Re: ARC VT100 repair and test.



Ian Iveson wrote:

Patrick wrote

Thanks, Patrick.

The Ariel square four was beautifully smooth as long as it was kept tuned in
perfect balance and everything was within tolerance. Otherwise it shook
itself
apart.

And the two rear cylinders cooked themselves to death.

OK with 4 separate pipes, normal low-compression and soft tuning.

The VHT needs a matched quad for the fancy dual LTP, otherwise I don't think
the
output will necessarily be balanced, and the cancellation of distortion will
be
incomplete and inaccurate.

There was no great matching of the 6DJ8 used in the amp during the tests I
made
with 6 good Ei 6DJ8 out of the 8 and a couple of olduns from my junk bin,
(even
Telefunkens).
There was virtually no 2H in the distortion spectra once I de-bugged the amp,
so
balance was fine.

That doesn't follow.

Well where the balance is crook the output stage grets driver harder on one side
than the other
so the result is some 2H produced in the output stage.



The THD was very normal for this genre of amplifier, ie, PP, UL and with
aaverage FB
around
a tubed input stage.

What genre? It isn't UL, strictly, because some of the load is distributed to
the cathodes.

VT100 is a 43% UL amp but with the speaker output coil also providing a
small % of the a-a signal as balanced CFB.
At 100 watts into 4 ohms connected to 1/2 the balanced output OPT secondary, there
is +/- 20Vrms applied to each output cathode. Anode load is 1.6k, so Va at each
anode
is 200Vrms, and 86Vrms is applied to each screen.
The cathode FB voltage amounts to 10% of the anode signal voltage, very like Quad II

but the lion's share of the local FB is the screen FB.
But there is more NFB in the output stage than a normal UL amp, i can agree with you
there.
So only 10dB of global NFB is then applied to make a total amount of about 20dB for
the above
load condition, which I maintain is rather poor.
The use of 8ohms connected across the 4 ohm outlets improves the load match
immensely,
and the gain of the output tubes is increased and the amount of UL local screen NFB
and the
effectiveness of the CFB also increases.

There may be the average total feedback for a PP valve amp (what
is the average?) but it is not global, and nor is it applied in a normal way.

The average global + local FB these days for UL PP amps is still regarded as between
16dB and 20dB with rated loads connected to
the load match taps for that load.
In the past many makers tried to use more NFB to get lower thd and sales.
McIntosh have a total of more than 30dB, EAR509 have 44dB total,
and the total varies depending on loading.

Fashion today is slightly varied because a plethora of SET amps have arrived on the
scene
with no global NFb, or only 6dB, so exactly what the average is is a little unknown.

But without any global NFB the VT100 would have Rout = approx 4+ohms, ie, DF = 1.
This is regarded as very poor, so global NFB has been added.
For those with sensitive 16ohm speakers, you could connect them directly to the
4 ohm outlet and disconnect the global NFB and have a DF = 4, quite adequate,
and there would be about 27Vrms available for 45 watts with a high % of pure class
A,
and THD would be no worse than with 4 ohms on the 4 ohm tap with 49 watts and with
global NFB.

Possibilities abound, but methinks the use of a higher than specified value load
connected
to the 4 ohm outlet would improve the sonic performance.




Also, each valve in the arrangement sees a very low resistance at its
anode...a
nearly vertical load line, which will maximise distortion.

The THD of the dual LTPs produce is negligible because the Vout is very low.

That doesn't follow at all. Vout can be made low by using a very small anode
resistor, for example, but that doesn't make distortion negligible. Quite the
reverse.

I can only reveal what I saw in my general test. I had no time to carry out
a full evaluation and analysis which would have taken all day. Who'd pay me for
that?




Altogether it will end up with significant higher order odd harmonics,
particularly in the parts of the bandwidth extended by the feedback.

I didn't see to much evidence of huge amounts of extra odd order thd products.
The biggest factor in the THD wa that the load for the output tubes is 1.6k
a-a for
8 ohms connected to the 8 ohm outlet.
This load is way too low IMHO, hence I suggest never using any outlet except
the 4
ohm one.

You have not measured the distortion components.

I measured the THD and monitored it while measurements were taken.
3H was the dominant THD product with levels of 5H well down, and at all levels of
operation.
This is very easy to see just looking at the CRO wave.
After vewing countless other tube PP amps I ddn't see much different, or anything
warranting a letter of complaint to Bill Johnson.



As you have recommended a different load from that intended by the
manufacturers, I wonder if you have checked if this is in line with warranty
conditions? I also wonder if you have checked the effect on full-power
bandwidth?

The load I have recommended is a higher ohm value one and one which
could only ease the effort by the amps since maximum anode load currents would be
reduced.

If people are going to be so bone headed about loads of if ARC were to refuse to
honour
warranty pledges upon admission by a customer that 8 ohms was
used across the 4 ohm outlet, then all I can say is that everyone can make up their
own minds.



Direct coupling without protection is another bad idea for an amp sold on the
open market.

I think so.

I know, I was agreeing with you,

Like the Ariel, probably the brainchild of a self-indulgent engineer.

An R69S BMW would have been a far better bike than a bloomin squaffer.
Smooth as silk, cool running, easy to work on......

I have an R100RS with Steib sidecar. Usually the only parts of that era BMW
engine/clutch/gearbox that are easy to work on are the cylinder assemblies and
ignition timing, but in my case the chair is in the way of one cylinder.
Gearboxes and clutches can't realistically be worked on at all without special
tools, so ppl replace them as units, which perhaps isn't the kind of easy you
had in mind.

I fully stripped my R75/5 gearbox in about 1975 which decided to
select two gears at once a long time ago.
I couldn't find anything wrong, and after re-assembling and re-installing it so
easily
without any special tools it ran for another 50,000 miles trouble free.
I have no idea what had caused the problem.
I sold the BMW in 1981 for what I had paid for it in 1972.


Someone gave me a copy of "Zen and the Art of Motorcycle
Maintenance" but I couldn't get past a page or two. The comparison between a
Honda and a BMW, maintenance-wise, is silly. Everything about that book is daft.
Even worse than Jonathon Livingstone Seagull, and the Zen of Pooh, or whatever
it's called, but I digress...

All I know is that all the british bikes left a trail of parts behind them
which required owners to stop for a search for footpegs, mufflers,
stands, saddlebags, you name it.
Seizings and chain breaks were common, and the vibration and noise from
the 2 BSAs and Matchless singles and twins I owned were appalling.
Brakes were laughable, and with the BSA these cost me a serious leg injury because I
couldn't
stop as I would have were I riding a BMW.
I didn't read the Zen book of maintenance, just the workshop manuals for the bikes I
owned.
I was the only one who serviced all those bikes. Maybe I should have written a
book...


I also wonder about screen current at high amplitudes, BTW.

Not a huge problem with 43% UL circuits.

Depends, but anyway, one of the distinctions between the ARC output stage and UL
is the increased likelihood of screen current, because of the combined assault
on Vak from the distributed load. I suggest next time you have access to an amp
of similar architecture, you measure it.

BTW, where you say it's not a hybrid because the gain comes entirely from
triodes, is this your own definition, or have you read it somewhere? If I use an
op-amp as a buffer for impedance matching, how does that fit with your
definition? Still not a hybrid?

The use of 5 mosfets in the gain stages of the VT100 is to act as extremely high
impedance
current supplies. There is no AC power consumed by the CCS, no gain
contribution, so the CCS can be considered as an extremely high value of resistance
from cathodes to some imaginary high value negative voltage supply.
Using a 6 mA CCS which may have a finite high resistance value of say 30megohms
is the same as having a 30Meg resistor taken to -150,000V, something that is
entirely
unfeasable to achieve.

But as soon as you have a solid state device acting as a buffer follower or gain
stage
then the circuit is a hybrid, IMHO.

Patrick Turner.



cheers, Ian

.