[Patrick Turner]

Snip for brevity,

I mentioned not every hi-fi manufacturer was a crook and then I said
.......
Sometimes perfectly wonderful people can design amps which would
easily withstand one or two serious improvements.

Perhaps but that isn't the topic I've been discussing.

The wonderful people wh have given the world ARC amps could do very
well to improve their products. It is the point I am trying to make,
but I am but a voice in the wilderness......

I managed to complete the servicing on the VT200 this afternoon.

But they have a 27 ohm 1/4 watt plus what appears to be a 1ohm
resistance between the anodes and OPT primary connection. It looks
like an old fashioned carbon comp resistance but may indeed be
inductive, but anyway two had exploded apart from too much heat. I saw
no coil of wire. The other good ones measured 1 ohm.
So during initial testing *measured an average of 67mA of anode
current without the screen current, so maybe the Ia + Ig2 was 75mA and
Pda even higher than I thought it was.

I found that they DON''T have 1 ohm R in the cathode circuits but have
0.2 ohm which are 5 watt rated, well made and none were faulty. Only
one screen resistance had failed. I set the bias by measuring the
voltage across 4 x 0.54 ohm 5W resistors in the anode supplies to each
PP 1/2 of each channel. Bias currents varied by a max of 50% and ther
maximum bias currents determined the final overal setting. Pda is now
less than 20W max per tube.
The tubes were all serviceable. Two which had cooked their series
resistances and one which I found to glow with a red plate all seem OK
and seem stable and happy with the lower bias current.

Glad to hear it.

God only knows why they'd deliberately make it 220VAC, and mark it
220VAC, when everyone 'knows' the harmonized voltage is 230VAC and
their published spec complies with harmonized 230VAC. That is, only
God and you 'knows'.

I don't believe there is any God except the God Of Triodes, and he
sends me a bad electric shock if I ever spout bull****.

I certainly would never want anyone to think I was on a par with some
imagined deity.

I'm just a humble bloke, doin my job, and tryna keep ppl in music
without smoke, noise, and distortion or sullen silence.

My first smoke reduction suggestion would be to match amp to mains, as
in 220VAC isn't 240VAC.

Yes but people don't like to have to spend on having a custom made
transfrormer to achieve what the manufacturer should have provided,
ie, an easy means of matching mains voltage to their product.

All I do know is that ppl here buy ARC stuff and don't care about the
mains voltage and then smoke happens.

Go ahead and tell me how you've done a survey of all ARC amps sold in
Oz and they all have 220VAC markings on them and burned up.

Well, I have at least one colleague who is never surprised with smoke
emanations from ARC amps.

I don't get to see many ARC amps, but those I do see have burned out
parts in them.

And why would they bring you one with no problems? I mean, seriously,
that's like a doctor claiming everyone is sick because that's all he
sees at the hospital all day.

AFAIK, there would be many ARC amps in Oz and which may be teetering
on the edge of their thermal stability.
I'm only one small time audio tech in a small city.
But other people tell me there is an unusually level of heat related
problems in ARC gear.

Some amps get sicker more often than other amps and thus need a
hospital job more often.



The pair of VM220 have a 220Vac sticker. Not very prominent, but there
it is, probably because there are two 110V windings in series.

The question of what a 220VAC amp is doing on 240VAC remains.

Good question. A colleague said ARC has its transformers made by
Magnequest.
Whether that has any relevance is a moot point.
The owner bough the VM220 amps second hand, and from where they came
before he bought them is unknown.
The amps which may have been purchased from China or somewhere with
220V mains.

There is no easy way to reset the mains voltage to suit what we have
here.

This problem is ARC's responsibility IMHO.

ARC should never ever let any amp out the factory gate unless it is
equipped with an easily adjustable mains voltage selector switch, with
selections of 100V, 110V, 120V, 220V , 230V 240V and 250V. This isn't
difficult in such highly priced amplifiers.


Much stuff people don't want elsewhere in the world ends up in Oz
after its has been sold on E-bay.

Well, since that stuff 'from elsewhere' isn't necessarily designed to
operate on 240VAC that's a problem, now, isn't it?

Whatever leaves the ARC factory gate needs to always have easily
adjustable mains voltages for all countries.

I would say much has 110V setting for the USA. Nearly every audiophile
I know has something he bought on E-bay and he has a 240V:110V step
transformer. With preamps it is not critical but with power amps it
only become critical if the mains voltage is too high while ppl ignore
it.

Must be *really* old stuff because my 1940 Sonora radio is rated 117V,
AC/DC.

I can't remember when the US was 110VAC although I can remember it
being *called* "one ten." But that's because people tend to call it
whatever they heard it called so grandpa might have called ...

110V, 115V, 117V, 120V has been used to nominate the USA mains
voltage.

Just what it actually is in any given area is what is really relevant.

Where you havea tube amp with fixed bias, and the Ea rises from say
400V to 450V, and the amp is tetrode, UL, or triode, and Eg2 = Ea,
then at dc operation the anode resistance of 6550 is about 1.2k ohms
at say 45mA of Ia. Pda at 400V would be 400 x 0.045 = 18W. Raising Ea
& Eg2 by +50V will increase Ia by 50 / 1,200 = 41mA. The corresponding
increase in grid bias might be say -47V to -52V and with gm at 0.005A/
V, this Eg1 change reduces Ia by 25mA, but a net 16mA increase of Ia
occurs to give Ia = 61mA, and Pda = 27.5W.
This seems OK for 6550, but where the same grid voltage is used to set
the current in 4 output tubes there may be a a bias current difference
among the 4 tubes of +/- 50%, or from 31mA to 91mA, and then max Pda
could easily be 41W, and I have witnessed one tube with red hot anode
in one of the VT200 monoblocs.
The situtaion is much worse if the initial Ia is set at say 65mA for
where Ea = say 410V with correct mains input voltage.

The VM 220 monos have a screen supply voltage from the same point at
the OPT CT. The screen voltage should be considerably lower than Ea,
say +350V, and be regulated by a SHUNT REGULATOR so that if a tube
malfunctions the extra screen current allows the screen voltage to
sag. The VM220 have their whole speaker secondary winding which is a
match for 16 ohms used as a CFB winding which gives almost identical
operating conditions as a Quad-II amp designed in 1953.
Quad did not bother to regulate either, but there was little need
because they used an Ea of only 350V and had cathode biasing which
tends to make Ra at dc much higher and therefore mains voltage changes
make **** all difference to Ia.

With a regulated screen voltage, the Ra of the 6550 at dc and with an
initial Ia = 45mA becomes about 30k, and hence Ia will change little
with Ea change.
With the addition of non regulated grid voltage bias the change of
bias voltage due to change of mains voltages will further help to
regulate ia and keep Pda of the tubes far more constant than is so far
evident in VM220 and VT200 amplifiers.

Lemme make sure everyone gets the message - ARC amps could be designed
to be far better than they are.

Patrick Turner.