Re: 70v. output transformer?
- From: J.P. <jpasano@xxxxxxxxxxx>
- Date: Thu, 07 Feb 2008 06:53:02 -0600
F0535025On Wed, 06 Feb 2008 22:45:00 -0600, flipper <flipper@xxxxxxxx> wrote:
On Wed, 6 Feb 2008 21:57:04 -0500, "Phil S."Schematic?
<psymonds_no_spam@xxxxxxxxxxx> wrote:
"flipper" <flipper@xxxxxxxx> wrote in message
news:sslkq3l4klqh94djsdau4v2g0hn31j3tel@xxxxxxxxxx
On Sat, 2 Feb 2008 13:09:32 -0500, "Phil S."
<psymonds_no_spam@xxxxxxxxxxx> wrote:
OK, I'm a sucker for cheap iron. $20 delivered....two 70v output
transformers on one chassis. I'm assuming the 70v is the input side and
the
outputs are labeled in watts: 1w, 4w, 8w, 16w, 28w (not 32), and 64w.
Clean
and heavy.
All the wires emerge from one side of the bell housing. Is it safe to
assume that the 70v side is the input (primary)? Everything is clearly
labeled. There is no CT, but I thought maybe these might make nice OT's
for
something single ended?
I realize, I may have f***** up on this, but for the price I figured I
couldn't get hurt too badly.
Advice/comments please before I start to apply some voltage to these to
see
what comes out the other side.
They sound like line to voice coil transformers but, from your
description, they appear to be made with the power taps on the speaker
side rather than the more common 70V side.
I use 70V line transformers as OPT on my 6BQ5/6GK6 PP amplifier and
they have the power taps on the 70V side. That way the 4x power tap is
CT.
Since the ratings are 'constant voltage', the impedance is easy to
calculate with (70.72^2)/Po (for a 70V line transformer). For example,
the .625W tap is 8k (one plate) and the 2.5W tap is 2K (CT) and, of
course, common is 0 (the other plate). That assumes you use the
specified speaker ohms and it would change by the ratio if you use
something else (I.E. 4k PP with 4 ohm speakers on the 8 ohm tap).
The 'challenge' is in finding a line transformer with a *low* enough
power tap because that's the maximum impedance available. 1W, like
yours, would be 5k. Or one made for a higher line voltage, like 100V
or 140V (no small task in the U.S.).
You can't do that, though, with the power taps on the speaker,
secondary, side, as appears to be the case with yours, because there's
no tap to sub as CT. The 'wrong side', for your purpose, is tapped.
Line transformers are also not designed to carry DC so they're not
suitable for SE, unless you did something like a parafeed.
As a side note, for PP operation I use a cathode bias current mirror
to keep DC balanced so the net DC magnetic flux is (very close to) 0.
Lastly, it's possible they're autotransformers but you can easily
determine that by ohming the windings.
If you want to play around with a low cost PP amp try the 10W SPECO
T-7010. Very cheap (about 4 bucks) and while the 'specs' are nothing
to write home about (if you could even find any) they do surprisingly
well, for 'Medium-Fi', especially considering the price.
The higher wattage SPECOs only go down to a 5W tap for 1K PP.
Interesting....I think these are autotransformers.
Could be and I suspected that because you didn't mention two commons.
But, then, you might have just not felt it worth mentioning.
On the 1-2-4-8-16-24-64W
side, the ohms to the common are 0.1, 0.2, .0.3, 0.4, 0.4, 0.7 (can't cut it
any finer...) and the common to the 70v side ohms are 2.5 and 2.7. I'm sure
I've got the doorstop.
One common sure sounds like an autotransformer but there's a chance it
could simply have both sides sharing the one common. Ohm from the
power taps to 70V. If there's continuity there then it's an
autotransformer.
It's as clean as if it just left the factory. No
dirt, no goo, no rust. You could almost eat off it. I'll be seeing what
eBay will bring.
Good idea because they're fine for the intended purpose, and there's
plenty of 70V systems out there, so surely someone is looking for just
that sort of thing.
Btw, the el cheapo line transformer sounds better than the specs would
suggest because, for one, as an OPT it's not being operated open loop
and because 'music' isn't a mono tone at max power. There's no 'spec'
for 'music', of course, and modern CDs are often compressed to hell
and back, but 'traditionally' (loose definition) average music power
(in amps of this power range) is on the order of a watt or two or
three, with the rest being transient peaks, so that the little line
transformer can't do full power at 30Hz doesn't have as much 'real
world' impact as the impression one might get from looking at just
'the numbers'.
Might not work as well for a guitar amp because there you are often
working at max power but, then, you wouldn't be going lower than about
80Hz either so, I dunno.
I am using a 5W SPECO (higher impedance) for a sub watt guitar amp,
and it works great there, but I'm using triodes and running it way
under rated (power wise, anyway) rather than max and over.
In addition to speaker out I've also got a built in dummy load (a lot
easier to do when under a watt) and line out so you can run it into a
SS power amp for any volume you like, but with tube distortion. So you
can 'quietly' over drive the heck out of it or blow the walls off.
Runs off a 12VAC wallwart that's internally transformered up to 200V
B+.
.
- References:
- 70v. output transformer?
- From: Phil S.
- Re: 70v. output transformer?
- From: Phil S.
- 70v. output transformer?
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