[James Romeyn]

This is a warning for anyone interested in biamping with tubes on their VMPS ribbons only (this warning is totally unrelated to bass amplification).

I have been around tube nuts for a long time. For many years my reference was a very fine 25w push-pull tube amp on the upper-range (a hand-me-down from Big B), with SS bass monos.  A degreed/professional electronics engineer friend owns the JBL K2 beryllium-diaphragmed horns (beautiful, over $10k, sold only overseas) and builds exotic tube amps as often as you & I get haircuts. One mono pair weighs several hundred lbs. & has way lethal voltages inside (BTW he heard & recommends the deHavilland 845s).  I have had enlightening conversations with Jack Elliano of Electra-Print Audio, maker of some of the finest custom tube amp transformers, circuits, amps, & preamps.

Though this may come as no surprise to you, it was a surprise to me to find out recently that if bass frequencies are fed to the input of a tube amp, there must be woofers hooked up at the amp's ouputs.  

What the heck is he talking about?  Put simply, you may NOT feed a fullrange music signal to your tube amp input, then have no woofers at the output.  I am describing what I thought was a common & acceptable practice for tube biampers: feed a fullrange signal to the tube amp input, then the tube amp drives only upper range drivers.  The tube amp drives a passive high-pass crossover, which filters the bass frequencies, then that filtered signal feeds only the upper range drivers.  In the case of VMPS, that would be the ribbon mids & tweeters from 166Hz up.  The above described scenario is a no-no.  

Here is what results: the tube is fed the bass signals & attempts to multiply the voltage, but there is no correponding woofer load for the tube to drive.  If the input signal is increased to a certain point, the tube could actually arc across its filaments & could eventually burn up.

This explains the several problems I had over the years blowing up a power resistor near a tube, & the blue glowing which would occur within my tubes during certain enthusiastic listening sessions.  When Mr. Elliano explained this to your humble correspondent, I was reminded of an ancient proverb heard years ago: never short a SS amp, always load a tube amp.  

I often wonderd about this proverb, & now after all these years the wisdom appears.  I never remembered reading in any of Bascom King's reviews any warnings about tube amp loads, but now it makes sense.  Yes, if you have the ribbons hooked up, you do have a load, but you don't have a load in the bass range.  If you feed the amp a bass signal, you gotta have a bass load.  Simple, yes?  

Anyway, I spent a long time listing all the many methods I could think of to fix the problem & if anyone wants it, send an email.  If enough of you want it, I'll post it here.  It's kinda long.  The most obvious fix is to use an active XO, but this has drawbacks realtive to fidelity, plus there are complications.  Peace to all.[/i]

[PLMONROE]

Jim, it might well be simpler to post your solution her rather than answer a slew of emails, I for one am very interested. Thanks.
Paul
pmonroe1@cox.net

[Juan R]

I was changing amps in my system, and I liked the biamp mode on the rm-40, my question is, If rm-40 will sound better with the new stereo ampzilla for mid and high, the 2000 for bass, than the 2000 only.

[James Romeyn]

I was changing amps in my system, and I liked the biamp mode on the rm-40, my question is, If rm-40 will sound better with the new stereo ampzilla for mid and high, the 2000 for bass, than the 2000 only.

Juan
I don't think anyone has heard the Stereo Ampzilla yet.  JB said the stereo amp had more recent circuits in it vs. the monos.  From something JB said at CES, I inferred the stereo will have less power but higher quality.  It's reasonable to assume the stereo will be at least as good.  It is not unusual to find power & quality inversely related.  Your proposition seems excellent (stereo for the ribbons w/ monos for the bass).  With the spkr level controls turned up & an appropriate input level attenuator on the stereo amp, the stereo/mono combo will produce higher output & less distortion vs. just the monos.

[John Casler]

I was changing amps in my system, and I liked the biamp mode on the rm-40, my question is, If rm-40 will sound better with the new stereo ampzilla for mid and high, the 2000 for bass, than the 2000 only.

The short answer Juan is Yes.  

All things being equal, sending more clean power to the individual drivers would certainly allow for better performance, via improved dynamic range, and lower distortion.

Spoke with James today and he said the new amp may be available in about 30 days.

[Sa-dono]

Spoke with James today and he said the new amp may be available in about 30 days.

Any chance you will be getting one of these amps, even if to sample? I would love to read what the rest of the mafia thinks

[JoshK]

What I am really surprised is that JB didn't come out with a trinaural (meaning 3 ch) amp to match the trinaural processor.  Or better yet a 3 channel amp with built-in trinaural processor accepting two channel inputs from a preamp.  That would really seize the day with his trinaural idea I would think.

[doug s.]

...The most obvious fix is to use an active XO, but this has drawbacks realtive to fidelity, plus there are complications...

a while back i queried brian cheney re: his opinion of getting a pair of rm-40's w/no passive crossover between woofers & midrange, so i could actively bi-amp.  he said this would only *improve* the sonics, provided i use a quality active 24db/octave x-over.  in fact, he offers such a device himself, an option for the ff-series', but it could be used w/the rm40's.  he said the marchand x-over would be excellent.  he suggested 200hz as a good starting point for setting the x-over...

doug s.

[Sedona Sky Sound]

Hello JoshK,
JB actually has three different multi-channel amps in the works right now. However, for the next month or so I think he is fully dedicated to getting the SoA out the door. Next in line out the door should be a killer pre-amp (CES timeframe?). The first of the multi-channel amps will then come after that.

Julian
www.sedonaskysound.com

[hmen]

Jim,
 I'm a tube biamper and I'm wondering if you plan to post your solution. I'm also very curious to hear what Brian has to say on the subject.

                                       Howard

[cbartolomei]

One solution, the one that I use is to place passive x-overs directly on the inputs of the tube amp (ala Vandersteen).  This also helps to reduce IM distortion that the amp would have produced (according to Roger Modjeski) without it.

[rkapadia@ROOP]

Jim - I wouldn't make such a blanket statement, as this is not necessarily true for all tube amplifiers.  Usually it's the poor circuit implementations that show themselves when passive biamping causes problems.  While certain tube amplifiers may certainly show problems, this isn't true for all of them.  No need to scare people from passive biamping; passive tube/ss biamping is a common process that's been going on for years.  If it is such a widespread epidemic, why haven't we seen hundreds of posts on A.A. or A.R. over the years by now?  A brief search in the archives results in nothing nearly as alarming as the tone of your original post.

Howard - What is your biamp configuration, into which speakers?  Granted, if you haven't experienced any problems, or anomolies yet after long-term use, I'd say chances are that you'll be fine.  

I've also passively biamped over the years, and have yet to have experienced any of the originally mentioned symptoms.

Kind Regards,

Rup

[hmen]

Rup - My preamp is an AES DJH signature model and I'm using 2 Decware Zen SE84C's for the mids and tweeters and a rebuilt Dynaco ST70 for the bass. My speakers are RM2's with auricaps and FST. I've had this configuration for about 4 months and haven't noticed any problems.

[James Romeyn]

Unique Requirement
According to a builder, seller, designer & engineer of premium tube circuits, transformers & amplifiers:  tubes require a load at all source frequencies.  In other words, if a tube amp is fed a full-range signal (including bass frequencies), it must be loaded properly at its output.  This means you may not feed bass signals to the amp & drive only the passive high-pass XOs & mids/tweeters. In this case, the amp has no woofer load to reproduce the bass frequencies at its input.  The tube internal impedance will rise to dangerous levels, eventually arcing or worse.  This explains the problems experienced by myself & others using their tube amps as described.  This is consistent with the old warning that tube amps must always be properly loaded (only played with a load at the outputs).  We just did not know the load must match all source signals.  

A two-pronged solution:
1.   The music signal is high-pass filtered before it is amplified (choose one of two methods below) and…  
2.   Residual unfiltered bass is converted to heat in a power resistor at the amp output.

High-pass Filtering

Active High-Pass Crossover
Requires a high-pass/1st order slope/100-120Hz range.  If the active XO slope is second, third, or fourth order, it may be possible to convert to first order.  Contact the XO manufacturer.  If they don’t help, seek assistance from an outside technician, with the schematic if possible.  The only case I can see it impossible to convert is if the slope is sourced within a chip that cannot be substituted.    

Altered High-Pass Filter Pole
Tube amps have at least one coupling capacitor per channel in series at or near the input.  It is a high-pass filter, blocking DC from the circuit path.  Replacing it with a lower value moves the pole frequency higher.  The amp manufacturer or someone with engineering knowledge may determine the correct capacitor value(s) for the desired 100-120Hz pole.  I know an engineer who may determine correct capacitor values if a schematic is provided.  Obviously, use the highest quality cap you can afford.

Because it introduces no extra active parts, I’d personally prefer the capacitor swap rather than adding an active XO.  I’d definitely use the best caps I could afford.  

Residual Signal
Some bass signals will still be amplified after either of the above steps.  Rather than cause the tube filament impedances to increase, these bass signals must be converted into heat with a power resistor.  The resistor is loaded directly across the amp outputs (speaker binding posts).

We can estimate the ideal values.  The best estimate is to start with a ratio of about 1:4 (midrange speaker impedance vs. the power resistor value).  If the mids are 4-Ohms, a 15 Ohm resistor is a good bet.  It is OK to listen, & try another resistor with a value close to the first.  

Alternatively, though unnecessary, a full audio-range AC Voltmeter & audio oscillator source could pinpoint a good resistor value.  Load the meter across the amp outputs with a 2-Ohm resistor in series.  The goal would be to test different resistor values to achieve as linear of a response as possible, especially in the range at the XO frequency (166Hz) & below.

 Wattage: 1 to 5 watts should be fine.  Make sure the resistor is not touching anything flammable.  After listening at higher levels, test the resistor with a heat-sensitive device.  Liability prevents me from suggesting you use anything except a proper test instrument.  If the resistor is too hot, move up to a higher wattage.            


Please post feedback of your results for our further enlightenment.    I pray for peace in America, this second anniversary of 9/11.

[James Romeyn]

Jim - I wouldn't make such a blanket statement, as this is not necessarily true for all tube amplifiers.  Usually it's the poor circuit implementations that show themselves when passive biamping causes problems.  While certain tube amplifiers may certainly show problems, this isn't true for all of them.  No need to scare people from passive biamping; passive tube/ss biamping is a common process that's been going on for years.  If it is such a widespread epidemic, why haven't we seen hundreds of posts on A.A ...

The two emails I got, plus my own problems, were absolutely consistent with the specified symptoms.  It could be that tube users generally expect more problems than SS.  "Poor circuit implementations" does not define this for me.  I am not a tech.  Jack did not say it is universal, but he also did not list any specific exceptions.  How could someone know from looking at their circuit whether it's "inferior" or not?  If persons notice absolutely no symptoms like blue toobes when pushed, or blown tubes or blown resistors near the tubes, then maybe they are OK.  My amp had these problems.  My circuit is as classic RCA (dual EL34) as there is.  I personally kinda think RCA circuits were generally safe & conservative, but again, I'm no teck, as you can see can't even spell it!  

Is the old addage that tubes must be properly loaded true or false?  If true, what is loading the tube at 40Hz, if there is a fullrange source signal, & no woofers are connected?  At 40Hz, there is no load, & the impedance is close to infinity, no?  If the load is infinity or close, what is going on inside the tube filaments, trying to drive & there is none?  Tubes are voltage devices, the opposite of SS (current devices).  Consistent with this is that SS should never to be shorted (saved by safety circuits).

Cheers & peace to all, second 9/11 anniversary.

[toxteth ogrady]

Based on the description of your amp I suspect you own an Audio Innovations integrated, which is pushing the tubes too hard to begin with. I know this because I own an Audio Innovations S500 that has been heavily modified and also sports the Border Patrol choke regulated power supply (absolutely fantastic by the way).

I'm not trying to discount what you're saying by blaming it on your amp because what you've described makes sense theoretically. But I will inform you that the AI amps at least in stock form do have their share of problems - mainly higher filament voltages. Audio Innovations believed that higher than nominal filament voltages gave better sonics so it was no surprise to find the filament voltage on my amp between 7.0V and 7.5V instead of a safer and more reliable 6.3V. Unfortunately, that design flaw has led to many poor sods watching their tubes glow like nuclear carrots before watching their amps expire in a burst flames.

Border Patrol who does all the repairs for AI gear in the UK recommends swapping the el34's for Sovtek 5881's simply because of their industrial strength and their ability to handle higher voltages - I tried the Sovteks in my amp but they sounded like utter shite, so I chose to correct the filament voltage instead. They are very nice sounding amps though, and with some simple modifications they can really sing. I definitely won't be selling mine.

Incidentally, if I've totally missed the mark guessing what amp you own, just ignore everything I said.

t. ogrady

[doug s.]

i bi-amp w/a mesa baron running my monitors, & electrocompaniet solid state amps on subs.  i use an active x-over, which is a 4th order x-over.  i see no reason why changing to 1st order would offer any reliability improvements - the tube amp isn't being loaded w/signal from the preamp below the x-over frequency, plain & simple.  sharper x-over slope means the tube amp is yust seeing that much *less* signal below the x-over point...

but, theory aside, the amp works fine, & it runs really stable...

if/when i ever can afford someting like an rm40, which would benefit from bi-amping, i will send my x-over back to marchand to be conwerted into a 3-way, so i can triamp, & run the barons on the ribbons.  i will use one ec amp/channel, w/one amps' channel on a sub, the other on the rm40's woofers...  again, i wouldn't anticipate any problems w/the tube amp.

doug s.

[PLMONROE]

Interesting -- For the past year I have passively bi-amped my RM-40's with tubes (VTL-250's) for the mids and highs and solid state (Music Fidelity 300) for the bass with no aparent ill effects. I am interested in hearing more on this because (apart not wanting to chance damaging any equipment) I am in the process of changing to active bi-amplication.

Paul

[doug s.]

paul, according to brian cheney, you will get a worthwhile improvement actively bi-amping.  if yure not using brian's dedicated active x-over (which i believe is adjustable, with 24db/octave 4th order slope), he suggssts a 4th order slope set at 200hz.  he said the marchands would be an excellent choice.

no need to tink about triamping, unless yure gonna get a pair of subwoofers...  doing it as i described above, would yust mean yure vertically bi-amping the bottom two-thirds of the system, w/one stereo amp used for each channel's low pass & mid-pass.  the amps have to be identical.  horizontal amping would have one amp for the subs, & another for the woofers of the rm40's.

doug s.

[zybar]

So what was the final result?

Am I at risk if I try and run my Kora Cosmos Reference Mono Blocs passively on the mid/treble and a ss amp on the bass?

GW