[sdiy] Tube question 1: transformers

[Czech Martin]

I just wanted to make sure that my ignorance
is not exploited in $.

Power transformer:

I'm not so sure about the EL95 pentode.
Perhaps I just use the EL84 and turn down
the Va to 200 V, or even less.
The data sheet indicates that the 
construction allows for that.
Another way to reduce output power.

In most shops you find transformers for 230V/40V
at most. Some end at 230V/18V.
Some of them have several taps.
They are cheaper than most "tube" transformers.
Since beauty is no criterion, I could
perhaps use two of them back to back.
It would be ideal to have a beefy 230V/6V
(could deliver filament heating, be it DC or not)
and then one 4,5,6V/230, lower power to step up
again. Ugly, but cheaper.
I would not be afraid to add a few taps on
the low voltage side.
This would give high voltage (which is decoupled
from mains) and selectable voltage with standard
gear. As long as the second transfomer will not
be abused (too much VA), to much non sinuoid input,
or much more then 230V output
effective, this should be allowed.
I could even use another trafo for separate
preamp supply if I can get a cheap one.
No need to waste power with cascaded RC in
the supply, then.

Years ago, there was some remaining stock
vendor (Voelkner) in
West-Germany, do you know any such
shop today?

Output transformer:

Some people say that classical tube amps
have not underdimensioned iron cores.
Some do not aggree.

So far I've only seen ELA transformers
with one primary coil, not suitable
for push-pull pentode stages.
Of course I could use two of them in series...
-no I can't because that would mean
DC current and DC core bias, something
the transformers without air gap aren't
made for...


m.c.


> -----Original Message-----
> From: René Schmitz [mailto:uzs159 at uni-bonn.de]
> Sent: Donnerstag, 28. August 2003 13:12
> To: Czech Martin
> Cc: Sdiy (E-mail)
> Subject: Re: [sdiy] Tube question 1: transformers
> 
> 
> Hi again,
> 
> > I found that output and PSU transformers for tubes
> > are quite expensive, arround 70 EUR.
>  >
> > Is this shameless abuse, or justified pricing
> > for low volume production?
> 
> The production is not mechanized, and involves many
> manufacturing steps, as the windings are "interspaced"
> (verschachtelt). Thus the labor cost is dominating the price I think.
> But this doesn't mean you could wind your own for that price. Simply 
> because you can't get the materials in the small volumes you need. So 
> you end up buying a whole spool of wire, costing you more than the 
> transformer if you buy it.
> 
> Still I think for what you're after you should try one of those ELA 
> transformers. The inductivity would be ok, just these aren't 
> interspaced. (It will maybe only go to 10kHz, but what the heck, the 
> speaker won't reproduce that anyway.) A smaller core will faster 
> saturate, since that is what you're after it doesn't make 
> sense to use a 
> bigger core one to avoid that. (I think you won't be able to saturate 
> the usual HiFi transformers with that EL95 tubes, because 
> there is not 
> enough current.)
> 
> Cheers,
>   René
> 
> 
> -- 
> uzs159 at uni-bonn.de
> http://www.uni-bonn.de/~uzs159
> 
> 
> 
>