CONVERTING 110V AMPS TO 220V OPERATION

[Dougdh]

Hello, my name is Doug and I greet you from a fairly cold Johannesburg, South Africa (11 degrees centigrade).

The Hoffman amp site is extensive and it's a privilege to be amongst knowledgeable gents such as yourselves.

I've been involved in converting a number of 110V valve amps to 220V operation and have had success.

However, I have run into a problem which I have not been able to find info about.
The amp is a Fender Twin Reverb II, circa 1984.
The original 110V transformer:
Primary 110V.
Secondary  320-0-320 centre tap (CT).
A separate winding of 60V for bias, and a separate winding of 6.5V for heaters.
The 220V transformer:
Primary 220V.
Secondary 320-0-320 (CT),
part of that winding is a 60V tap for bias
and there is a separate winding of 6,5V for heaters.

The 110V transformer had one end of its bias winding grounded.
The other end was connected to a resistor-diode-smoothing cap for bias.
The HT winding went to a 4-diode bridge rectifier, with its negative to ground.
The HT centre tap went to the junction of two caps with resistors across them.
The full HT is from the top cap to ground.
The 110V transformer centre tap appears to be floating.

With the 220V transformer, this arrangement doesn't work.

The 110V circuit schematic makes the above clear.

Do you have any suggestions of how to solve this problem?

Kindest regards,
Doug
(FIRST TIME POST)

[acheld]

Is this the schematic for your amp?    https://el34world.com/charts/Schematics/files/Fender/Fender_twin_rev_ii.pdf

And which PT model are you using?  (Difficult for me to read the standard export model # off of this schematic).

[sluckey]

I've been involved in converting a number of 110V valve amps to 220V operation and have had success.

However, I have run into a problem which I have not been able to find info about.
The amp is a Fender Twin Reverb II, circa 1984.
The original 110V transformer:
Primary 110V.
Secondary  320-0-320 centre tap (CT).  ***WRONG***
A separate winding of 60V for bias, and a separate winding of 6.5V for heaters.

The original 110V transformer has a secondary of approx. 180-0-180. The easiest solution will be to get a proper replacement export transformer.

There is another more technical solution that will allow you to use the 220v PT you have. Just replace the full wave bridge rectifier with a conventional rectifier. Refer to the Fender AB763 Twin Reverb schematic.

[sluckey]

This schematic shows how you can use your PT...

[tubeswell]

There is another more technical solution that will allow you to use the 220v PT you have. Just replace the full wave bridge rectifier with a conventional rectifier. Refer to the Fender AB763 Twin Reverb schematic.

If this is suggesting what I think*,  the OP would also need to rewire the tube socket filaments for 12VAC heater operation

and change the bias supply VDC output

(*if this is talking about using a PT designed for 110V on a 220V mains, because all the secondaries would be putting out 2 x the VAC that you'd normally get on a PT with a 110VAC primary. Of course, I could have the wrong end of the stick here)

Another issue one might face if using a PT built for 100VAC @ 60Hz, is the higher power dissipation that will be encountered if the PT is run on a 220VAC 50Hz mains (because of the bigger area under the 50Hz sine wave function curve)


Otherwise, simpler to use a PT designed for 220Vac mains, or a step-down power transformer

[pdf64]

No, the idea was so that the 220V replacement PT that has been purchased

The 220V transformer:
Primary 220V.
Secondary 320-0-320 (CT),
part of that winding is a 60V tap for bias
and there is a separate winding of 6,5V for heaters.

can be used in this amp.

[sluckey]

If this is suggesting what I think*,  the OP would also need to rewire the tube socket filaments for 12VAC heater operation

and change the bias supply VDC output

(*if this is talking about using a PT designed for 110V on a 220V mains, because all the secondaries would be putting out 2 x the VAC that you'd normally get on a PT with a 110VAC primary. Of course, I could have the wrong end of the stick here)

It's not what you think. The OP bought a PT that's meant to work with a conventional rectifier. If he puts it in that amp with a FWB he will have over 900V B+ and his bias circuit will not work.

[PRR]

.....all the secondaries would be putting out 2 x the VAC that you'd normally get on a PT with a 110VAC primary. Of course, I could have the wrong end of the stick here....

Any time you think you will have 2X the design voltage, you probably have a wrong end stick.

Voltage (with frequency) is a key design factor in transformers. Voltage too high, iron saturates, inductance vanishes (1/1000), current goes to infinity. There's some leeway to cover high line voltages. But 10%-20%, not 100%.