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Old 11th August 2006, 07:16 PM   #1
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Default Yamaha VFET amps

I have a couple Yamaha B-2 VFET amps. Uses 2SK76/2SJ26
pairs. The extensive input and speaker switching circuits
have been getting really noisy and intermittent lately and
does not sound tubelike at all lately.(very solid state sounding).
I'm thinking about doing away with the rear connections including
the cheap speaker connectors and removing all switches/relays
from sig path. I'm even thinking about taking both amps
apart if pairs match close enough and paralleling them with
a new drive stage (want to safely drive my Carver ribbons),
maybe even running lower voltage for pure Class A.
Any thoughts about a Zen, balanced Zen or other simpler
input and drive stage?
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Old 11th August 2006, 08:30 PM   #2
anatech is offline anatech  Canada
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Hi coloradosound,
Just a thought. Those amps are worth more as Yamaha's than Frankenamps. The outputs are unobtainium on top of all that.

To play, use fresh, available transistors and a proven design. If something blows up, okay. Fix it. Not possible with the Yamaha outputs.

Quote:
I'm thinking about doing away with the rear connections including
No speaker disconnect protection? It's a brave new world I guess. I wouldn't consider doing that.

In short, fix the Yamaha's, build new amps if you want to.

-Chris
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Old 14th August 2006, 02:24 AM   #3
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Good point on the protection circuit, Chris. Just some thoughts
I had without the schematics in front of me. The switching
circuit for the speaker select is really noise though and I'll
only be using one set of speakers, but still don't think these
would be suitable for driving the Carver ribbons with the
4 ohm rating. Something like 4 stages of bipolars driving
the VFETs also (I think the old "heavy global feedback cures
all distortion" hilosophy was used.
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Old 14th August 2006, 03:16 AM   #4
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"global feedback" philosophy. My spell checker wasn't
working earlier.
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Old 1st October 2006, 01:16 PM   #5
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As someone who uses B-2 amps to drive my main system I, too, would hate to see these 'reworked' with the new Deoxit (CDA or something like that - supposed to be carbon wiper friendly). You can do a good job of cleaning all the switches and contact points. Or simply by-pass the controls you don't want. You can also replace the speaker terminals without butchering the back. The back is hinged, so can be tipped open about 30 degrees to help access. Also, you can pull the covers off the relays and clean the contacts, as well.

I've done a recap on mine. If you want the parts listing of the caps changed you can PM me. Now that I say that, Its been so long since I've been over here I don't even know if that is possible. If not, reply here and I'll post the list. Also replaced the the bias trim-pots with 10-turn Bourns.
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Old 1st October 2006, 08:55 PM   #6
anatech is offline anatech  Canada
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Hi gpdavis2,
Man, I wish people would stop sticking 10 turn pots in amplifiers. I can understand since Yamaha has too much range with most of their adjustments. All a 10 turn pot is , is a pot were the wiper is geared down. The contacts are not rated for DC current in a bias circuit. I would put anything but a standard 270 trimmer in one.

-Chris
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Old 1st October 2006, 09:37 PM   #7
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I see nothing here that states that these 10-turn posts are not rated for DC service. And, in many years of use I've not had one fail (other than a few that started out bad and I did not realize it until they were in the circuit).

http://www.bourns.com/components.aspx?cmsphid=7631383|7163299|895905
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Old 1st October 2006, 09:42 PM   #8
anatech is offline anatech  Canada
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Hi gpdavis2,
Bad link.

Anyway, I've seen various 10 turn pots fail. There is nothing that is poor quality about a 270 pot. I just have a problem thinking that 10 turn pots are better just because they cost more.

Never use a lower wattage part to replace a higher wattage part. Most 10 turn units are rated to a lower power than the part they replaced. I am talking in a general sense here.

-Chris
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Old 1st October 2006, 09:49 PM   #9
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Not sure what's up with the link. Appears the DIY server is truncating it for some reason. Anyway, it gets you to the Bourns homepage as it is and you can click on Products and Trimming Pots to see all (from 1-turn, up). Bourns are high quality and have worked well for me.
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Old 1st October 2006, 09:55 PM   #10
anatech is offline anatech  Canada
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Hi gpdavis2,
Quote:
Bourns are high quality and have worked well for me.
I have no complaint with the brand.

I just bought some 270 models a month ago. For new design. I do use 10 turn trimmers where they are needed. Test gear for example.

-Chris
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