Re: Estimating transformer quality by size
- From: Patrick Turner <info@xxxxxxxxxxxxxxxxxx>
- Date: Sat, 31 Dec 2005 16:37:53 GMT
Andre Jute wrote:
> Ian Iveson wrote:
> > Seems to me it should be possible to estimate the likely quality of
> > a competently built amplifier by comparing the size of the OPT with
> > that of the output valves.
> >
> > What would be a healthy volume ratio for EI transformers, and how
> > much would this change with toroids, C-cores, etc?
> >
> > So many ready built amplifiers, particularly from the Orient, appear
> > to me to have very small OPTs.
> >
> > cheers, Ian
>
> This is a smart post. All the good transformers I have ever had (over
> twenty different brands) were hefty. All the lightweight ones were
> crap. Unfortunately, it is also possible to make a big heavy
> transformer incompetently, so we cannot put our minds in neutral and
> judge by the scales alone.
The formula relating turns, core cross sectional area, F, Bmax, and voltage
is at my website under the output transformer page; it has been
addapted to work in millimetres, and derived from RDH4.
Just about all the main amp makers since they started making tube amps have
ignored the recomendations of RDH4 with regard to having
the frequency of saturation at full 1 kHz level at onset of clipping
at F < 14 Hz, which is what everyone *should* aim for.
The specification for the original Williamson OPT
for 16 watts into 10k is in RDH4, and the wt/size is a good example
of where reasonable quality begins.
Hardly anyone complied with the design; too hard to do, too much
labour.....
Quad II and Leak and Dynaco etc sure didn't.
Then in most OPTs there is usually never enough interleaving.
>
> All the military surplus transformers and chokes I ever had, all good
> quality stuff, were hefty.
Some are, and some just ain't, because makers for the military
routinely tried to rip the military off.
I once bought some well potted preamp trannies, and although they are
reliable 1950s gear, the Rout of the DC from a cap input filter is
1,200 ohms, way too high to expect any sort of decent DC regulation.
Really thin wire on the HT secondary has been used, and the core ,material
is not grain oriented E&I or C-core, it must be very plain iron.
Its **so** easy to make trannies that will be far better than
nearly al the old "milspec" gear, which usually was a simple
plain design but better sealed up to keep salt water out
or last a bit better in a tank.
>
>
> All the good modern commercial tube amps I have sport massive pieces of
> iron.
The CJ and ARC amps I have serviced use 30% smaller OPTs for higher power
ratings
than I would ever use. Using the power to weight ration in RDH4,
both these leading brands fall short of the design optimums.
>
> However, at the commercial end, vintage gear now highly regarded didn't
> necessarily have what today we would regard as even adequate iron,
> never mind exceptional. An example that sticks in my mind was is the
> Peerless TFA-204, said to be the official replacement for the WE91 OPT.
> The pair I had, claimed to be wound by Magnequest as a faithful copy of
> the original off the Peerless blueprint were twee things. The primary
> inductance was around 6H ("around" because the pair I had varied by
> 10%).
I have never seen any examples of OPTs made before
today that i would feel happy buying, especially the old junk
as you mention above, and Magnequest perhaps.
Anything made to be a dead copy of some crummy 1950 design is best used
for a boat anchor.
>
> The problem with an undersized tranny isn't just a lack of bass, it is
> that it sounds unbalanced because the human ear expects a balance
> between bass and treble, so that an amp without anything below 50Hz
> should probably not have much extension above about 12kHz.
Well, its a very poor tube amp that won't go below 50Hz, ( -3dB pole ).
Whether NFB is used or not will have a huge effect on BW, and amps with no
loop
NFB may well indeed have bw from only 50 to 12 khz, the 50Hz at
full po being where saturation begins at say 1.2 tesla and the
high leakage L and shunt C cause a pole at 12 kHz.
But at normal usage levels, 1/10 of full po, bass -3dB
is often 5Hz in the OPT due to saturation, and if its still 50Hz,
its because the amp was designed for pa, and has time constants in the
CR couplings between tubes that are all short TCs.
This is particularly true with SE amplifiers.
I have an SET EL34 in my kitchen radio which produces
magnificent table rattling bass from a rather poor OPT taken out of an old
tape recorder; bw with no FB isn't great in the OPT,
but I have 12 dB of NFB loop, and the circuit is critically damped just
right
and so i get about 30Hz to 30kHz poles at 4 watts, more than enough
triode power for the sensitive speaker I am using.
Sound is truly beautiful, bass is edible, treble in only limited by what is
transmitted.
I found the 4.5Kg Hammond 1670 series OPTs for SET
amps to be very good, exceptional compared to the rest
of the range.
But see, I used 1670 for 2A3, only 4 watts, and 4.5Kg to 4 watts
is a very favourable weight; and my measurements found the
winding losses were low, well below the 10% design used by
old makers, only ok for the rated load, and if the load drops to
1/2 the rated like it does in nearly all speakers at some point,
losses increase to 20%, YUK, so having a large core, large window,
and large wire size is imperative to get the best bass,
subjectively or measured, and the best reliablity.
Bean counters in most amp making companies attack the
weight and complexity of the OPTs almost always to save
on weight, something that never sells easily, and causes
the chassis size to be too big, and the shipping costs to be too high.
> One of the
> reasons old tube radios with poor amps and poorer speakers sound so
> good is that the amp and the speaker are so well matched to expectation
> at the ear.
Expectations were never very high in 1950; even a cheap
radio cost a week's pay. Nearly all used a pentode without NFB,
and a speaker whose response treid to compensate for the appalling
BW of the crummy electronics inside the nice looking timber or
bakelite cabinet which itself had resonances that were less disturbing
than todays thin unbreakable high impact plastics.
Sure they souded better than radios made now, until you turn up
the volume past 0.1 watts, which btw was loud, because speakers
needed so little power.
But some of the best radios of 1950 used plenty of NFB
around a pentode + triode amp and had at least a 2 way speaker system
and some electronics capable of 20Hz to 8 kHz
instead of the usual crappy 80Hz to 2.5khz which is all you get
from most AM radios.
But of course *deluxe* radios in the higher quality model range
also had better and far bigger cabinets and some even had FM, usually only
mono.
The FM electronics was bleeding awful in most sets, due to
bean counting dumbing down the design to reduce the tube count.
Ppl paid through the nose for the extra features, some of the large
Grundig radios of the mid 1950s fall into the "deluxe" category,
and they sound better than much other real garbage sold
to the public.
Many SS AM radios have lower bass, but cut off at 1.5kHz
with NFB....its because the IF stage has such poor BW.
But the FM from many a ghetto blaster is often far better
than any old AM radio, despite the plastic sound;
The N&D is simply so much lower than the earlier tube sets....
To make any tubed AM or FM radio sound really well
and measure well you must use techniques you will never find in RDH4
but that's another story...
Patrick Turner
>
>
> Andre Jute
.
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- Estimating transformer quality by size
- From: Ian Iveson
- Re: Estimating transformer quality by size
- From: Andre Jute
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