WEM Custom 15

[Bushbaby]

The transformer wiring is:
- 0V blue, 230V pink, 240V brown & 250V grey
- 2 x red 550V CT red/yellow 125mA
- 2 x green 6.3V CT white 3A
- 2 x black 6.3V 1A
I haven't had a chance to look at this over the weekend but will try to do so as soon as I can.

Best Regards

Rob

[valvekits]

Thanks for the clarification Rob.
The black wires are the 6V3 heaters for the EZ81. You might have noticed that the EZ81 heaters are tied to B+ and that is because there is a maximum heater to cathode voltage  allowed. (It's not shown on the circuit diagram)
This connection means that the heaters and cathode are basically sitting at the same potential so that maximum voltage won't usually be exceeded.
I would still check that there is 3v ac between ground and the green wires though, because there is always the possibility that the black and green lead out wires got swapped during manufacture.
After that, you can forget that bit of the circuit.

Eddie

[Bushbaby]

Hi All,
I've had my first opportunity to have another look at the amp.
With the chassis now out of the cabinet and all valve removed, I have checked with power on:
- chassis to earth connections on the transformer side - 0V (expected of course) and <1R
- green heater wires to earth - 3.3V with no load
- black rectifier heater wires to earth - 6.2v with no load.
I have a new JJ dual 50uF 500v cap and a new 16uf 475V cap so I am now thinking its not worth testing the old ones now for working leakage but just replacing them and see what effect they will have.

Best Regards

[Bushbaby]

Replaced the 16uF........still got mains hum!!! I'm beginning to think that I will find the same if replace the dual 50uF cap too but will do this tomorrow.

[Bushbaby]

I have also rechecked the reading from pin 2 of V3 to ground which has a reading of 1.03M R which suggests that the 33k R resistor is in the circuit.
This now appears to be beyond my limited capability assuming that the change in the dual 50uF doesn't make any difference.

The only changes have been to the original amp has been:
- change in power transformer
- earth to heater centre taps at the start of the circuit as well as the end
- direct connection of the 240V primary to the mains lamp and on/off switch (avoiding the use of the voltage selector)
- changed the 16uF 450v cap
- addition of a snubber cap across the mains switch
I cannot think of anything else
Stumped!

R

[Mike Watterson]

- earth to heater centre taps at the start of the circuit as well as the end

Things ought to be earthed at only one place. Also with decent layout earthing the centre tap of the heaters makes little difference, or no difference.

There are three main earth schemes:
1) Star wiring to a common point at the main electrolytic.
2) bus wiring, either an actual bus bar (US sets with an isolated chassis, but otherwise "live"), or point to point with no loops, progressing from main electrolytic and OV of transformer, past power stage, stage by stage to the input. So no output or smoothing currents are flowing on input earth wiring.
3) The "good" Aluminium chassis. No earth wires at all. Each set of tags or earthed part is earthed by the chassis at the nearest point.

What you can't have is just connecting points that are all OV/Earth in any order.

Single sided PCBs are usually laid out using type (2) philosophy. Double sided (or more layers), especially if for RF, will use philosophy (3), one layer will be earth. A four layer board may use an entire layer for Earth/OV and and one for +5V. More complex designs might have a logic Earth layer and an analogue signal earth layer.

[Bushbaby]

Ok, the earth connection to the chassis is now back to the original design having now disconnected the incoming earth lead/CT's from the chassis adjacent to the power transformer. The hum has gone down in volume BUT it is still there!!!
I have yet to replace the dual 50uF cap but loath to doing this if the hum will still be there afterwards!!!
Is there any thing else I should look at?

Best Regards

[Mike Watterson]

Layout.

Also you can connect a cap across a cap to test as long as it's not the main reservoir on the rectifier. So you can check the other caps on HT rail.

[valvekits]

When you pulled V3 and you lost the hum, did you try replacing V3 with one of the other ECC83s?

You could disconnect the transformer heater centre tap, to see what effect that has on the hum, ie does it get better or worse?

[Bushbaby]

I did replace the dual 50uF cap with a brand new JJ dual 50uF 500v- the hum is even louder!!! I have used JJ caps before and I have to say they are excellent.
I did replace V3 with a Mazda ECC83 which made no difference.
Which heater tap shall I disconnect?
The only thing I can think of now is that there is something seriously wrong with the power transformer OR that the new replacement transformer is showing up an issue that may have existed in the original amp design?????