Valve amp transformer bodge

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By bodge, do you mean what us west of the pond folks call kluge?

Anyway, the voltage rating of the capacitors on rectified output will determine if the rectified 240V is excessive or not. The tubes can take it. Also, you will have double losses going from your 240 input to 6V and back to 240V, so you will probably only have 230V on the output or maybe less. Tapping the 6 V for filaments is reasonable, I would connect a couple of resistors in series across the 6V and connect the midpont to ground to approximate their grounding scheme, it probably is to reduce hum from the input amplifier tubes. Make the resistors 10K or so to not overload the first 240 - 6V transformer. After things are running, check both transformers that you are not overheating anything.

Hi Gareth,

Have you no isolation transformers on your side of the pond?

Remember that the maximum current you can draw is the ratio of the 6.3V to its amperage compared to 240V. So if the 6.3VAC was rated at 10A, then the other side of your proposed pair would be 240V @ 262.5ma (assuming 100% efficiency). I suspect you will be having trouble with your idea if the 6.3V output current is even less!

John :-#)#

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Hmm, I haven't really thought this through, I didn't consider an isolation transformer!

I guess an isolation transformer and a heater transformer would be a lot less iron overall.

Thanks,

Gareth.

OK, so this amp, according to my calculations, requires a minimum of just over 15 Watts of 6.3v heater power, having 3 x ECC83 @ 300mA and 2 x EL84 @

760mA.

In the real world, how many ballpark Watts of power supply transformer should you be thinking about for the rest of the amplifier?

Cheers,

Gareth.

"hr(bob)

Also, you will have double losses going from your 240 input to 6V and back to 240V, so you will probably only have 230V on the output or maybe less.

** With no load, the output voltage will be close to 240V as the combined turns ratio is exactly 1:1. If two 50VA trannys are used, each will have a regulation loss under load of about 10% - so the actual output voltage will be under 200V.

A far worse problem is the primary magnetising current which the first transformer must supply to the second. The typical amount for a 50VA conventional tranny is 60mA on a 240V primary, but if operated in reverse it increases according to the turns ratio.

It becomes 40 times 0.06 or 2.4 amps - eating up almost 30% of the VA rating of the first tranny.

Things get better if two toroidal trannys are used, as the magnetising current is far lower.

.... Phil

"Gareth Magennis"

** Go for a 240V / 220V isolation step down of say 50VA rating.

Plus a 6.3V, 3 amp tranny.

.... Phil

If you have the original transformer you can get some sort of an idea of the current requirements by checking the wire gauge for each winding.

That's about all I can think of other than figuring out what style of output you have and then calculating (based on) the maximum plate dissipation (usually in watts) and convert that to current as you roughly know the DC voltage.

Beyond that, pull out some tube amp design books and see what they recommend for transformers for your style outputs...

John :-#)#

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    (Please post followups or tech enquiries to the newsgroup) 
  John's Jukes Ltd. 2343 Main St., Vancouver, BC, Canada V5T 3C9 
  Call (604)872-5757 or Fax 872-2010 (Pinballs, Jukes, Video Games) 
                     www.flippers.com 
       "Old pinballers never die, they just flip out."

That sounds like the plan.

Thanks,

Gareth.