Re: ARC VT100 repair and test.


Patrick Turner wrote:
Bret Ludwig wrote:

Having the fan blow first onto the regulator that gets hot before
wafting around everything else is a good
idea. And indeed waft around is all you'd get,
because it would be difficult to direct air to all the output tubes.

I am in no hurry to copy the circuitry used in the VT100.

Considering that it is a harsh sounding amp this seems wise.

I cannot see why it should be a harsher sounding amp compared to any
other.
At low levels the THD is commendably low like nearly all other
PP amps with 4 x 6550/KT88 tubes.


Analysis isn't something I have spent a great deal of time on any ARC
circuit. What I like to do is to hook up a stereo amp to a guitar cab
(I have a single 15" PV in an Altec green util cab originally made for
a 604) and using a homemade pre run a guitar or bass into it. I am a
shitty guitar player, but no matter. I have a switchable L-pad
arrangement so I can have an 8 0hm load with varying power attenuation.
This is no scientific test but it intuitively tells a lot, particularly
with the tube amps.

I also listen to recorded music through this test rig. There is little
treble of course but it's surprising what it can tell you.

I have heard various ARC, c-j, Quicksilver, VTL, Mc, Marantz, and
modded Dynas through both conventional stereo setups and my test rig.
The Marantz and early VTLs-the current ones stink, the new opt is
worse-are far and away the best. Modded Dynas do halfway decent,
surprisingly. Stock Dynas are for all intents and purposes, guitar amps
anyway. Mc's poop out, but when run with outboard Kepco or Labbda B+
supplies are surprisingly stout. The Quicksilvers are OK until their
tube rectifiers half-mast the power supply. ARCs and c-js are just very
poopy and grating sounding when padded down and running hard.

The all time champion amp I have ever tested this way was a homebrew
built exactly to Acro TO-350 handbook plans but with a huge PS
repurposed from one built long ago by a ham (probably went SK\ before
JFK and MM did) for a modulator running 4E27's. It had one swinging but
two smoothing choles, one for each channel-chassis, and ran 6159s at
745V B+. The balls, definition and dynamics were flawless. Even
driving a pair of (pre-Nautilus) B&W 801s at loud volumes it sounded
fantastic. When it blew up a year later, though, it reduced the
Cornwalls it was driving to cabs-the DC surge took out the OPT's on
both channels, killing the tweeter and xo on one and all the drivers on
the other, plus the entire power supply. It was a smokefest to the max.

.