#124 Questions

[martin manning]

well having the nut will raise the cover off the chassis which might have some decoupling effect??? the transformers I use have nylon washers so they would have the same effect possibly.

I think reduction of the leaked magnetic field passing through the chassis is a good reason to space the core away from it.

[pompeiisneaks]

Jelle and a few others told me that using the heater center tap is actually not the preferred method. They indicated using a virtual center tap with two 100 ohm resistors is preferred because if something shorts to the heater rails the 100 ohm resistors short out and don't burn up the transformer itself. If you prefer the 100 ohm resistor method, then yes, shrink wrap it and zip tie it off near the transformer somewhere.

~Phil

[martin manning]

Jelle and a few others told me that using the heater center tap is actually not the preferred method. They indicated using a virtual center tap with two 100 ohm resistors is preferred because if something shorts to the heater rails the 100 ohm resistors short out and don't burn up the transformer itself. If you prefer the 100 ohm resistor method, then yes, shrink wrap it and zip tie it off near the transformer somewhere.

One or the other, but not both. In this case, if the transformer core makes electrical contact with the chassis, you have both.

[pompeiisneaks]

Jelle and a few others told me that using the heater center tap is actually not the preferred method. They indicated using a virtual center tap with two 100 ohm resistors is preferred because if something shorts to the heater rails the 100 ohm resistors short out and don't burn up the transformer itself. If you prefer the 100 ohm resistor method, then yes, shrink wrap it and zip tie it off near the transformer somewhere.

One or the other, but not both. In this case, if the transformer core makes electrical contact with the chassis, you have both.

Oh is that what the ground in the middle of the line means? I guess you'd have to isolate the transformer to avoid that then eh.. sorry something new to me.

~Phil

[Bombacaototal]

Many thanks dor the help as always Martin and Phil.

So a PT heater center tap should always either the internally grounded or use the 100R approach. In the case of my transformer which is internally grounded I should make sure it doesn’t touch the chassis, so is the approach of heatshrink the lead and set the wire aside correct one?

[norburybrook]

I'm not sure if we've got some wires crossed here if you pardon the pun, or am I not getting the point.


if a PT has a heater center tap then you can either 1; ground it, to the chassis, which is what the whole point of the center tap is or 2: tie it off and use two 100R resistors to act as a false center tap which is what has to be done is there is no center tap on the transformer.



M

[martin manning]

if a PT has a heater center tap then you can either
1; ground it, to the chassis, which is what the whole point of the center tap is or
2: tie it off and use two 100R resistors to act as a false center tap which is what has to be done is there is no center tap on the transformer.

And, if the CT lead is internally grounded to the transformer core, use option 1, since the electrical connection from the transformer core to the chassis may be unreliable.

[Bombacaototal]

Many thanks Marcus and Martin! Very clear now.

I will ground the heaters CT at the same spot as the red/yellow wire from the PT

[Bombacaototal]

I just want to double check the wiring of the PT.
1) Each of the 2 Red wires go to each of the diodes on the rectifier
2) Red/Blue goes to the resistor on the rectifier board
3) Red/Yellow and Green/Yellow to ground
4) The two Green wires go to the first power tube socket at the heaters

I am using a multitap for the PT and therefore
5) Brown/White goes to the 220V tap
6) Gray goes to 230V tap
7) White/Brown goes to 240V tap

Brown wire is only used for 100V and therefore unused on my amp. I will heatshrinkl the lead.

So far so good, now missing are the Black, Orange and the COM from the multi-tap.
I am thinking the following:
8 ) Orange to ground

Regarding Black and COM, I am a bit confused where exactly to place them?

I have an IEC mains with Middle to ground, live to the fuse middle, and the butt of the fuse to the mains switch (lug 2 - input) and Neutral of IEC to the mains switch (lug 3 - output). Will COM go to the same lug as the live from the fuse (lug 2 - input) and Black wire of the PT go to the IEC Neutral (lug 3 - output)?

Attached the spec sheet of the transformer.

[martin manning]

This is 40-18097 Classic tone PT?

Ground the Orange shield wire with the Grn/Yel and Red/Yel CT leads.
Inlet, fuse and and voltage selector:
IEC Inlet Earth ground -> dedicated chassis bolt
IEC inlet neutral -> PT primary (Black)
IEC Inlet Line -> Fuse holder tail terminal
Fuse holder side terminal -> Voltage selector common

[Bombacaototal]

This is 40-18097 Classic tone PT?

Ground the Orange shield wire with the Grn/Yel and Red/Yel PT CT leads.
Inlet, fuse and and voltage selector:
IEC Inlet Earth ground -> dedicated chassis bolt
IEC inlet neutral -> PT primary (Black)
IEC Inlet Line -> Fuse holder tail terminal
Fuse holder side terminal -> Voltage selector common

Thank you very much Martin for the help. Indeed the 40-18097 Classic tone PT

1) [CHECK] Ground the Orange shield wire with the Grn/Yel and Red/Yel PT CT leads.
2) [CHECK] IEC Inlet Earth ground -> dedicated chassis bolt

Now is where I have questions, how does the mains switch come into play.
3) IEC inlet neutral -> PT primary (Black) I have an extra wire from that connection to the Mains Switch Lug 3 (output)
4) IEC Inlet Line -> Fuse holder tail terminal I think I got this reversed as my Line is going to the Fuse holder side terminal. I will fix this.
5) I currently have a wire on the Fuse holder tail terminal, which after correction above will be Fuse holder tail terminal going to the Mains Switch Lug 2 (input) -> Connected to the COM (Voltage selector common)

I guess if my takes on the Mains are correct the only thing needed is to reverse the fuse holder connections?

[martin manning]

If you use an SPST mains switch, put it between the fuse holder side terminal and the voltage selector common. If you use a DPST, wire one side as for the SPST, and the other side between the IEC neutral and the PT primary (Black). I think CE standard is DPST, and that gives you a tie point for the PT primary lead.

[Bombacaototal]

If you use an SPST mains switch, put it between the fuse holder side terminal and the voltage selector common. If you use a SPDT, wire one side as for the SPST, and the other side between the IEC neutral and the PT primary (Black). I think CE standard is SPDT, and that gives you a tie point for the PT primary lead.

I am using he follwing switch:
https://www.mouser.co.uk/ProductDetail/ ... %2F0d4Pw==

It is SPST ON-NONE-OFF

[martin manning]

In general it looks like this:

IEC Inlet Earth ground -> dedicated chassis bolt
IEC inlet neutral -> Mains switch (if using DPST) -> PT primary (Black)
IEC Inlet Line -> Fuse holder tail terminal
Fuse holder side terminal -> Mains switch -> Voltage selector common

Note correction above to DPST for two pole switch.

[Bombacaototal]

In general it looks like this:

IEC Inlet Earth ground -> dedicated chassis bolt
IEC inlet neutral -> Mains switch (if using DPST) -> PT primary (Black)
IEC Inlet Line -> Fuse holder tail terminal
Fuse holder side terminal -> Mains switch -> Voltage selector common

Note correction above to DPST for two pole switch.

Perfect. That is exactly what I have except the fuse which is in reverse. I will fix it!