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ARC VT-100 Mk III, now I'm confused

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Posted on December 31, 2005 at 07:11:19
middleground
Audiophile

Posts: 23409
Location: Sherbrooke, Quebec
Joined: December 23, 2001
Some will say it doesn't take much; probably true.

I just got the new output tubes yesterday from thetubestore.com (great service, third time I order from them; this time and last time I ordered in the morning, they confirmed shipping late afternoon of same day and I got the tubes by Canada Post Expreesspost the very next day!).

I was lookig at the instructions on rebiasing posted by Abe Collins a while back on the Mk II and, as I knew, there are differences between the two models.

I popped open the hood this morning and what I see are four trimpots in light blue all on the same side of the amp (left side) towars the front, whereas the older model had one on each circuit board either side of the amp. What gives? Are two for one channel and two for the other? How does one adjust these trimpots? I understand from the instruction manual that one should: "adjust the bias in 2 places on each channel for 65 mVDC (.056 VDC) between test poits (across 0.5 ohm resistor)", this leads one to believe that of the four trimpots, there are two per side and not fur onthe same side of the amp.

I have not yet bought a multimeter, tweaker tool, clamps, etc. to do this. Many people tell me it's simple to do, I now wonder if I should pay the money to get it done by a tech.

Seemed so easy at first...

 

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Re: ARC VT-100 Mk III, now I'm confused, posted on December 31, 2005 at 08:51:55
mtdking


 
It is very simple. You do need a multimeter. The MkII is different then the MKIII when it comes to biasing them. Call ARC and they will tell you how to do it.

 

Re: ARC VT-100 Mk III, now I'm confused, posted on December 31, 2005 at 08:54:14
middleground
Audiophile

Posts: 23409
Location: Sherbrooke, Quebec
Joined: December 23, 2001
Thanks, but ARC is closed and those new tubes are burning a hole through their boxes as I write!

 

The Mk III apparently works a bit differently, posted on December 31, 2005 at 08:54:28
Bambi B
Audiophile

Posts: 3264
Location: Los Angeles
Joined: March 24, 2005
middleground,

I'm not by any means an expert in this world, and have not heard a VT100/III, but my understanding is that the ARC VT100 MKI and MKII had one trimpot for each pair of 6550s. The assumption was a certain standard of matching of the 2-6550's for each channel, and in MKI and MKII, the one pot adjusts both tubes of each side.

When ARC redesigned the VT100 for MKIII and changed drivers from E88CC to the 6H30, one of the goals was to use those higher current drivers and careful design to extend tube life- both drivers and output. To further this goal, to get everything just right, the MKIII now has four bias trimpots- one for each tube- or two per channel if you like. This would add some fuss to biasing- four measurements instead of two, but as you know, Audio Research will jump in with something they feel improves the gear whenever they feel like it.

Again, I'm a technical Bambi, but that's what I think is going on!

The VT100 (and I've considered changing my D-115 to a VT130) I think is among the best newer Audio Research amplifiers.

What did you use before the VT100 and what other pieces are using with it now?


Cheers,

Bambi B

 

Re: The Mk III apparently works a bit differently, posted on December 31, 2005 at 09:13:25
middleground
Audiophile

Posts: 23409
Location: Sherbrooke, Quebec
Joined: December 23, 2001
Bryston 7-B STs which I plan on keeping and an LS-25 (I removed the NOS Amperex tubes in favour of JJ Teslas recently since the system is being used by the kids for HT) and a PH-3 phono section.

Sources are Rega P-9/1000 arm with Ortofon Kontrapunkt B and Ortofon T-20 Mk II transformer and a Sony SACD XA-9000 ES.

 

Re: ARC VT-100 Mk III, now I'm confused, posted on December 31, 2005 at 13:54:25
AbeCollins
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Posts: 22644
Location: USA
Joined: June 22, 2001
Contributor
  Since:
February 2, 2002
The VT 100 MkII had just ONE bias pot per four 6550 tubes. In other words ONE bias pot per channel for the power tubes.

The VT 100 MkIII one bias trimpot per TWO tubes. This doesn't give you an adjustment per tube but it's better than the MkII which gave you only one adjustment per four tubes. The two tubes should be matched pairs if possible.

You will need a digital multimeter and clip leads and a tweaker tool. The power tube bias adjustments should be fairly straight forward per the ARC Owner's manual.

 

Correct! an important clarification, posted on December 31, 2005 at 14:56:14
Bambi B
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Posts: 3264
Location: Los Angeles
Joined: March 24, 2005
AbeCollins,

Yes- an important clarification.

With the VT100/ MKIII, there are 2- bias trim pots per channel and each controls a matched pair of 6550s, giving the opportunity to use matched pairs rather than matched quads.

My D-115 (1987) with similar output (105W/CH) also uses 4- matched pairs (8) of 6550s that are also biased by pair.


Cheers,

Bambi B

 

Re: ARC VT-100 Mk III, now I'm confused, posted on December 31, 2005 at 18:19:34
boneman
Audiophile

Posts: 57
Location: Washington State
Joined: November 15, 2001
I have the VT100 MKlll and find it quite easy to Bias. As stated by others there is a trimpot per pair of 6550's and the resistors which need to be measured and trimpots are numbered. You should have received a ARC screw driver and bias adjustment tool with the amp. The 6h30's don't need to be adjusted so you just need to bias the 6550's. "For the VT100MKIII adjust the bias in 2 places (ie the numbered resistors and trim pots-this is my add) on each channel for 65mVDC (.065VDC) between test points (across 0.50 ohm restor). Caution: resistor is 420V above ground."-these last two sentences are from the manual. Make sure you are in a zero-signal condition and you have warmed the amp up for at least 15 minutes. Finally the work that Abe did to outline how to bias was extremely valuable to me even though I have the MKIII. Good luck

 

Re: ARC VT-100 Mk III, now I'm confused, posted on December 31, 2005 at 18:29:50
boneman
Audiophile

Posts: 57
Location: Washington State
Joined: November 15, 2001
I found a good multimeter with retractable hook connections to be the most helpful in biasing. I was a total novice when I started and just by following the manual and Abe's tutorial I was able to figure it out for myself. I was and continue to be very careful to watch what I am doing and not get in a hurry.

 

Re: ARC VT-100 Mk III, now I'm confused, posted on December 31, 2005 at 19:40:30
middleground
Audiophile

Posts: 23409
Location: Sherbrooke, Quebec
Joined: December 23, 2001
Thanks, I just looked in with a flashlight and saw that the pots are, indeed, numbered. It’s incredible what you can see just by looking! What with all the grocery shopping, etc. today I didn't have time to buy the multimeter, etc. so the whole exercise will have to wait until after New Year's Day. I bought the amp used and the tool wasn't included. What does the tool that ARC supplies with the Mk III look like? I can see the tool that is used on the Mk II in Abe Collins' instructions, but since the trimpots are now all at the front and the transformer is just next to them I presume the tool must be a lot shorter? Hope I can find something locally in a short, non-conductive screwdriver.

Thanks too all of you for your help.

 

Re: ARC VT-100 Mk III, now I'm confused, posted on December 31, 2005 at 21:35:33
boneman
Audiophile

Posts: 57
Location: Washington State
Joined: November 15, 2001
The tool looks like a tan thin pencil with a small flat screw driver blade on the end. I think it is actually plastic so that it is not conductive with just the very small metal blade on the end which fits the trim pots nicely.
Chuck

 

Re: ARC VT-100 Mk III, now I'm confused, posted on January 1, 2006 at 08:51:43
middleground
Audiophile

Posts: 23409
Location: Sherbrooke, Quebec
Joined: December 23, 2001
Thanks, I'll see what the local shop has.

All the best in the New Year!

 

Here's the Tool, posted on January 1, 2006 at 20:38:23
AbeCollins
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Posts: 22644
Location: USA
Joined: June 22, 2001
Contributor
  Since:
February 2, 2002
Some are black, some are tan, whatever... Basically it's just a plastic shaft with a metal blade protruding out on one side. The other side also has a metal blade but it is recessed within the plastic shaft. This end works great for those tiny trimpot screws. Since the blade is recessed, the tool doesn't slip off the trimpot screw while you're turning it. You can try RadioShack, Jensen Tools, or most electronics shops.

 

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