2. amps-preamps
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Audio Research vt100mkll question

I wonder if someone has experince of this ??

Two questions really.

I've just bought a vt100 mkll and am running it with single ended input from the source , with-out the pins in the balanced input. It seems to work ok just wondered if there was any down side to this ?? Should it work at all ??

Also My amp has been re-valved with 6h30's instead of the original 6922s. Is this a good thing as the 6922's are a lot less current hungry than the 6h30's ?? Will it work properly ??

Over-all the amp is proving harsher than I expected on leading edge vocals and sounding a touch bright. (could the valve be causing this ??) I'm driving proac 3.8s . Bass is good and some things sound very, very good.

Any opinions would be appreciated.

Thanks for your time.
borg7x9

Showing 5 responses by gmorris

gmorris
351 posts
 
I am currently using an ARC VT100 mkII in my system. The 6H30 tubes are used in the VT100 mkIII amp. As mentioned previously, you need a set (8) 6922 tubes. Upscale audio sells the 6H23 (NOS 6922) with the required tolerance/balance that are necessary for the VT100 MKII. Poorly selected input tubes will cause your 6550C output tubes to arc and take out the bias resistor.

The above article by Collins is excellent in providing guidance in setting the bias of your amp correctly.
gmorris
351 posts
 
Borg

I think it is important to note again that the VT100 MKII requires input tubes with closely matched tolerances. You may encounter problems with arcing of the output tubes with a loss of the bias resistors if the tubes are not properly pre-selected.
gmorris
351 posts
 
Borg:

If the input tubes are not properly balanced and adjusted you will not be able to adjust the output tubes to the specified 130 mV.

I would first re-checek the input tubes before shipping off the VT100 for repairs.
gmorris
351 posts
 
You should also check the 1 Ohm resistors R37/R38 AND R43/R44 (in series with the anodes of the output tubes. (Sometimes they look ok from general inspection, but can be defective.)

Also check the 100 Ohm resistors R39/R40 AND R45/R46 (same comment as above)

I keep a bunch of these resistors (can order from ARC). These are the first to go if there is an arching problem.
gmorris
351 posts
 
Borg:

I am happy for you. As I had suggested the resistors were the culprit.
The VT100 MKII is a fabulous sounding amp with surprising power. Enjoy

George
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