Audio Research VT150SE Output Tubes

[JAN Amps]

I’m getting ready to replace the output tubes on my Audio Research VT150SE amplifiers and, I was wondering if anyone out there is using KT120 tubes, in place of the 6550, in these amps? I’m aware of the fact (and I'm somewhat concerned), that they do draw a higher heater current. During my conversation with ARC, they said it’s “not supported”, on that product. I’m not sure if “not supported” means don’t use them or, it’s just their safe answer to a question, that has not been researched. Unfortunately, ARC would not provide any kind of service documentation (such as a schematic), which might have helped me make, a better decision.


Thanks…Gary

 

[dcgillespie]

Gary -- I would offer these considerations:

1. Will they physically fit? That should be easy enough to determine.

2. Will they bias properly within the range of the existing bias controls? Likely, only trying some out will tell this tale.

3. To your original concern -- will their heaters overload the power transformer?

ARC power amps already have the reputation for running as hot as a firecracker, and while I don't know about the VT150SE specifically, anything creating more heat in that kind of environment cannot be a good thing.

Wish I could be of more help, but just offering some points of consideration. I think your second thought represents what "not supported" means though. They've moved on and have either not had the time or inclination to investigate how appropriate these tubes might be in this amp.

Dave

 

[peterh]

I’m getting ready to replace the output tubes on my Audio Research VT150SE amplifiers and, I was wondering if anyone out there is using KT120 tubes, in place of the 6550, in these amps? I’m aware of the fact (and I'm somewhat concerned), that they do draw a higher heater current. During my conversation with ARC, they said it’s “not supported”, on that product. I’m not sure if “not supported” means don’t use them or, it’s just their safe answer to a question, that has not been researched. Unfortunately, ARC would not provide any kind of service documentation (such as a schematic), which might have helped me make, a better decision.


Thanks…Gary

Just curious : Why ? not replace with what the amp is designed for ?

 

[JAN Amps]

Gary -- I would offer these considerations:

1. Will they physically fit? That should be easy enough to determine.

2. Will they bias properly within the range of the existing bias controls? Likely, only trying some out will tell this tale.

3. To your original concern -- will their heaters overload the power transformer?

ARC power amps already have the reputation for running as hot as a firecracker, and while I don't know about the VT150SE specifically, anything creating more heat in that kind of environment cannot be a good thing.

Wish I could be of more help, but just offering some points of consideration. I think your second thought represents what "not supported" means though. They've moved on and have either not had the time or inclination to investigate how appropriate these tubes might be in this amp.

Dave

Dave, thank you for the reply.

Yes, the tubes will physically fit with no problem so, I'm off to a good start.

This amp uses an auto bias control. You manually adjust the bias on one tube and the amp automatically sets the rest of them. Will it work with KT120s? Good question.

My VT150SE do not run anywhere near as hot, as my ARC D90-B. However, that's probably do to the fact that these are mono-blocks so, there's only 4 output tubes per chassis as compared to 8, in the D90-B.

Can the transformer withstand the extra current? I think most likely it can however, having some specs or a schematic, would certainly help with that question.

Thanks again for sharing your thoughts, it was very helpful.

Gary

 

[JAN Amps]

Just curious : Why ? not replace with what the amp is designed for ?

It's my understanding that the KT120 is a far more robust tube, then the 6650. In theory, the KT120 should live a longer life, under the same operating parameters, then a 6550. In my opinion, ARC amps have a tendency to drive output tubes quite hard and when they go, it's not always pretty. If your lucky, it just takes out a resistor but it can get worse from there. ARC now uses KT120 in place of 6550 so, they must believe there's some advantage to using them.

I just want to make sure that I'm making the best choice I possibly can, for this amp.

Thanks....Gary

 

[dcgillespie]

Your description of how the bias works is interesting. Three of the tubes must use separate servo controls, whose reference is established by the manual current flow setting of the one tube. ARC would certainly do something like that. That would tend to put the odds in your favor then that they will in fact bias correctly.

As to heater current draw, you're looking at an increase of about 1 amp in current flow over 6550s. Do you know if the amp was rated to run KT88s? The original heater spec on that tube is 1.8 amps, and it would only be leap of a couple hundred milliamps of heater current from there to KT120s, so that could possibly be a clue as well.

Good luck with it!

Dave

 

[JAN Amps]

As to heater current draw, you're looking at an increase of about 1 amp in current flow over 6550s. Do you know if the amp was rated to run KT88s? The original heater spec on that tube is 1.8 amps, and it would only be leap of a couple hundred milliamps of heater current from there to KT120s, so that could possibly be a clue as well.

Good luck with it!

Dave

I didn't specifically ask ARC if KT88s are approved so, I can't say for sure. However, I'm thinking they would have mentioned it, if there were other tubes that I could use. Then again, maybe not! It's hard to get any information from ARC, unless it's through e-mail. Even then, it's very brief and to the point. If I could ever have a phone conversation with someone at ARC, it sure would help.

Thanks again,

Gary

 

[GordonW]

Here's my take- I'd bet that KT90s would work fine. They have 50 watt plates (whereas the KT88/6550 are rated at 35w normal, 42w max), but the same heater draw as the KT88/6550. I'd think that they'd be more "robust" in the ARC amp than KT88s (since ARC tends to run tubes near the "ragged edge" many times- the extra plate thermal capacity would help the tube run with a higher safety margin).

KT120s? As Dave mentions, you're on your own with the heater draw thing...

Regards,
Gordon.

 

[JAN Amps]

Here's my take- I'd bet that KT90s would work fine. They have 50 watt plates (whereas the KT88/6550 are rated at 35w normal, 42w max), but the same heater draw as the KT88/6550. I'd think that they'd be more "robust" in the ARC amp than KT88s (since ARC tends to run tubes near the "ragged edge" many times- the extra plate thermal capacity would help the tube run with a higher safety margin).

KT120s? As Dave mentions, you're on your own with the heater draw thing...

Regards,
Gordon.

Thanks for the info. If all else fails, it sounds like the KT90s might be a good alternative to the 6650. Or, I might just roll the dice, and give those KT120s a try. I'll see what other information I can dig up, and then make my decision. It just makes me a little nervous, when your talking about amps that in their day, cost about as much as a car.

Gary