Bob Carver ‘Black Magic’ VTA 20S Tube Amplifier Review from Dagogo.com ...

joeinid said:
Carver’s circuit topology is fairly simple. The input signal first passes through a 250k ohm volume control and is then amplified by one half of a 12AX7. The second half of the 12AX7 is configured as a cathodyne phase-splitter. That’s it—no driver stage is used. The push-pull output stage is pentode connected and uses a pair of self-biased El84Ms per channel. I estimated the anode dissipation to be about 11 watts, which is well within the 14W maximum rating of an El84M. Bob Carver indicated that the bias is actually set for a nominal plate dissipation of 10.5 watts to allow for tube variation. Note that the El84M is a mil- spec version of the standard sovtek El84 with extended voltage tolerance, improved plate dissipation, and rugged construction.

I would not recommend substitution of a standard El84, as it is only rated for a maximum dissipation of 12 watts.


Bob Carver VTA20S ?Black Magic? Power Amplifier | The Absolute Sound
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Oh yeah; well I think 'ole Dick Olsher, is just scurred. ;)

No...it's a point well-taken Joe; and thanks for the heads-up. I'll discuss it, with some gurus; and make sure they don't think I'll blow my little Magic up.
 
Chris

You might want to contact Roger Modjeski or his people at his tube store and see what they say. I know that Roger's tube testing standards are probably the highest in the Industry and he has been selling the same EL-84 based amp for over 25 years with an output of 35 WPC. He may be familiar with Carver's design and can give you some guidance as to what tubes to stay away from.

Music Reference | RAM Tubeworks
 
+1

Excellent advice Jack.

Jack said:
Chris

You might want to contact Roger Modjeski or his people at his tube store and see what they say. I know that Roger's tube testing standards are probably the highest in the Industry and he has been selling the same EL-84 based amp for over 25 years with an output of 35 WPC. He may be familiar with Carver's design and can give you some guidance as to what tubes to stay away from.

Music Reference | RAM Tubeworks
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Hmm...from the man himself:

The Russian EL84M tube types are a copy of the 7189 Military Spec tube type that have a larger bottle, and more double the getters than a standard EL84 tube type...

These EL84M sound better than the standard EL84 tube type, and are highly recommended.

But what fun is it, if I can't r o l l?
 
If by the "man himself" you mean Bob, I wouldn't pay much attention to it. I would seek the opinion of a long term tube amp designer instead of one that moves from one business model to the next. His "simple" circuit should be easily interpreted by someone who has designed tube amps long term or a competent tube amp tech.
 
Jack said:
If by the "man himself" you mean Bob, I wouldn't pay much attention to it. I would seek the opinion of a long term tube amp designer instead of one that moves from one business model to the next. His "simple" circuit should be easily interpreted by someone who has designed tube amps long term or a competent tube amp tech.
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Ouch; harsh words, for Mr. Carver.

But actually...that was lifted from Roger Modjeski's site.
 
Chris

Not being harsh about Bob, but he has in fact "moved around" a lot with a lot of different company structures. As to the tubes if Roger says the EL-84M's are more durable, then I would believe him. In every tube class their were consumer tubes and Military spec'ed tubes. The Mil spec tubes had to have a guaranteed life span according to the contract. Consumer tubes had no such guaranty.
 
Jack said:
Chris

Not being harsh about Bob, but he has in fact "moved around" a lot with a lot of different company structures. As to the tubes if Roger says the EL-84M's are more durable, then I would believe him. In every tube class their were consumer tubes and Military spec'ed tubes. The Mil spec tubes had to have a guaranteed life span according to the contract. Consumer tubes had no such guaranty.
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Well...durability is one thing; but he says...and I quote "these EL84M sound better than the standard EL84 tube type". That's pretty definitive stuff!

Oh well; maybe I'll have to buy a second EL-84-based unit...just to do some rolling :)
 
If the EL-84M's have to meet a contract spec and guaranty then they would be hand picked. The same thing as buying a quad of tubes as opposed to a "matched quad." If you want to try a long term tested EL-84 based amp then try to find a used RM-10. I have owned mine for 14 years and never been disappointed. At 35 wpc, it works with speakers 85db and above. It was a perfect match with Reynaud, Soliloquy, Platinum, Meadowlark and Green Mountain. It even pushed the Vandersteen 2CE Sigs to over 90db in a moderate sized room.
 
Jack said:
If the EL-84M's have to meet a contract spec and guaranty then they would be hand picked. The same thing as buying a quad of tubes as opposed to a "matched quad." If you want to try a long term tested EL-84 based amp then try to find a used RM-10. I have owned mine for 14 years and never been disappointed. At 35 wpc, it works with speakers 85db and above. It was a perfect match with Reynaud, Soliloquy, Platinum, Meadowlark and Green Mountain. It even pushed the Vandersteen 2CE Sigs to over 90db in a moderate sized room.
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Thanks Jack; I'll keep an eye out. If I decide to stray from the BM; I might try a cheapie Jolida JD102CRC.
 
Chris

I think you can tube roll fairly simply with the Carver. I just think he runs his EL-84/6BQ5's at a higher voltage, which means there life expectancy is less. Rolling the driver tubes won't hurt anything they just may not last very long. At the price of some of the tubes mentioned earlier in the thread you just need to know that going in. Since that amp is "auto biasing", what are they auto-biased at. Roger's amp is biased at 30mv. I would suspect the Carver is a bit higher. I still think I would contact Roger directly at his forum on AC and ask him. He seems to be willing to answer any question about anything tube amp related.
 
I didn't realize it's self-biasing so you can't just set the bias low before putting in new tubes and then bring it up to the proper level. Like Jack said, you don't want to go blowing out $100-200 tubes as soon as you put them in.



Sent from my mobile.
 
Update - Omegas are here! But you'll have to give me a few days, to digest and comment; they are not at all, what I expected! :huh:

Besides...I need to build 'em, some proper stands this weekend.
 
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