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).