Re: ARC VT100 repair and test.

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.

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. 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.

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.

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.

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?

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. 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...

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?

cheers, Ian


.

Relevant Pages

  • Re: ARC VT100 repair and test.
    ... perfect balance and everything was within tolerance. ... There was virtually no 2H in the distortion spectra once I de-bugged the amp, ... It isn't UL, strictly, because some of the load is distributed to ... An R69S BMW would have been a far better bike than a bloomin squaffer. ...
    (rec.audio.tubes)
  • Re: ARC VT100 repair and test.
    ... perfect balance and everything was within tolerance. ... There was no great matching of the 6DJ8 used in the amp during the tests I ... It isn't UL, strictly, because some of the load is distributed to ... The use of 8ohms connected across the 4 ohm outlets improves the load match ...
    (rec.audio.tubes)
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    ... Its usually hard to make a triode amp that sounds poor, ... recheck the balance but did not. ... But if you have a capacitor load of say 0.22 uF, and no other load, ...
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  • Re: 6BX7 amp
    ... > Its usually hard to make a triode amp that sounds poor, ... recheck the balance but did not. ... But if you have a capacitor load of say 0.22 uF, and no other load, ...
    (rec.audio.tubes)
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