked that way for me...

[Bieworm]

Ok Sluckey... you were straight out right about the rectifier! Again, thank you!

The amp is operational now and I biased the EL84's at 85% ( Va 328V / Vk 12.48V / Rk 180R ) after putting 34V of zeners between  B+ and 1st filter cap.
The way it's built right now there is very low volume on tap (maxed out) when the FX loop is in the circuit. (Volume's normal when I bypass it with alligator clips and a wire)
I saw I had put a 47k resistor at R56 instead of 4k7 by mistake, so I will correct that. But it puzzles me the way V4-B is wired. B+ (C-node) is fed directly to the cathode... anyone know this kind of circuit? I can't find anything on this topic. It's not a CF...
When I change R56 to 4k7.. isn't there going to be even less signal going to the next grid? Like in: even less volume?
I personally think this is a mistake on the schematic. I think the plate and cathode of V4-B are switched. There are more mistakes on that schematic. Look at the numbers on the pots... everything would operate backwards if wired like that. Or is this a UK thing, like driving on the wrong side of the road? 

thanks

[sluckey]

I personally think this is a mistake on the schematic. I think the plate and cathode of V4-B are switched. There are more mistakes on that schematic.

I agree.

[Bieworm]

That cathode/plate switch on the schematic is clearly an error. All is perfectly fine now..