100w plexi PT without center tap?

[eki1988]

Hi! So, I’m about to build yet another plexi clone. I aquired a transformer set, but it has a flaw: it doesn’t have center tap with ht wires. So how I’m going to wire this, as the layout has ht ct wire but my pt has not?

Thanks for advance!

 

[abbey road d enfer]

What is the voltage/current rating of this xfmr?
Typically, non-CT secondaries are used with a bridge rectifier.

 

[Tubetec]

You need a bridge rectifier .
Its possible with the original Marshall layout to reconfigure the diodes into a bridge on the turret board .
Most certainly do check the transformer specs and make sure its right for the job ,as Abbey says .

Look carefully , the CT connection goes to the O/P transformer primary , it looks like that board is indeed configured for a non centre tapped winding with a bridge rectifier.

 

[eki1988]

What is the voltage/current rating of this xfmr?
Typically, non-CT secondaries are used with a bridge rectifier.

Have no idea of current, but ht voltage is 370v.

 

[abbey road d enfer]

OK, so it's about the correct voltage for a tad under 500V B+.

 

[eki1988]

You need a bridge rectifier .
Its possible with the original Marshall layout to reconfigure the diodes into a bridge on the turret board .
Most certainly do check the transformer specs and make sure its right for the job ,as Abbey says .

Look carefully , the CT connection goes to the O/P transformer primary , it looks like that board is indeed configured for a non centre tapped winding with a bridge rectifier.

I think it should work, as it is bugera 1960 PT

 

[eki1988]

You need a bridge rectifier .
Its possible with the original Marshall layout to reconfigure the diodes into a bridge on the turret board .
Most certainly do check the transformer specs and make sure its right for the job ,as Abbey says .

Look carefully , the CT connection goes to the O/P transformer primary , it looks like that board is indeed configured for a non centre tapped winding with a bridge rectifier.

Right you are! There is already a diode bridge. So it should work without that center tap.

 

[juan86]

Don't forget to add 2 resistors to first 2 filter caps (where the CT would go)

 

[abbey road d enfer]

Don't forget to add 2 resistors to first 2 filter caps (where the CT would go)

I'm not sure we're talking about the same thing here.
Which one of these arrangements are you referring to?

 

[Tubetec]

Two large electrolytic caps in series require a resistor divider to balance the voltages .
It can also act as a bleed resistor ,so the caps gradually discharge when the amp is powered down .
it doesnt waste much power when the equipment is energised , but it can help prevent you geting a nasty shock if your poking around inside the chassis later .

Its case #2 Abbey , but two 100uf caps are run in series,

 

[abbey road d enfer]

Don't forget to add 2 resistors to first 2 filter caps (where the CT would go)

Does it mean the original arrangement is that? It would indeed take care of balancing capacitor voltage.


EDIT: I checked the Plexi schemo, it is indeed that.
However, they have not considered balancing voltage for the caps that go after, although the total voltagheis about the same (not much DC drop in the choke)..

 

[eki1988]

Two large electrolytic caps in series require a resistor divider to balance the voltages .
It can also act as a bleed resistor ,so the caps gradually discharge when the amp is powered down .
it doesnt waste much power when the equipment is energised , but it can help prevent you geting a nasty shock if your poking around inside the chassis later .

Its case #2 Abbey , but two 100uf caps are run in series,

Resistor divider? To ground? What size would that need to be?

 

[Tubetec]

In this case they supply a non center tapped transformer and the caps are connected in series to 0V ,
So one resistor from the ht line across the upper cap ,then another across the lower to 0V .
220-270kohms is the usual value , best check dissipation to make sure but 1 or 2 watt components are usually enough .

 

[abbey road d enfer]

Resistor divider? To ground? What size would that need to be?

Typically, one resistor -68 to 220k/2 to 5W -across each capacitor.

 

[Tubetec]

Err on the side of caution ,
generously over rating like Abbey suggests means less thermal stress on the component ,
that affects MTBF ,

At the current rate of consumption of the planets resources it might not be such a bad idea to think long game ,especially with something as personal as an amp to a guitarist .

 

[KIDVOX]

im just builing a reworking of a kld amp at the moment the tranny on that does not have a center tap its just the two outputs from the tx straight to a bridge rectifier with the negative side of the rectifier going to ground. i have a kld amp it works well and sounds great. one of the best amps i have ever had

 

[eki1988]

So quick question, does this wiring seem legit to you?

 

[SteveL]

View attachment 114037
So quick question, does this wiring seem legit to you?

Why not directly to the pot from the "IN"? Not sure what the point of taking the signal from the Cathode is. Is this for a parallel loop?

 

[Tubetec]

I see no reason to use a pot ,it needlessly loads down everything .
If you show the schematic instead of the build diagram its easier to see at a glance whats happening.

 

[SteveL]

I see no reason to use a pot ,it needlessly loads down everything .
If you show the schematic instead of the build diagram its easier to see at a glance whats happening.

Yes, it would be a lot easier to see the entire schematic and know where this is in the circuit and how it's supposed to function!