It's kinda tough to find "plate transformers" anymore. Yes, there's
mail order, but try this:

There's those "semi toroidal" transformers with 4 bobbins, a pair
on one side, and another pair on the other side. Square
looking things, using C and I laminations. 120/240V
primary, and whatever secondaries at X volts x twice the amps
or twice the volts at just the amps. Thing is, there are two separate
primary windings, and two separate secondary winding. What you
could do is use just one primary winding to connect to the powerline,
and use the other "primary" winding to get some B+ after rectification.
And the secondaries can feed tube heaters. However, you need to
select a transformer that has twice the VA rating of the amount of
VA's you expect to consume. Because you're using only half of
the normal primary windings, and the one winding you do use for
the primary will do all the work instead of half the work in a
"normal" application.

Use a voltage doubler rectifier circuit if you want something
like 300VDC instead of 150VDC.

You probably don't want to use a transformer that has on a
single bobbin the 120/240 dual primary windings. As the
insulation between the two primary windings wouldn't be good
enough to pass "hi pot" requirements (think transient spikes and
arc over). That's why I specified the square toroidal transformers
with 4 bobbins. The kind with C and I laminations, not E's and I's.
The "flat pack" PCB mount kind.

"Robert Casey"

It's kinda tough to find "plate transformers" anymore. Yes, there's
mail order, but try this:

There's those "semi toroidal" transformers with 4 bobbins, a pair
on one side, and another pair on the other side. Square
looking things, using C and I laminations. 120/240V
primary, and whatever secondaries at X volts x twice the amps
or twice the volts at just the amps. Thing is, there are two separate
primary windings, and two separate secondary winding.


** Yes, but operating in pairs on the same side.


What you could do is use just one primary winding to connect to the
powerline,
and use the other "primary" winding to get some B+ after rectification.


** The other "primary" is physically separated and only coupled by the
iron core - expect massive leakage inductance and hence loss of voltage
under load.


And the secondaries can feed tube heaters.


** One can - the other will drop volts like a stone when under load.

Have you actually tried this ?




.......... Phil

I use shaver isolation transformers in a full-wave doubler circuit. They
can usually be had for $2.50 to $5 from the local MCC thrift, depending
on condition.

--
Gregg
*It's probably useful, even if it can't be SPICE'd*
http://geek.scorpiorising.ca

here's a thought, and it is something that i have done.

get a grunty 6.3v filament transformer, enough to drive the heaters
for your project,

then get a smaller 6.3v transformer, and connect the 6.3 secondaries
in parallel.

if you can find a transformer with
uk/nz/european/australian/singaporean primary, 230V, use that as the
second transformer, et voila, easy plate ht transformer. yoiu may
even find a filament transformer with split primaries, ie 115/115 if
you are in the states, these can be connected in series to give you
230V. failing that, ( I am assumming that you are in the USA) do the
voltage double thing on the rectified primary of the 2nd tranny.

make sure that the VA rating of the first transformer can take the
current for the heater and the secondary transformer.

another idea is to get a step down isolating transformer from a tool
warehouse, connect it the wrong way round and rectify the output,
these things can usually be bought second hand at a sale, auction.
car boot sale etc. i got one here, 230-115 volts for an valve
tester, cost me $19NZ, about $10US.

hope this helps.

kind regards

bill ramsay

iPS. t does not matter if the transformer is a toroid, EI etc, in
fact you can use one of each if you wish.


On Mon, 06 Oct 2003 04:25:27 GMT, Robert Casey
wrote:

It's kinda tough to find "plate transformers" anymore. Yes, there's
mail order, but try this:

There's those "semi toroidal" transformers with 4 bobbins, a pair
on one side, and another pair on the other side. Square
looking things, using C and I laminations. 120/240V
primary, and whatever secondaries at X volts x twice the amps
or twice the volts at just the amps. Thing is, there are two separate
primary windings, and two separate secondary winding. What you
could do is use just one primary winding to connect to the powerline,
and use the other "primary" winding to get some B+ after rectification.
And the secondaries can feed tube heaters. However, you need to
select a transformer that has twice the VA rating of the amount of
VA's you expect to consume. Because you're using only half of
the normal primary windings, and the one winding you do use for
the primary will do all the work instead of half the work in a
"normal" application.

Use a voltage doubler rectifier circuit if you want something
like 300VDC instead of 150VDC.

You probably don't want to use a transformer that has on a
single bobbin the 120/240 dual primary windings. As the
insulation between the two primary windings wouldn't be good
enough to pass "hi pot" requirements (think transient spikes and
arc over). That's why I specified the square toroidal transformers
with 4 bobbins. The kind with C and I laminations, not E's and I's.
The "flat pack" PCB mount kind.

I just get old ones from cheap old radios (nothing thats possibly
collectible)

Hey Robert,
I have done that a few times with various projects.It works fairly
well,but you do need to oversize the transformer a bit or the voltages
can sag. I have used either half wave,bridge,or voltage doubler
output,and it works great.
I got two "sticks" of about 10 fairly large PCB mount x-fmrs 120/240
to /20V@600ma,just like you mentioned. I usually just use a
7805,with a diode or two in the ground leg for the 6.3v filaments of
small projects. Maybe use the other 10V winding for bias (if you don't
need it for fil's.) Works great for preamps,headphone amps,effects
pedals,and small RF circuits. ;-)

Patrick.



Robert Casey wrote in message ...
It's kinda tough to find "plate transformers" anymore. Yes, there's
mail order, but try this:

There's those "semi toroidal" transformers with 4 bobbins, a pair
on one side, and another pair on the other side. Square
looking things, using C and I laminations. 120/240V
primary, and whatever secondaries at X volts x twice the amps
or twice the volts at just the amps. Thing is, there are two separate
primary windings, and two separate secondary winding. What you
could do is use just one primary winding to connect to the powerline,
and use the other "primary" winding to get some B+ after rectification.
And the secondaries can feed tube heaters. However, you need to
select a transformer that has twice the VA rating of the amount of
VA's you expect to consume. Because you're using only half of
the normal primary windings, and the one winding you do use for
the primary will do all the work instead of half the work in a
"normal" application.

Use a voltage doubler rectifier circuit if you want something
like 300VDC instead of 150VDC.

You probably don't want to use a transformer that has on a
single bobbin the 120/240 dual primary windings. As the
insulation between the two primary windings wouldn't be good
enough to pass "hi pot" requirements (think transient spikes and
arc over). That's why I specified the square toroidal transformers
with 4 bobbins. The kind with C and I laminations, not E's and I's.
The "flat pack" PCB mount kind.

Phil Allison wrote:





** One can - the other will drop volts like a stone when under load.

Have you actually tried this ?




Invoking rule 10 for debate cheating: "Your mama!"
Now that is out of the way ;-)
You may be right. I haven't tried it yet, so I need to.

"Robert Casey" wrote in message
...
Phil Allison wrote:


** One can - the other will drop volts like a stone when under
load.

Have you actually tried this ?


Invoking rule 10 for debate cheating: "Your mama!"
Now that is out of the way ;-)
You may be right. I haven't tried it yet, so I need to.



** Recommending something you have not even tried ??

Naughty....



............ Phil

Bill Ramsay wrote:

here's a thought, and it is something that i have done.

get a grunty 6.3v filament transformer, enough to drive the heaters
for your project,

then get a smaller 6.3v transformer, and connect the 6.3 secondaries
in parallel.



I just did this, except with 24V centertaped transformers. One thing I
did was
ground one side of the 24V windings, and tie the 120V secondary of the 2nd
transformer to the top of the 24V windings. If phased correctly, you then
get about 145VAC to make B+ from.
The tubes I used have 12V heaters.

"Robert Casey" wrote in message
...


I just did this, except with 24V centertaped transformers. One thing I
did was ground one side of the 24V windings, and tie the 120V secondary of
the 2nd
transformer to the top of the 24V windings. If phased correctly, you then
get about 145VAC to make B+ from.


** This is a dodgy practice - most AC supply transformers are not built
to be run in the reverse direction.

Reverse operation means the magnetising current is transferred to the
secondary which is not going to be sized to take the extra current, plus
the voltage output with two identical transformers back to back will be less
than the voltage input - ie there is no extra voltage to allow for the
regulation factor of the tranny.

I just tried a 20 VA ( E-core ) tranny in reverse and found that 75% of
the secondary current rating was used up by magnetising current - leaving
only 25 % for useful output.

Toroidals and C cores have way lower magnetising currents - so there is
much less problem with them. The loss of voltage under load is still an
issue though.




........... Phil

Bill Ramsay wrote:

here's a thought, and it is something that i have done.

get a grunty 6.3v filament transformer, enough to drive the heaters
for your project,

then get a smaller 6.3v transformer, and connect the 6.3 secondaries
in parallel.
[...]

I use this approach often. Even the little transformers salvaged from
12-volt, 500 mA "wall warts" work fine for small preamp or stomp-box
circuits. Example: http://www.dogstar.dantimax.dk/tubestuf/mchowit.htm .

By using somewhat larger transformers, even small power amplifiers can
successfully be built. Here's an SET capable of a watt or so:
http://www.dogstar.dantimax.dk/tubestuf/miniblok.htm . For the 3+ watt
push-pull version, I used a couple 24 V, 40 V-A Class 2 "bell
transformers". http://www.dogstar.dantimax.dk/tubestuf/mnibl2-1.htm .
Works a treat.

Beyond that you reach a point of diminishing returns. For not much more
than the cost of two new 40 VA bell transformers you can get an "actual"
plate/filament transformer with comparable capability. But it of course
depends also on what you've got in your junk box, how you do your
shopping, and how much excess iron you're willing to plunk onto your
chassis to compensate for the lower efficiency.

This kind of thing lends itself best to the empirical approach; if you
think it might work, try it and see! I do suggest letting your test rig
run for awhile, to be sure that the first transformer doesn't heat up
excessively.

Cheers,
Fred
--
+--------------------------------------------+
| Music: http://www3.telus.net/dogstarmusic/ |
| Projects: http://dogstar.dantimax.dk |
+--------------------------------------------+

"Robert Casey" wrote in message
...
It's kinda tough to find "plate transformers" anymore. Yes, there's
mail order, but try this:

There's those "semi toroidal" transformers with 4 bobbins, a pair
on one side, and another pair on the other side. Square
looking things, using C and I laminations. 120/240V
primary, and whatever secondaries at X volts x twice the amps
or twice the volts at just the amps. Thing is, there are two separate
primary windings, and two separate secondary winding. What you
could do is use just one primary winding to connect to the powerline,
and use the other "primary" winding to get some B+ after rectification.
And the secondaries can feed tube heaters. However, you need to
select a transformer that has twice the VA rating of the amount of
VA's you expect to consume. Because you're using only half of
the normal primary windings, and the one winding you do use for
the primary will do all the work instead of half the work in a
"normal" application.

Use a voltage doubler rectifier circuit if you want something
like 300VDC instead of 150VDC.

You probably don't want to use a transformer that has on a
single bobbin the 120/240 dual primary windings. As the
insulation between the two primary windings wouldn't be good
enough to pass "hi pot" requirements (think transient spikes and
arc over). That's why I specified the square toroidal transformers
with 4 bobbins. The kind with C and I laminations, not E's and I's.
The "flat pack" PCB mount kind.


Hi Robert -
How's about an industrial microwave (oven) transformer? (only half-kidding).
Anyhow, I've sucessfully used both isolation transformers (I've seen a
homebrew where the guy used 3 in series on the secondaries), primary
windings on the 240/120 wired transformers as isolation transformers (have
to be 2 windings, which they usually are, to use all the primary turns for
120), and the 120-240 step-up transformers (common & cheap around here,
bign chinatown), though many times they're not true isolation, but CT'ed -
the CT is usually trivial to split, without any problems. I'm sure that
theoretically this is a no-no, especially in the step-up transformers, where
the windings are stacked on top of each other without proper insulation
(like tape or paper), but, in reality, no problems.
Now the microwave transformer... Just scavanged one... What a great
start for a HT power supply...
-dim