Replacing power transformer on a Marshall 1974X question

[joeyello]

Hello everyone. I am new to this forum. I am replacing the power transformer in my Marshall 1974X. Of course the new transformer’s wires are a different color than the old. I have been able to determine how they match up except on the input side.

I am in the US and I’m not sure if the AC input should go to the 120 tap or the 240 tap on the transformer.

Any help would be appreciated.

Thanks. Joe

[JMPGuitars]

Hello everyone. I am new to this forum. I am replacing the power transformer in my Marshall 1974X. Of course the new transformer’s wires are a different color than the old. I have been able to determine how they match up except on the input side.

I am in the US and I’m not sure if the AC input should go to the 120 tap or the 240 tap on the transformer.

Any help would be appreciated.

Thanks. Joe

Domestic AC is 120V. Welcome to the site!

Thanks,
Josh

[joeyello]

Thanks Josh! As I suspected but every schematic shows the higher voltage and I wanted to verify.

I guess the schematics are Uk based.

[geoff 1965]

hello Joe,
is your amp an original 1974X or the reissue? appreciate if you could add some pics it's very rare we see a genuine marshall!
i know they had problems with the early reissue PT's and the dagnall transformers in the newer reissues have a 150mA HT rating.
does your amp have the 91R el84 bias resistor?

[joeyello]

hello Joe,
is your amp an original 1974X or the reissue? appreciate if you could add some pics it's very rare we see a genuine marshall!
i know they had problems with the early reissue PT's and the dagnall transformers in the newer reissues have a 150mA HT rating.
does your amp have the 91R el84 bias resistor?

The amp is a reissue!

[geoff 1965]

Which replacement transformer have you used?

[JMPGuitars]

is your amp an original 1974X or the reissue?

1974X is the reissue. The original is 1974.

[joeyello]

Which replacement transformer have you used?

I got the ClassicTone Vintage from Stewmac, which is made by Magnetic Components Inc. I heard they just went out of business earlier in the month.

I hope to install it this week. Just going through the wiring now. I hate having to undo the beautiful wire wrap job that Marshall did on the wires from the transformer to the tube sockets.

I hope it still sounds good!

[geoff 1965]

so is the HT 275-0-275 the same as the Marshall?

[joeyello]

It’s supposed to be a direct replacement. The reviews were very favorable.

[geoff 1965]

Just looked up the specs and the classic tone is 290-0-290 @120mA HT so be careful! your B+ will be higher and you will have to rebias the el84’s, the 91 ohm bias resistor will be too hot for 290-0-290 without dropping your B+,check out Josh’s “B+ Voltage reduction” thread.
Those reissues are biased hot and i’ve a feeling that’s why they upped the HT rating to 150mA,keep us posted and if you are unsure about biasing and B+just ask.
good luck
note; we have the schematic for the reissue but no reference to voltages.

[joeyello]

R23 is the 91 ohm resistor. What should it be changed to?

[geoff 1965]

Most of the 18W’s built on the forum have between 150-180 ohms depending on your B+ and dissipation wanted,but like I mentioned earlier we have no reference to the reissue voltages so it’s hard to relate to the 91R without knowing what B+ & plate voltage the reissues are running.

[geoff 1965]

Here’s the reason I think you need to be careful using the classic tone as a “direct replacement” for your amp.
my HT is 275-0-275 @ 120mA and here are my averages at idle for both el84’s with a 160R bias resistor.
plate voltage;334
cathode voltage;12.4
plate current;35mA
dissipation;11.5W
so take into account that lowering the resistance of the bias will lower plate & cathode voltage but will “increase” plate current & dissipation,so with the reissue 91R you can imagine the increase in current/dissipation compared to 160R.
you have to be mindful of the current ratings of both the HT and heater supply when replacing transformers.

[bvayling]

Welcome to the minefield!
Seriously, lots of help available here, we want more 18W amps in the world! (The best amp for the modern world!)