[Patrick Turner]

Hi all,

I have a pair of VM220 200W monoblock amps here for a service, and a
stereo amp, VT200, 200W per channel.

Both sets of amps belonging to two different owners complained about
overheating tubes and burnt resistors and malfunctions.

My experience is not wonderful with all ARC amps because they end up
in my repair workshop because of their serious design shortcomings. I
have had to totally rewire a VT100 in the past to make it reliable and
easy to adjust bias settings.

But let me talk about the VM220 monblocs first, so that owners in
Australia might at least be able to live more peacefully with these
amps.

The VT220 has a sign on the rear panel which says the mains voltage
needed is 220Vac. There are probably 2 x 110Vac windings which would
be paralleled in the USA with 110Vac mains but here in Oz the nominal
mains = 240Vac which may exist of cold nights but otherwise often
measures 250Vrms during the day.

The result of mains = 250Vac with a mains tranny designed for 220Vac
is that the HT winding voltage rises and the B+ becomes +452Vdc
instead of +400Vdc.
All these ARC amps have screen supply voltage = plate supply voltage
so if the plate supply goes high, then the screen supply goes high
thus raising bias current alarmingly!

The multiple 470uF x 450V rated electrolytic B+ capacitors work with
+452Vdc across them.

The working measured heater voltage rises to 6.7Vac instead of the
correct 6.3Vac.

The amps come from the ARC factory with bias set for the USA
conditions where B+ would normally be just over +400Vdc.
When plugged used in Oz straight out of the box the bias currents in
each 6550 output tube is about 67mAdc, and the Pda per tube averages
about 31W at idle,
I have never seen any publically easily available information for the
biasing of all ARC amps and there is nothing printed on their amps
about biasing, and often the means of biasing is difficult and
extremely dangerous and confusing to many owners. I get so many ****ed
off owners of expensive ARC crap I have to say that ARC is overly
optimistic about their amps being able to survive very long, at least
in Oz.

I'm not sure what the recommended idle current should be in ARC amps
but it seems to me that Pda = 31W is much too high and invites
troubles.

In the pair of VM220 monos, I mounted a 16 ohm x 60W rated resistance
on a heatsink between the mains and output transformers. This is
connected in series with the 340Vrms HT winding and the diode bridge
charging the multiple electrocaps. I then biased each 6550 to 40mA at
idle and B+ settled at +430Vdc which s unlikely to bother caps rated
for 450Vdc. Pda + Pdg2 is now 17.2W, and the amp runs much cooler and
everything will last a lot better.

I will gurrantee the sound will NOT be degraded in any way, and I
found 200W was available at clipping with a sine wave, despite the
added series R in the B+ supply.

The VM220 was only produced between 2004 and 2008, and to me it was a
better design than VT100, VT120, VT200 etc.

Just why ARC would discontunue a better than before design is a
mystery, indicating their designers have **** for brains.

The VM220 has separate RC coupling of 6550 output tube grids to the
6N1P driver tubes. There are 8 pairs of voltmeter jacks allowing easy
monitoring of bias currents while twiddling each one of the 8 bias
pots located near each output tube, via a hole in the top plate.
However, its very easy to not find the pot's screwdriver slot because
their is no guide for a screwdriver, and there should be one along
with a philips headed pot shaft which should of course be metal and
much more rugged. But at least the VM220 has separate pots, one for
each tube, for acuurate bias setting.
The RC coupling is FAR MORE reliable than the stupid VT100 method ARC
has with coupled 6550 grids to driver tube cathodes, with biasing
voltage applied to driver cathode follower grids. When such cathode
follower tubes arc over inside because the driver tubes have too much
Ea betwen anode and cathode, the Vdc applied to output grids can go
very positive, and saturate output tubes which try to conduct 10 times
their idle current thus perhaps causing fuses to blow, but maybe not,
maybe burning out cathode resistors and or screen resistors.

There is no active protection circuitry in ARC amplifiers - its a
hostile form of negligence in this day and age where owners just wanna
plug and play without smoke ever happinging!!!!

Anyway, I suggest my little mod to the VM220 monos will allow them to
last a whole lot better than they otherwise might.

Now the owner of the VT200 just wants me to repair but not modify
because he says there isn't enough power.

The first dreadful and ****ing awful thing about the VT200 is the
weight. What on earth goes on in ARC designer's heads when they fail
dismally to to produce goods which cause serious spine problems every
time some unfit audiophile tries to move such a thing??????

The VT200 is like having 2 x VM220 inside the one box, but instead of
2 power trannies, there is just one big one.
But lifting VT200 is a two person job. The VM220 monos are too heavy,
but trying to deal with something twice this weight is a challenge,
and damned dangerous to health. The sooner someone sues the arse off
ARC, the better.

Anyway, The VT200 had caused at least two clouds of smoke to appear
while being used in Jindabyne, not far south of here.

I remove the 6mm al plate side covers to get access to the underside
of the 2 pcbs where there are 8 x 1 ohm resistors per channel, one for
each output tube, between each cathode and common connection to the
OPT secondary windings which are used for local cathode FB, rather
like Quad-II amps. These cathode R allow the monitoring of bias
current in each OP tube, but its a terrible shame one has to remove a
cover to check the bias current for each tube. Where are the plug in
test points??

I find two of these 2W cathode resistors have burnt to a crisp before
going open, hence there must have been two clouds of smoke.
I replaced one, and turned on the amp.
The tube with its open cathode 1r has no bias current, and I have yet
to see if the tube is stuffed, or the screen series R has also gone
open. But a tube nearby has a red glowing anode, although its current
is only 70mA, so probably its screen has overheated in the past and
screen wires have become warped deformed in the heat and the tube
function is ****ed.
But otherwise I find Ik = 60mA average everywhere with B+ at +445Vdc.
Heaters are at 6.7Vac, tolerable, but should be 6.3Vac.

Immediately I adjust bias down so Ik averages 33mA on the 14 out of
the 16 x 6550 tubes which appear to be working at all.

B= then rises to just over +450Vdc. ARC have all the VT200 tubes
laying horizontally so a natural breeze flows upwards past each tube
allowing good natural cooling. A single fan is fitted. It didn't come
on when the amp is turned on so I guess it only runs if things get
really hot.
I have only had the VT200 turned on for 15 minutes with only 14 tubes
working while my shed temperature is 30C on this summer day. Boy, I
got worried about the VT200 overall temperature. Fan didn't come on,
and even if it did, probably it is as useless as tits on a bull
because fans hardly ever do anything to prevent badly biased tubes.

The VT200 has the same awful crap method of direct coupled 6550 grids
to a 6922 cathode follower tube.
The 6922 have the same stupidly too high Ea to be reliable against
arcing.

The VT200 has a stupidly similar complex driver amp with many solid
state TO92 sized devices, mainly j-fets, for constant current sources.
In the past I have witnessed the failure and fusing of these fragile
bitsa crapology.
When is ARC going to ever learn to do without so many un-necessary
sordid state turds littering its tube boards?
There are also many adjust pots on the input-drive amp pcb and all
should NOT BE THERE, if ARC really knew how to build simply and
effectively.

The owner of the VT200 just wants a repair before selling the crap
ASAP, and without any mods.
This means the Electrolytics will be suject to a working Vdc slightly
above their stated rating.
BUT, at least I will have the damn amp running cooler, and heating of
electros and other circuitry located too close to output tubes will be
less severe. Looks like maybe 3 tubes need replacing. I found Svetlana
winged C 6550 have been fitted, and I don't know if these are
originals, or a set of replacements.

Now in the VT200 there are two bias adjust pots per channel. There are
holes for a small dia flat bladed screw driver to allow adjusting of
the bias pots. There are pcb mounted guide tubes for the screw driver.
But I found it very difficult to get a small screw driver to engauge
the the slot to make the adjustment because of poor alignment of guide
to pot. This would baffle non technical audiophiles.
But worst of all the VT200 only has ONE bias pot to adjust 4 x 6550
tubes, so the bias currents in each of the 4 tubes cannot ever be the
same. Even with a matched quad of 6550, there will be Iadc variations
for the one applied Vdc to the 4 grids. As the tubes age, they become
unmatched and it is not uncommon to find 4 output tubes which are
quite serviceble but which I require grid bias voltages which vary
within a range of say -53Vdc to -60Vdc in order to get the same Ikdc.
4 older but good tubes with the same applied grid bias voltage may
have Ikdc varying between 30mAdc and 80mAdc, so those with 30mAdc are
too cool, and those with 80mAdc are teetering on the brink of thermal
run-away. So one then MUST bias all 4 tubes at an average of say
33mAdc to allow for ageing drift, or for some clown who connects low
impedance insenstive speakers to the wrong labelled outlets, then has
a joint, too many beers, and turns up the volume to a silly level.
This is how some **** happens.
So the bias setting is determined by the tube which conducts the most
of the 4, and one must be sure to measure the Ikdc of each tube, and
all this is a painful thing which causes people to hate ARC.

ARC, it seems, don't need lessons in shooting themselves in the
foot.

I will proceed to repair the VT200, and hopefully it won't smoke while
the owner demonstrates its capabilities to someone willing to buy such
a thing.

I would have to say that on the VT200 ARC have improved their manner
of OPT matching to loads.
There are properly set out taps on the OPT sec to allow many properly
balanced speaker connections.
It means one may connect an 8 ohm speaker to where its says 4 ohms,
retain proper balance, and get good class A performance.

With 8 x 6550 per channel, and with Pda at idle = 31W, ( and plain
wrong, as explained above ), one has total Pda per channel = 248W, and
the maximum possible PURE class A1 is about 44% of 248W or 109W, and
this should be enough for anyone, but of course not for everyone.

The VT200 with high bias current setting will produce a total idling
heat = 700W+ for the two channels including the filament heater power.

So the VT200 and other similar concoctions are excellent amps if you
live in cold climates.

With bias set for 33mA average, the total Pda is only about 462W.

Class AB PO will be the same, but possible maximum pure class A =
approx 55W, also usually enough for anyone.

In all my experiences with local audiophiles there are none who
require 200W capability and most rarely have levels exceeding 5 watts
for more than a split second, so 2 x 6550 per channel for 50W class
AB1 usually gives superb performance and having another 6 x6550 tubes
per channel makes no difference to sound quality.

Regards to all, for the record, Patrick Turner.