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PostPosted: December 24th, 2021, 6:34 pm 
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Don't think many others here frequent Audiokarma. A link to this thread at Audiosciencereview came up yesterday and for those who haven't seen it, I thought you might find it...um...interesting, to say the least.

https://www.audiosciencereview.com/foru ... nts.29271/


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PostPosted: December 24th, 2021, 9:29 pm 
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Is that Paul Joppa or Paul Birkeland from Bottlehead?


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PostPosted: December 24th, 2021, 10:53 pm 
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pboser wrote:
Is that Paul Joppa or Paul Birkeland from Bottlehead?


Oh, maybe Paul Birkeland.


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PostPosted: December 27th, 2021, 9:51 am 
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Joined: February 28th, 2013, 1:19 pm
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The schematic of the amplifier in question is posted on Bob Carver, Corp website:

https://9d0691c9-2bb0-4a51-acf2-a4e121c ... 28d241.pdf

There was some speculation about whether there is anything from a circuit perspective to compensate for the low power iron used, but this circuit looks to be a modified Mullard front end circuit with a pentode PP output stage, and with more complex feedback/compensating networks than usually seen.

1. Zobel across the secondary
2. Zobels from the B+ CT on the primary across each primary winding.
3. Fairly complex feedback network from transformer secondary + output to cathode of input tube.
4. A Zobel at the output of the input tube (R35+C18)
5. Compensating network (R20||C22 into R42) at each output tube grid. This is similar to what is done with the Hafler/Keroes 6550 UL Williamson
6. Interesting way of lowering the grid leak resistance when drawing grid current, with (R45+D3)||R42 at the output tube grid. I guess this helps maintain bias stability. This is interesting.

It appears there is alot of work to try to make the small output transformers work. BTW, the biasing instructions indicate that the 4 output tubes are biased to a combined 100mA current, or 25mA per tube (for KT120s). This means the tubes are significantly underbiased by conventional standards.

David


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PostPosted: December 27th, 2021, 11:57 am 
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I'm glad you chimed in, David. Is there any way that amp can make 20Hz at 75 watts? I can't see how. And what is this "tube restorer" feature? Supposedly, if a tube flashes over, it's simply impurities in the tube and the "tube restorer" will cure the problem. How does that work?


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PostPosted: December 27th, 2021, 2:20 pm 
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Grover,

I think the tube DC restorer is nothing more that the fact that the tubes idle operating current is so low, they do not degrade over time. I don't think what Bob Carver describes in terms of a flashover and how to test afterwards is no different than what you do with any other amp. The schematic does not indicate any special circuit that performs this function, except perhaps the grid leak circuit with the diode/33K resistor in parallel with the 68K grid leak, perhaps that performs that function.

As to making 75W at 20Hz, that is hard to believe. A few things to consider:

The transformer input impedance with the Edcor transformer is 2600 ohms. I am not very familiar with tetrode operation, but at least with triodes, one can get more output power (and more distortion) with a lower impedance load, but obviously, the lower inductance implied by the transformer impedance will impact the LF cutoff. Obviously, Carver is using alot of feedback and compensation, so probably is dealing with the distortion aspect. Now if you use a very high permeability material for the lamination, either entirely, or in combination with M6 Silicon Steel laminations (pin-stripping), one may be able to reduce the size of the transformer core. Look at the Table on p.208 in RDH4. Carver's marketing material suggests that special laminations were used. Lower idle current means the copper gauge can be reduced in size (lower DC in the core), and combined with the higher permeability of the core, fewer turns can be used and inductance can be more easily increased for LF response. This will also decrease the transformer size. Of course the governing issue is saturation, however if the 75W is a considered only a transient condition instead of a continuous output, it might just be possible with exotic laminations. I really do not know enough to say, since I am just basing this argument after a scan through RDH4, Chapter 5, Section 3 (pretty dense reading...so much to learn!)

David


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PostPosted: December 27th, 2021, 2:24 pm 
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Should like to add that Dave Slagle can get alot of performance out of a small nickel core (permalloy). Now this is regarding small signal or interstage "iron", I admit.

David


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PostPosted: December 27th, 2021, 6:19 pm 
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Grover,

I read the thread on AudioKarma you posted on and completely agree with your assessment. This just reeks of deception. It looks to me that the dealer, Jim Clark, is in cahoots with the deception. The picture of Bob Carver winding a "transformer" is a joke, it is clear to me that it is a crossover inductor that was stuck on the end of the shaft. Usually there is a tensioner and turns counter, as well as a bobbin to support the winding. And the winding will not be round.. The dealer and /or owner of Bob Carver, Inc. was sending this out to give the illusion Bob Carver was actually involved in test winding.

This kind of thing really torques me off about some commercial audio. The hyperbole is so over the top, and the claims defy physics.

David


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PostPosted: December 27th, 2021, 6:51 pm 
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Aaaaaah! I thought you were just being discreet. ;-) So now you see what has folks riled up.

Paul has actually purchased a production copy (he plans to return it with a generous restocking fee) and will have it for testing by next week.

ETA: Although apparently he had some trouble tracking one down. One dealer told him the amps were "discontinued." (!) Did Paul's test give them the willies?

Good catch on the winding photo op. Pretty embarassing.


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PostPosted: December 30th, 2021, 11:47 am 
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Joined: March 12th, 2013, 11:12 am
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LOL!


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