Power Transformer For Marshall 18 Watt

[merlin]

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

That sounds to me like you measured it unloaded. So it's probably a 160V, 5V, and 6.3V transformer under load.

If you have something like an 80V transformer you could add the voltages together, giving you 160+80 = 240V effective.
But maybe then the project is growing like Topsy.

 

[amplexus]

Valve amp voltages are usually considered in spec if they are within 20% of what’s listed- in this case anything from ~230-340vdc B+ would be quite fine and you would be unlikely to hear much, if any, difference in performance between a bang on 290 and say 325.

Of course officially an EL84 plate gets a little unhappy if you go too far over 300vdc- but datasheet values spec for a Va of 300vdc in a push pull pair to get you 17 watts so its fair to say a decent el84 should take that without difficulty.

You do not need a giant resistor to drop your B+ after doubling. If your xformer is centre tapped you can use a couple 5W zeners or the attached “adjustable” zener supply off the CT to drop your voltage to exactly where you want it. More precise and less heat, much more efficient (and probably less expensive) than a giant resistor. A few people have knocked this circuit in to 18W builds for power scaling with good results so for your needs it should work just fine.

If

 

[Liutmod]

Maybe I'm wrong, but the above circuit is used to reduce the voltage in a configuration where I should instead increase it (172 Volts).
The need to reduce the voltage arises where I use the transformer without the center tap (344 Volts)

 

[CJ]

Doublers have lousy regulation which might emulate a tube rectifier very well.

Ripple is higher so you will need some hefty caps.

And I believe current capacity gets cut in half as there is no free lunch.

Actual voltage output might be closer to 1.7 tines original which would be about right for EL84 types.

Build it and see, if no good then at least you get experience.

 

[Matador]

Agree with CJ: it's going to be hard to pre-guess which configuration you'll say is better.

If it were me, I would just build it to run at ~250V and see if it was loud enough for my situation. The EL84 datasheet lists a 250V operating point at 11W. Remember that even half power is only 3dB loss in level, which could easily be made up for by a 3dB-more efficient guitar speaker. The tubes themselves would also greatly appreciate being run at the lower voltage as well (in terms of tube longevity).

 

[Cqwet Dbdfte]

If "mojo" is an RC network, it can be added to any stiff powersupply.

 

[Liutmod]

I built the power supply and unloaded I measured 484 Volts.

Surely with the load the voltage will drop but to know how much I will first have to realize the amplifier, and it will take some time since I will do it in my free time

Eventually if it si necessary I can use a power scaling

 

[CJ]

The caps will charge to peak value unloaded so simply divide 484 by root 2 for ballpark figure when loaded.

Then subtract DCR loss by measuring secondary resistance and multiplying by current draw, like 100 ohms x 89 ma = 8.9 volts.

 

[Liutmod]

Thank you CJ!

In the meantime I build the amplifier I want to understand which output transformer to use.

I have seen that there are many replicas of the old RS transformer used by Marshall.

The 18 watt was a cheap amplifier and probably also the transformer that was used and I wonder if a better quality transformer could increase the possibilities of the amp, even if in this way the project would no longer be a real Marshall clone.

Furthermore I realized that the final section is very similar (if not the same) as many other amplifiers including the VOX AC15 which uses a different transformer (6k2 ohm input impedance instead of 8k4 ohm of the Marshall)

Does anyone have any suggestions?

 

[CJ]

Depends on what speaker you use, if you use like a 12 inch good for 60 watts then you can use a big output xfmr that has maybe a EI100 core, about 2.5 x 3 inches,

if you use a 20 watt 10 inch then maybe use something smaller like a 75EI or 87EI core which has a 3/4 inch stack and 7/8" stack respectively.

You can also use a transformer from an old stereo receiver, my favorite output is the one that you find in the Fisher receivers that use the EL84/6BQ5 tubes. but they are rare and expensive.

but don't stress it too bad, the speaker and the enclosure will do more to shape the sound than the output xfmr, and NOS Amperex tubes will also make a big difference.

Most guitar transformers are just primary/secondary wound since you do not need extended hi end so a cheap transformer will work fine, you can always upgrade later.

 

[Liutmod]

I want to build the amplifier as a head to match with one of the cabs I already own, one with a Celestion Vintage 30 and one with a Celestion Gold (both 12 inches)

With these speakers what is the difference between the Marshall 18 watt and VOX AC15 transformers?

How does the difference of their input impedance affect the sound?

Thanks in advance

 

[merlin]

Both amplifiers used an 8k output transformer.

 

[Liutmod]

From what I found online, the output transformer in Marshall 18 Watt has an input impedance of 8k Ohm while the one in Vox AC15 has an impedance of 6k3 Ohm.
Also, the AC15 transformer is bigger ...