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.
I suspect the harshness you hear is from the 6h30's. Call Leonard at ARC service dept - he will give you the straight scoop.
Well, a popular line of thinking is that the MkII with 6922s should be somewhat warmer/sweeter and less edgy (less definined, but more musical?) than the "white-ish/analytic" sounding MkIII. Whether that is because of the use of the 6922s vs 6H30s or whether that is because of other circuit design considerations in the all tube design of the MkII vs the hybrid design of the MkIII is something I can't say - but I'd think you'd want to hear the MkII with the 6922s to see it what is capable of and intended to do from an original design standpoint. Both the MkIII and the MkII should sound very good, but you might really like the MkII if you hear it as it was originally released with the 6922s.
The VT100MKII is not designed to be used with the 6H30 tube. There's a good chance you've damaged the amp by using those tubes in that amp. Also, if memory serves me correctly, the MKII version added a toggle switch to go between RCA and XLR inputs. What you may have is the original version of that amp. In any case, what you are doing is highly ill advised. As Newbee suggests, call Leonard at ARC. He will no doubt be horrified.
You have a wonderful amp. I have owned everything and think the AR is the best. I don't agree that you have damaged the amp. Just call ARC and try the recommended tubes. Enjoy your teriffic system.
Thanks for your replies Guys,
I bought this amp on the weekend second hand. It came re-valved with 6h30's so not my doing. They have only been in there a couple of weeks so hopefully no damage has been done. I work in the Telco industry and there is someone here that I can borrow 8xmatched 6922's off and give them a go. Should be fun. However .....

Common sense tells me that this circuit should work best with the tubes for which it was designed. I just hope that no permanent damage has been done.

I think the Mklll not the Mk11 had the toggle balanced/single ended switch. see:-

I'll give Leonard a call in the morning and see what he has to say.

Thanks for your excellent advice.

Just a word of wise. The VT100 Mk 2 requires a 2 stage bias set up. You must bias the 6922's prior to inserting the 6550's and then set up final bias. Leonard will be able to send you the biasing imformation which does not come with the original manual.
But all things said, congratulations on your purchase.
Yes, be careful, the bias scheme for the 6922 is involved.

Go Here:

very good article on this.


Thanks again guys. I tried to talk to Leornard this morning but he's not in untill Tues. :-) I'll give him a call tonight.

Any thougts on the single ended operation without the balanced jumper pins ?? Should I perhaps be thinking that connections have beed internally modded as the single ended inputs work without the jumpers in place ??

I really hope its not been internally modified and that everything is as it should be.

Thanks agian,
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.
Myron: Please tell us. What did Leonard say?
Hi Guys,
I managed to talk to Leonard who advised that there could be some perminant damage due to the use of the wrong valves, so weeping, with a screwdriver in hand I took the cover off the VT100 and found that the valves were in-fact 6922's not the 6h30's as i'd been advised by the previous owner. So kind of good news there. I suspect that the valves in situ are the original factory supplied valves and hence 5 years or so old and that the previous owner was trying to "up sell" this amp by claiming a recent upgrade etc.(not so good).

I've just bought a matched octet of jj electronics ECC88 so I'm going to replace these and bias up the amp this weekend. Looking forward to it.

After that I'm going to replace the 6550's with a matched octet of jj electronic KT88's which leonard has confirmed will work fine.

I'll let you know how I go.

i really like this amp and have heard it sound great in a well balanced system...

btw, you might find the jj e88's a little lacking in resolution....

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.
Still, the previous owner may at some point have used the 6h30 tube (or done something else equally stupid) and damaged the amp. I'd be very suspicious. I recommend that you send it to ARC -- or someone familiar with the amp -- to make sure everything is OK. Otherwise, you'll always wonder if you're hearing the amp as it should sound. And it really is a superb amplifier.
Thanks for all your advise guys.

Just to let you know that the person who supplied my tubes has matched/supplied them up in relationship to their use in the circuit described in the schematic that Leonard sent me(together with the original biasing instructions).

I'm going to do the job this weekend, I'll let you know how I go. Thanks for your interest and comments.

Ok just in case anyone is still checking this..............


I warmed up the amp and tested across the resister on the Power valves (130Mv) and was only able to adjust to 98 Mv max on right channell and 122mv max on left channell. they'd both been adjusted to 98 mv.

Adjustment on the input output valves were shot too. I adjusted these to within the tollerances specified (60v on relevant testing points)without any drama's but this didn't influence the output of the power vlves at all.(expecetd)

So I guess the short story is that the guy who sold me the amp fibbed about the recent Valve replacement ( bad bad man ).

So my plan is to ship the amp to the local distributor who will revalve and service My amp. It will end up having cost me 4000 USD in the end. So while I'm a bit negative about the experience overall, if I end up with a smooth, lovely sounding amp I still think I will have done OK.

Thanks all for your time. Any encouraging comments would be appreciated :-).


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.
Thanks for the info. The guy who gave me the jj tubes has a test bed that I can use. He has offered to go through the adjustment processes with me. I'll do that this weekend. He also has a large collection of tubes that he's offered to lend me to try, which might be interesting.

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.
Borg,I have a pal with your amp,driving NOLA(ALON)Circe's.Fabulous sounding amp.I have heard it,also,drive Verity Parsifal's to a stunning degree!!

BTW-loved your "classic" matches with MacEnroe,and Connors!!Did you really have icewater in your veins??
Update.............. More Help please.

A technician friend and I have dialed in the 6922's almost perfectly. Plus we tested individual tubes to look at plate current etc. All looks good apart from one 6550c running a bit hotter than the rest. (see I even know the terminology now).And one 6922 being quite suseptable to microphony.

However the problem I have is this.... on the .5 Ohm resister which is the testing point for the bias adjustment on the power tubes I can only get max .098Mv on one channel and .122mv max adjustment on the other channel.

Has anyone had simialr problems ??

By the way Sirspeedy, Thanks for your comments Its a shame my clothing empire wasn't as successfull. Maybe I'll try hair/grooming products next.

Final update...............................

Every thing turned out fine in the end. I've had my amp in with a fantastic Auckland based Technician who has replaced my burned out resisters and set everything up for me. The amp now sounds great. It cost me about $60 to get done I'm super happy. :-).

The things I've learned from this are :

1. JJ EC88's do not work in a setup where they are mounted horizontally eg ARC vt100.

2. Always take out the 6550's when adjusting the pre-tubes. This can potentially save you dollars in the long run.

I'm moving into new house soon and I'm looking forward to setting up my room. In the end I've really enjoyed the whole experience (other than the burning out/ blowing up the screen resisters).

Thanks everyone for your advice and help.


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

Hi! Please help me How can adjustment the bias output tube and pre-tubes.
Thank you