Re: Signal to Noise Ratio: <P: Iain>



Iain Churches wrote:

"west" <restccq2@xxxxxxxxxxx> wrote in message
news:pfDUh.960$Qp.456@xxxxxxxxxxx

Iain: In your opinion, is the Radford STA 100 the best UL/PP amp, in
that
power class?

It stands head and shoulders above the other I have heard.

If yes, what are some of the reasons that distinguish it from
other well known PP/UL?

Probably the quality of the transformers. As mentioned elsewhere
Arthur Radford was an instrument designer - transformer winding
was one of his areas of expertise.

He may well have had nice trannies.

But the drive amp with ECC88/6DJ8 cascode input and EF184 pentodes in an
LTP
can easily be bettered if the DJ8 is reduced to a simple common cathode
stage with
both halves paralleled, and the LTP built using a CCS and strapped as
triodes.
This will further remove any higher numbered odd order pentode
artifacts.
The STA 100 has 3 nine pin input tube sockets and so a range of
topologies could be tried, and using EL84 in triode instead of the EF184
would make the sound more dynamic IMHO...

Input sensitivity won't be as good, but this doesn't matter because a
preamp is needed,
and 2V sensitivity is OK for a powerful amp.




In the BBC technical spec, Radford states, "the amplifier can
run indefinately at full power into open circuit or short circuit
load"

Hmm, I wouldn't be too blase about this idea.

Clouds of smoke and huge repair costs are usually on the way for those
who manage to design an amplifier
to run at "full power" into a short circuit.

What does full power to a short circuit mean? If there is no voltage
at the load, because the load = 0.0 ohms, there is no power.
But there is a bit of power dissipated in the tubes.
So with NFB, the amp vainly tries to increase its error signal so
a heck of a boost is given to the error signal so the coupling caps
become charged up and bias is effectively increased, and the output
wave into the short circuit is a square wave and the tubes are either
off, or
turned fully on, and most experiments I have done show that
as the load is reduced towards 0.0ohms, pda goes upward and the tubes
will fry,
despite the bias increase due to overload.

Like all old tube amps built before cheap silicon bjts were available,
let alone scrs et all, there was no easy way to provide active shut down
protection for
amps left screaming their heads off to death with a shorted or damaged
speaker or leads.


How does it stand up to a, lets say, Curcio, etc.
modified Dynaco MKIII?

It was built as an improvement on the Dynaco.
Curcio is much later, I believe.

Is it worth building something like it?
I looked at the schematic for a Radford STA-100 and it doesn't
look too difficult

Certainly, if you can get the transformers. The schematic
looks deceptively simple:-) Just a cascode, then a pair of
pentodes for the phase inverter driving the KT88s. Easy
and fun.

but I don't know what OPTs to use.

The OPT was the key component in the whole design.
I sent one once to European winder to be copied. They
reported back that it was "incredibly complex" A Hammond
might not perform as well.

When a tranny winder reports back to a client that something is
"incredibly complex" it is usually total BS.

What it means to be told something is IC is that
perhaps they lack the ability to copy the simple thing thay were asked
to copy,
ie, they are incompetent, or they have not got the time, or they think
you won't pay the re-wind cost for the extra few terminations,
or they cannot figure a way to dumb down the design
and use less turns and random wound windings instead of neat layer
windings
with insulation layers between each layer, and they can't
vacuum impregnate,
and they don't like providing terminations to math
or better the original methods.

The STA 100 circuit diagram showing the OPT layout is
perfectly simple. Nothing is complex. But there are a lot of simple
things in there, and these days it is unprofitable to rewind it.
In 99.99% of instances where I inspect old OPTs from ancient old amps
touted as being marvels, I find they can be easily bettered.

However, Hammond may indeed be inferior.
Sowter or Lundahl is where someone should go and be prepared to
pay more than a penny.

Or perhaps see if you can buy OPTs as spare parts for Manley Labs, CJ,
ARC
amplifiers of the same power.

Patrick Turner.



I have an STA100B which I listen to often. It is
quite stunning. It took me >20 years to find one.
I also have a second amp, stripped down minus
all transformers.

You might be well advised to build something like the
Radford STA25 MkIII. The schematic is on my site
at:
http://www.kolumbus.fi/iain.churches/Schematics/Radford/STA25.gif

These transformers are available to original spec.
I have built a couple of these amps. Now that less input
sensitivity is required for CD sources, a triode can be
used at the front end.

Regards
Iain

Thanks in advance my
friend.


Fondly,
west





.

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