[Patrick Turner]

snip for brevity,


In the Land of the Free aka as Home of the Brave, *when the mains
voltage is is off 10--20% we use Variacs . And no need to monkey
around with unnecessary 'modifucations' .

Technicians and do it yourselfers, maybe, but the vast majority of
users do exactly what his user does: plug it in and turn it on.- Hide quoted text -


The vast majority of users are technically illiterate, through no
fault of their own.

They cannot be blamed for buying ARC and other major "reputable" brand
names of audio gear while thinking they can use it without special
mains arrangements to get reliability if the mains voltage is 13.7%
above the labelled input mains voltage on the unit they buy, ie,
252Vrms instead of the labelled 220Vrms.

The damn hi-brow makers AND low brow makers SHOULD ensure that users
are made aware of the pitfalls of incorrect mains voltages. But they
all know any mention of anything technical or of any possible negative
outcomes is detrimental to sales figures. They think its better users
burn their gear to bits, with an attitude of utter carelessness about
their customers.

Usually most makers might test their gear during the prototyping
process so that mains voltages of +10% or -10% may be applied
indefinately while retaining 90% of the reliability of the item when
the mains voltage is exactly as specified.

ARC and many other makers in the US and in China etc fail this golden
rule dismally.

When the local shop Duratone Hi-Fi began importing Jolida amplifiers
they found the B+ in their UL amps with EL34 or 6550 would rise to
+530Vdc and way above the normal 470V and with a big increase in Ia,
and Pda went just above rated limits. So after several cloud-of-smoke
events and angry customers they were forced to modify all the stocks
they imported with a changed arrangement of electrolytic caps which
shorted out. But they also then had to re-bias any all amps to a lower
Ia to avoid a high Pda, and this allowed the B+ to rise even more. I
had the job of fixing two Jolidas after OPT failures, probably caused
by tubes saturating, heating OPT primaries, and then causing shorted
turns.
In one case O opened up tha Jolida 502 OPT to see where/how the short
had occurred, and as stunned my the ****ing lousy quality of the OPT
winding method, so it didn't take much to make the OPTs fail.
The Jolida brand is a is a US-China joint venture, and both the
Chinese and their US exploiters don't give a **** about quality.

The correct way to provide for mains voltage selection is as
follows...

There should be TWO mains tranny windings, both meant for 120Vrms, but
so Bmax is 1 Tesla even with 50Hz and 135vrms.
Taps should be placed on both for Common, 100V, 110V, 120V.

Then you can get low loss matches of parallel windings for 100V, 110V,
120V.

Series connection can give 200V, 210V, 220V, 230V, 240V.

B+ should never rise to more than 95% of the Vdc working voltage
rating of electrocaps even if 1/2 the recommended Ia is used for bias
and if the mains voltage rises 5% above the nominal value of the
country in which the amp is used.

So, in Oz, where mains is sometimes 255V, and nominal voltage is
240Vrms, and where cap rating is +450V, then cap working voltage must
never rise to above
+428Vdc. The best way tfor any maker to avoid problems of exceeding
Elcap V rating is to use caps in series, say rated for 350Vdc each so
that rating is +700vdc and there is never a problem. And the easiest
best way to achieve this is with a voltage doubler type of rectifier.
See my website for many examples.

Nicholson brand soldering irons made in China are widely sold in Oz
and cost about $25 for 80W versions. They are designed to run on
220Vac.
But here and in other parts of Oz we have 255Vac mains and the irons
won't last longer than a month if just plugged in here without a step
down tranny or a series resistor, which I uses, in a metal box under
the bench.

I fused 3 Chinese irons years ago until I made the step down
resistance box and then when I know there is about 210Vac at the iron,
they run fine for maybe 2 years or more used daily for long periods in
my workshop, with the hot end inside an open jam tin as a guard
agaisnt this falling on the hot iron.
screwed to the bench.
I had used Weller brand mains irons but could never get more than 12
months from them and they cost 3 times as much as the Chinese types.

The ARC uses hi-temp soldering which probably is Rohs complient, ie,
lead free, and of course difficult to solder with irons meant for for
old 35/65 standard solder.
But with the Chinese irons plugged straight into my 250V mains here I
found I could remove the open circuit cathode and screen resistors off
the ARC pcbs, and then solder in replacements using old type solder.

The ARC VT200 amp has what appear to be 3 Watt wire wound 100 ohms
series screen resistors. There appear to be old style wire wound.
There are carbon composition 1 ohm x 1W resistors between the 6550
cathodes and the OPT speaker secondary winding which is arranged to
provide 10% local cathode feedback to the two lots of 4 x 6550 on each
side of the PP circuit, ie, in a very similar manner to a Quad-II
amplifier.

To generate enough heat in a 1W x 1 ohm R to make it burst into
flames, one might have to have say 2W of power for some time so the
current must be 1.41 Amps, a huge amount considering the tube curves
indicate 0.5A max with grid bias at 0.0Vdc. To make a 3W x 100 ohm
also fuse open with 6W, then I must be 244 mA, way above the normal
6mA ort working screen current. So obviously when tubes decide to go
beserko, rather high amounts of energy are generated in the tube, and
sometimes it is in the form of very strong RF oscillations which the
"stopper" resistors don't stop. Many makers like ARC and and ALL
others afaik just don't give a **** about the damage shorting output
tubes may cause, and they never would bother to fit active protection
schemes against collateral caused by a failing OP tube.

Perhaps the ARC 100 ohm and 1 ohm resistors are *meant to fuse open*,
ie are fusible resistors made to go open but its all secret ARC
business.

On many amps I have put in a normal metal film 270 ohm x 1/4W
resistance between B+ and the screens. If it flames there is usualy
very little damage, and there is no need to replace it with a special
resistance type which is dfficult to get. If Ig2 rises from say 6mA to
12mA, Pd = 0.39W, and the 270 ohms will last OK if the time spent at
0.39W is a small duty cycle.ARC have what looks like a loose fitting
fibreglass sleeve around the 1 ohm cathode resistances. This falls
apart once fried by a flaming resistance. The presence ofthe sleeving
is somewhat useless IMHO, and partly prevents ppl easily measuring Ik
after removing the side covers off the amp to examine the idle
currents in each and every output tube.

Patrick Turner.