[Housteau]

I have always actively biamped and so this has not been a concern until now.  With the RM-V60 I plan on using my 100W VTL Delux 100 Monos (EL-34s) to run them either to their full range down to 70 Hz, or in conjunction with the PBS.  By that definition, I will be passively biamping to the sub bass system for certain and possibly to the V60 itself as well.  I was reviewing some older threads here and ran across one I had forgotten about until now:

http://www.audiocircle.com/index.php?topic=4483.0

This was first posted back around 8/31/2003.  What I am wondering is from that time until now, has anyone suffered from this potential problem?  I assume that many of you routinely biamp passively with tubes.  At the CES Brian passively biamped with tubes.  Have any of you taken measues to guard against this?  Did you even know of this potential problem?  What are your thoughts on this issue now after so much time has passed.  I don't recall the subject being brought back up again.

Dave

[seadogs1]

I intended to do the passive triamp route with the PBS to the Subs a SS amp passive from 70-280 and then passive with a tube amp for the mids and tweeter. Will this cause TROUBLE? I hope either John or Brian jump on this one soon.

[Bill Baker]

I hope Big B doesn't mind if I post my thoughts on this subject. If so, it could be removed without any ill feelings.

 Being one that deals exclusively with tubes I would have to say this issue would concentrate more on the impedance of the driver(s) being driven by the tube amp. The output transformers of a tube amp do not like impedance loads that are all over the chart. The passive crossover should be adequate enough to keep damaging low frequencies from the midrange and upper end drivers.

 I do not have any personal experience with this type of setup so cannot offer any comments. I have build many tube amps for people running various ribbons including VMPS owners but to my knowledge, when the system is being bi or tri amped, an outboard active crossover is being utilized.

 I can say that I have had to custom build a few units for a passive biamp arrangement and build the unit to be optimized for the application. In other words, it was designed with low frequency limitations built in.

 I will also be eager to hear Brian's comments as it will help me to know what to recommend when building amplifiers for VMPS owners.

[BobRex]

Consider this: a midrange crossover will provide a (theoretical) high resistance at low frequencies.  To the output transformer that will mean virtually no current flow.  So there is no impact on the tube.  Except for the frequency, what's the difference between running the 60s to 70Hz and running a mini monitor? Do you think ever mini-monitor/tube combination blows output tubes?  Here's a hint - I ran Acoustic Energy AE1s for almost 20 years.  First with a Conrad Johnson MV75A1 and then Welborne Moondogs (2A3 SET).  The only output tube that ever blew was one of the  2A3s and that took six years (amps built in 2000, tube blew in 2006), I doubt it was because of the mismatch.  Really, it's not an issue.

[John Casler]

I doubt there is any reason for concern.

Seems to me this earlier thread started in one place and ended in another, with most all saying that there was nothting to worry about.

While many tube afeciondos will tell you that tubes can be finicky, I don't think they need a dummy low frequency load to be safe or perfrom well.

I can't say how many run tubes for the ribbons/tweeter, and SS on the woofs, but it is a lot, and they have done so for years, and years.

[JoshK]

Others are right, there is no alarm for concern.  If you were going to permenantly make the tube amp run only mids on up then the coupling cap inside the amp can be smaller size so that higher end caps for reasonable money can be used, but not necessary. 

[Brian Cheney]

The V60 presents a minimum 4.5 Ohm impedance up to 7kHz where it rises to 6 Ohms.  If your tube amp has a 4 Ohm tap on the output transformer there should be no problem.  The woofer is on a completely separate circuit with its own amp.

The Atmaphere MA2/II we used at CES is an OTL and had no trouble with the low impedance of the main speakers.  Indeed we seldom exceeded a 5W average level even during very loud passages in a 14x19' room.