Power Transformer For Marshall 18 Watt

[Liutmod]

Hello, I want to build a 18 Watt Marshall clone for which I'd like to use a transformer that I already have, but which has an output of 172 Volts that are a bit few to power the EL84 considering that all the schematics I have found require a power transformer with a voltage between 215 and 290 Volt

The transformer beyond this limit has all the right specs to be used for this project

Anyone can tell me if there is a solution to use this transformer or is it better to buy a new one?

Thanks in advance

 

[13engrsapper]

Hello, I want to build a 18 Watt Marshall clone for which I'd like to use a transformer that I already have, but which has an output of 172 Volts that are a bit few to power the EL84 considering that all the schematics I have found require a power transformer with a voltage between 215 and 290 Volt

The transformer beyond this limit has all the right specs to be used for this project

Anyone can tell me if there is a solution to use this transformer or is it better to buy a new one?

Thanks in advance

I would just go to Mercury Magnetics and get the proper transformer to begin with.

 

[Liutmod]

and this is the least convenient solution

 

[CJ]

Get a new transformer but go to Antique Electronic.

If you marry the wrong woman you will have trouble for the rest of your life which will lead to drinking, divorce, alimony, high blood pressure, gout, heart disease, insanity and then death.

 

[tk@halmi]

If you marry the wrong woman you will have trouble for the rest of your life which will lead to drinking, divorce, alimony, high blood pressure, gout, heart disease, insanity and then death.

Happy Saturday CJ!

 

[Liutmod]

There are no chances of changing the wrong woman into the right one?

Jokes aside I forgot to say that the output is 172-0-172 Volt and I could double the voltage but in this way the voltage would be about 50 volt in excess as I'd have 344 volt instead of the 290 volt maximum required by the amplifier

 

[tk@halmi]

There are no chances of changing the wrong woman into the right one?

Based on collective experience, no…

If you want a real clone you either need to rewind your transformer or buy the right one.

If you start adjusting the amp circuit then it is not a clone, but something different.

 

[Alva Goldbook]

If you're going to build amps, it's good to understand how to spec and source iron. So this will depend on what kind of amp you want to build. Since an 18 watt Marshall can be a few different things, I'd need you to narrow things down more. For instance, some 18w Marshalls have a tube rectifier and some do not. Some have a tremolo channel and normal channel. Some use a simplified single channel. Some do a sort of AC30/Marshall hybrid type design using an EF86 on one channel. Some modify the amp to have bass middle treble controls.

Unless an amp has solid state stuff going on, like SS reverb, SS FX loop or SS channel switching, then the things drawing current are the tubes. If using an old Fender style pilot light you can add that on too.

First figure out all the tubes in the amp. Write down their heater current draw. Add them up. Give a bit of room for extra so the power transformer isn't over stressed.

For instance, most 9 pin preamp tubes run 300mA at 6.3 volts. 150mA at 12.6 volts. Power tubes will vary, and you can find the number by googling the data sheet. Hint: always look up the data sheet.

Suppose you calculate you need 2.3A of heater current. Shoot for iron with 3A. Don't get an iron with 2A. 2.5A will technically work, but you want a bit more wiggle room than that. 3.5A is pushing it. 4A is too much.

Then calculate the B+ current. Preamp tubes vary quite a bit, but for 12ax7's it's easier just to calculate 1mA per triode. For power tubes, go with the maximum current draw. For the plates and for the screen grids. Add them up. Add a little extra for a safety margin. Suppose you calculate you need 100mA. Look for iron that can do 125-150mA. Don't go for 75mA unless you want to replace your power transformer quickly. Don't go for 250mA unless you want your B+ voltage to go too high and also stress your power transformer.

Suppose you want a rectified B+ voltage of 350. Here the magic number is 1.3. Really it's 1.414, but under load it's 1.3. 350/1.3=269.23. You want a transformer winding of 270-0-270. Or with no center tap, 540 total.

Write down what specs you need for the amp you plan to build. Then match them to a power transformer you can buy off the shelf. There's no need to buy some insanely expensive thing from Mercury. Hammond makes plenty of transformers, and one of them will work for your build. You just have to check the specs. EdcorUSA also makes quality iron. As does Pacific Audio and Sour Sound. I could of a few UK based ones if you're in Europe.

 

[Matador]

If it's 172-0-172 that's close to 240 with a full wave rectifier.

 

[capacitorless]

If it's 172-0-172 that's close to 240 with a full wave rectifier.

+1, you may be fine. There's a power scaling circuit called VVR that's being used to reduce B+ with good tonal results (see eg ampgarage.com). On the 18w they're using it on the whole amp, preamp tubes as well. Shoot, you might prefer the tone with a little browning in the mix.

I also looked into how to boost the B+ on an existing transformer, and came up with the nutty idea of applying positive voltage and the appropriate supply of current to the center tap. I decided to instead just use the existing B+ and call it a day.

I would just build it with the power transformer as is and a full wave bridge rectifier and see what transpires. 18w amps are a good sounding build.

 

[Liutmod]

The amplifier I want to realize is really simple with no frills as you can see from the attached schematic

The AC voltage required is 215 Volt while my transformer delivers 172 Volt (344 Volt doubling the voltage)

I can try to power the circuit with 172 Volt or I think may be better doubling the voltage (344 Volt) using the attached schematic and then reducing the excess voltage with a 50 Watt resistor or better with the VVR power scaling circuit suggested

Any other suggestions?

I know that it would be easier to buy a new transformer, but I’d like to use the one I already have which is also a good transformer

 

[Liutmod]

This is another schematic to double the voltage of a center tap transformer
I don't know which of the two could be better

 

[Alva Goldbook]

The amplifier I want to realize is really simple with no frills as you can see from the attached schematic

The AC voltage required is 215 Volt while my transformer delivers 172 Volt (344 Volt doubling the voltage)

I can try to power the circuit with 172 Volt or I think may be better doubling the voltage (344 Volt) using the attached schematic and then reducing the excess voltage with a 50 Watt resistor or better with the VVR power scaling circuit suggested

Any other suggestions?

I know that it would be easier to buy a new transformer, but I’d like to use the one I already have which is also a good transformer

Why bring the voltage up with a voltage doubler just to bring it back down again with a massive 50 watt resistor?

Tubes are very forgiving of B+ voltages. Too low and you won’t get the full output, but it sounds like you’re not looking for full output anyway.

To make sure the transformer can be used you need to know what current it can provide and the current draw of the amp.

 

[Liutmod]

The current produced by the transformer is more than that required by the amplifier.

The only problem is the voltage that is lower than required

 

[Matador]

The AC voltage required is 215 Volt while my transformer delivers 172 Volt (344 Volt doubling the voltage)

Why won't the circuit function at the lower rectified voltage?

 

[Liutmod]

because I'd get less than 18 watts of power and in the rehearsal room it would be overwhelmed by drums
I'd also have less headroom for clean sounds

344 Volts in conjunction with a power scaling circuit may be the best solution

 

[Cqwet Dbdfte]

Look at the tube specs.
Remember sound is a series of peaks, not a steady power output. Peak to average could be 10:1. Average 1.8W peak 18W. Speaker efficiency is critical, you don't want 85dB/1W/1m but 100dB/1W/m speakers.
Some sound engineering may be needed to dampen that drum kit.
If you want more power, EL84 is not a top choice tube. 6550 or EL34 have more capacity. Self bias is obviously not in line with maxing out power, fixed bias with individual adjustment for each tube would be better.
Think thru that power requirement.

 

[CJ]

Is that 172 volts measured with a load? Never seen that stamped on a xfmr. Does it have a 6.3 wind? 5.0 wind?

 

[Liutmod]

Yes, I measured 172 volts and it has an output at 6.3 volts and another at 8 Volts

 

[Matt Syson]

Part of the 'mojo' that is an AC 30 is the way the HT supply sags when driving, even though it is not far off Class A so if you want an AC 30 build it CORRECTLY with all the correct parts, otherwise it is just another valve amplifier for which the circuit schematic is pretty much the same as so many others and it is the components used aND the layout of the parts that define it to be an aC 30.