Audio Research vt100mkll question

Gmorris,
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.

Myron.

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:- www.arcdb.ws/VT100/VT100.html

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

Thanks for your excellent advice.

Myron.

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,
Myron.

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.

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.

Myron.

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

DISASTER NOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO

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 :-).

Myron.

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.

Myron.