use-able loss

[Andy Le Blanc]

I was re-examining a power supply today where the secondaries of two
low-volt transformers are connected to get a filament voltage and then the
primary of the second is connected to a voltage doubler to obtain a plate
voltage, a back to back supply......
I noticed that the AC output of the second transformers primary had droped
significantly
the input voltage was typical for my area at 123 V AC
the output of secondary connected transformers was 117 VAC
both were 3A trannies with 12V secondaries
This appeared to me to be a good way for those with old amps to isolate and
reduce the todays hotter AC to more acceptable levels in an affordable way.

[skyboltone]

I was re-examining a power supply today where the secondaries of two
low-volt transformers are connected to get a filament voltage and then the
primary of the second is connected to a voltage doubler to obtain a plate
voltage, a back to back supply......
I noticed that the AC output of the second transformers primary had droped
significantly
the input voltage was typical for my area at 123 V AC
the output of secondary connected transformers was 117 VAC
both were 3A trannies with 12V secondaries
This appeared to me to be a good way for those with old amps to isolate and
reduce the todays hotter AC to more acceptable levels in an affordable way.

Can you sketch it out Andy? Sounds like free lunch. How much load did you put on the 117V back fed transformer? It will be about 10 times that load (times about 1.25) from the 3 amp secondary of the first transformer. It may not handle it along with it's filament duties.

Dan

[Andy Le Blanc]

I've used this kind of hookup with success to power preamps
if the demands are only a couple tubes, wire the fils. series to half the draw
Ive seen shematics of large pre amp projects and small practice amps useing
beefier trannies, but hadnt thought to use it unloaded to reduce volts
it does sound like a free lunch and I share the concerns about loading
but it might be a usefull for small amps.....

fighting with the scanner a bit ..... think is enough...

[skyboltone]

I think I see there that you're getting 104 on the "secondary" side of the backfed transformer. I think you're just up against iron losses here. Remember that you've got two magnetic gaps to cross. There is always loss of energy (about 20%) across the core. With a doubler you still might get some use out of it but I predict you won't have good regulation. Use BIG caps. Toroid transformers suffer less core loss but are still pretty expensive for what you are trying to do.

Dan

[Ears]

I can't quite make out your scan Andy.
Is it similar to Fred's scheme to use wall warts?
See: http://www.dogstar.dantimax.dk/tubestuf/mctube.htm

[Andy Le Blanc]

It is along the same approach...... hadn't seen the Mctube yet though.....
Ive also seen the same with wallwarts too but was worried about the draw
Im getting 104 DC on a bridge rectifier unloaded..... add some caps and
it will looks like it could go as high as 165 (peak) VDC.... with a doubler
that puts things around 320 VDC.....which is why its an attractive supply
It does work in practice too for a couple of tubes.... I havent tried a small amp yet.... small SE might go....
but the loading is an issue....

Im trying to look at it as way to reduce wall voltage and run largely unloaded
I saw the voltage drop and thought may be.... they push the volts AC in
maine for several reasons ..... the most likely reallity is that most domestic wireing is
old and hasnt been upgraded to large-er amp service this magnifies losses... I did the math...
droping the volts by 5 v would save the
consumer $$$ a few hundred a year....

anyway...... that drop (core losses?) through the trannies could be used to
buffer high running AC..... with 3A trannies what is "safe" loading?

[skyboltone]

anyway...... that drop (core losses?) through the trannies could be used to buffer high running AC..... with 3A trannies what is "safe" loading?

Well, a watt is a watt regardless of which side of the equation it's on. That is, less core loss, hysterises, copper loss etc; a figure that we can call 15%-20%, sometimes less. So 3 maps times 12 VAC is 36VA (equivalent to a watt in a single phase circuit). On the other side we've got 36VA (-10%-20% in this case). So multiply your voltage times the load in amps and make sure you don't exceed 36 VA. If you double, cut that half.

But what are you trying to accomplish? If it's a high filament voltage, (I'm a very strong advocate of not exceeding 6.3 v) you can place a suitable small value resistor in series with one of the legs of the filament supply to reduce voltage to a safe level. By small value I mean for instance a 6 amp supply with only 2.5amps on it might produce 6.78vac or so. A .2 ohm one watt resistor will tame things down to 6.25vac or so.

Dan

[Andy Le Blanc]

the idea is to use the losses inheirent in this tranny hook up to safely drop
the wall AC voltage just a bit ...... to ease the electrical conditions for older
tubed equipment...... some older gear was designed and rated around a
110 AC...... the AC in my area fluctuates from 123 to 128 so when you pull
grampies amp from out of the attic theres a good chance it wont last too long
and in HI-FI gear where one really invests to retube NOS 300b and may want
to strickly adhear to the units voltage requirements

[skyboltone]

the idea is to use the losses inheirent in this tranny hook up to safely drop
the wall AC voltage just a bit ...... to ease the electrical conditions for older
tubed equipment...... some older gear was designed and rated around a
110 AC...... the AC in my area fluctuates from 123 to 128 so when you pull
grampies amp from out of the attic theres a good chance it wont last too long
and in HI-FI gear where one really invests to retube NOS 300b and may want
to strickly adhear to the units voltage requirements

variac

[Stanz]

A question, how many tubes do you think a power supply like this could power? One 12ax7? More?

[Andy Le Blanc]

a variac would be nice ...... but if you can do the same for $20 it is a free lunch

Ive seen projects with a plate only supply useing 450ma trannies powering six tubes
a couple caps were wired as a filter to reduce noise between the second
trannies center tap and secondary winding legs too
Ive done supplies that power fils. and B+ for two tubes useing 1.2 A trannies
id use much larger amp ratings or a seperate supply for DC fils.
its not too bad an alternative to $$$ low power and fil. trannies

[Andy Le Blanc]

heres one...... by Joseph Norwood Still