5E3 Build questions - short?

[Tremolux55]

Now at this point I am going snow blind and considering pulling the circuitboard out and starting again.

I have just completed a Marsh 5E3 kit with two modifications

1. A standby switch which is connected to pin 8 of the 5Y3 and then connects through to the first filter cap and 4.7K resistor.

2. Moved the 0.022 coupling cap from the board and connected directly between pins 1 & 7 on the 12AX7 phase inverter.

Now when I power up with the standby off the current draw through my limiter (light) is reasonable. When I turn the standby on then the limiter lights up which I assume means there is a short. There is no sound, but can get a faint pop when I touch the anodes of the power tubes and pin 6 of the phase inverter.

The voltages are right down on the pre-amp and phase inverter

Pre-amp

A1 = -0.4
G1 = -0.15
A2 = 0.4
G2 = -0.19

Phase Inverter

A1 = -0.25
G1 = -0.193
A2 = -0.4
G2 = -0.4

Power tube voltages measure as

V3

A = 264
G1 = 254
G2 = -0.003

V4

A = 265
G1 = 255
G2 = +0.004


I have been over the circuit a couple of times with the layout diagram and highlighter and picked up a couple of mistakes but I am fairly sure that I have got the layout right.

Any tips what to look for? If I can't resolve it by troubleshooting I 'll pull the board and start again.

I should add at this point strangely enough I am not upset or frustrated and am enjoying the challenge of trying to sort out what is going on. As they say you learn more from your mistakes which is what makes it interesting.

Any help from the brains trust much appreciated.

[MWJB]

Your voltages are way down accross the board, 6V6s too.

Looks like a short to ground somewhwere in the B+ rail. Triple check for blobs of solder that might be making contact with the chassis/any ground wires, starting from rectifier working forward. Remove the power supply resistors & preamp plate resistors & replace one by one, starting with the 4.7K/5.1K, see if you get normal voltage at the 6V6 plates, then at the screens, then at the junctions of the plate resistors - ascertain at what point things go haywire, pin down the short.

Post some pics.

[tubeswell]

Is one of the bolts on your eyelet/turret/tag board touching a part of the signal path maybe?. (Try checking the signal path to ground with your meter at various points looking for a short?)

[Tremolux55]

Hi Tubeswell and MWJB,

I did end up pulling the board last night and the problem was the connecting wire from the filter caps to the first pair of plate resistors. Visually it was in place but the solid core wire had snapped at the eyelet. Easy fix once I worked out what it was.

Fired up beautifully except that I get a hum when I turn the standby switch off after playing. I think this may be due to the fact that the OT is connected directly to pin 8 of the rectifier so I'll move this to the first filter cap so that when it is put in standby there is no current going to the OT.

If you've got any thoughts re this they would be appreciated.

[MWJB]

So what are your plate & screen voltages for the 6V6s now? A broken B+ wire wouldn't cause the voltage drop you had.

[Randall]

I made the same mistake with the OT CT the first time, and had the same results. It's an easy thing to do working from the original schematic. I ran the OT CT to the backside of the standby switch, but it's the same point.