Carver and Tube Life

I’ve been looking at Bob Carver’s statement tube amps, the Siver Seven 700’s.  These massive tube amps designed by Bob  have been around for years and the latest version of his 700’s can now accommodate KT120 output tubes and his best output transformer design.

EACH mono block uses 20 KT120’s! That’s 40 KT120’s total when it comes time to ‘re-tube.’  However, according to his description these KT120’s should easily last “50 Years”.  Is that truly possible???  Can the output tubes really last that long ?

Hi Stickman,

Thanks for your question. It so happens that I had addressed that exact question in a post dated 11-17-2017 in this thread, although in connection with a different Carver amplifier. What I said was as follows:

Regarding the statement by Mr. Carver about the 50+ year longevity of the power tubes in his amplifier, I would point out that in general there are statistical approaches that are commonly used in deriving published MTBF (mean time between failure) specifications which produce misleading results.

For example, many of the mechanical (non-SSD) hard drives used in computers have published MTBFs of 1,000,000 hours. Or in the case of some Western Digital Gold Drives I recently purchased, 2,000,000 hours. 2,000,000 hours corresponds to 228 years of continuous operation. Does that mean these drives can be expected to last anywhere close to that amount of time? Of course not. What it means (and I’m choosing the following figures for purposes of illustration) is that if say 100 drives are operated simultaneously one of them can be expected to fail after slightly more than 2 years.

Similarly, **if** Mr. Carver’s 50 year figure is based on similar statistical methodology, and given that there are 12 power tubes in a stereo pair of these amplifiers, it would mean that one tube can expected to fail after a bit more than 4 years.

On the other hand, though, if we assume an average of say 1 hour of use of Mr. Carver’s amplifier per day his 50 year figure would correspond to 18,250 hours. I suppose that if the tubes are driven very lightly, and his design includes good soft start provisions, numbers approaching that figure might not be totally unreasonable. For example, I recall reading that the Western Electric version of the 300B power tube, when used under recommended conditions, can last for 40,000 hours. (I’m not sure, though, if that applies to the original 1930s version, or to the relatively recent reissue, or to both).

In any event, personally I would consider Mr. Carver’s claim as simply indicating that the KT120 and KT150 tubes that are offered with the amplifier will last considerably longer than in most other applications.

Best regards,
-- Al
Al, thanks for that analysis.  It’s difficult enough to pay that much coin for a ‘statement’ tube amp, then spend thousands over the next ten years re-tubing it.  But, if I understand your comments the tube replacement rate for this amp would most likely be considerably less than most high powered tube amps.

On that thought.  If you’re driving a power hungry speaker such as the Magnepan 20.7 or the new 30.7, assuming other parameters are equal does it make sense that an amp like the Silver 7 would lead to more sonic joy?  My current mono amps, the Carver Black Beauty 305’s, pump out about 350 watts into the Maggie’s 4ohm load.  A pair of Silver 7’s would more than double that wattage, close to 900 Watts!  Typically does that doubling of watts lead to a more powerful and bigger sound field in your room?   The 305’s don’t strain or struggle with the 20.7’s, but the 30.7’s add a significant amount of area to the speaker membrane, about 25% more bass panel area specifically.  Maybe more power equals better bass overall, a larger and even denser sound field, and a greater ability to push that Huge Power Response into a bigger room.  My room is fairly large at 21 x 29 x 11.

John (Roxy54), thank you kindly :-)

Stickman, I wouldn’t want to speculate on the differences that would result between the two amplifiers with respect to soundstage, dimensionality, bass, or pretty much any other sonic attribute. But given that your 305s don’t seem to strain driving the 20.7 I would think that whatever differences the Silver Seven 700s may make would result mainly from circuit differences and parts differences (including the completely different small signal tubes it uses), rather than from the 3 or 4 db difference in maximum power capability in itself.

And likewise with respect to the 30.7, **if** its sensitivity (for which I couldn’t find a spec or measurement) is similar to that of the 20.7, or higher.

Best regards,
-- Al
Doesn't Bob offer not only a 20 year warranty on the amps themselves, but tubes also?  That would really ease my mind.

KT120/KT150 tubes aren't cheap, especially if you have to replace a bunch.

+1 on Al, (he's our treasure), no you can't have him :-)

At this retail price point, they’re a must “try at home” before you buy product for me.  Maybe one day...

It was a very sad, but also sweet to see Almarg's contributions to this thread. When we lost him, it was an immeasurable loss.
It was a very sad, but also sweet to see Almarg's contributions to this thread. When we lost him, it was an immeasurable loss.

It was indeed.

Fortunately, his contributions and lessons live on. 
Carver makes a similar claim for his most recent amps. The output tubes apparently are operated at a cooler temperature than other traditional tube amps. The output tubes do get hot to the touch, but not enough to cause a burn. The other tubes are more typical types, such as 12AX7, 12 AT7, probably are hotter and may need replacement in a few years IMHO - never got around to ask Bob.
I cannot comment on the Silver Sevens. I do, however, own the Carver Crimson 275 amp, which carriers a long warranty on tubes.  I have made some amps and modify stuff for fun.  I can tell you that, while I'm not familiar with the circuits involved in that amp, in the 275 he uses a DC restorer circuit that is essentially lifted from television technology of yesteryear.  People were tired, presumably, of their TVs being "on the fritz" so they introduced the DC restorer to that product.  It may have been created way before for all that I know.  

The DC restorer regulates power so that big power tubes-a la KT 120s--can idle at <10 watts.  This results in efficiency and the tubes last for years.  
I have the deepest respect for Mr Carver but guaranteeing tubes for 50 years is a "politician's guarantee"; by the time the obligee calls upon the obligator, the obligator is long gone.