VTA Board Wiring Question

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VTA Board Wiring Question

Postby zonker92 » Fri Apr 15, 2011 12:05 pm

[EDIT: BOB LATINO HELPED ME WITH THESE QUESTIONS OVER ON THE AK FORUM]

Boy, quiet board here today. Anyway, following up on my thread yesterday, if I do upgrade my driver board, I'm leaning toward the VTA, and I have this wiring question. (I've also sent Bob Latino an email about this, but I think he's still on vacation.)

Again, my amp is one that Chris Keller recently restored (he sells them on eBay). It has a new OEM-style driver board. The cap can is a Weber 50-50 (500V) model, with only two “output” lugs, then two more caps are under the chassis. I attach a few photos. The cap can wiring has me confused.

If you look at the pictures below, you can compare the wiring on my amp to the wiring that the VTA board uses, and I'm focusing on the two wires leading from my cap can to my OEM-style driver board.

Here's the stock amp, with two leads coming from the cap can to points 19 and 20 on the stock board:

Image

Here's my amp, with the same two leads coming from my Weber 50-50 can to points 19 and 20:

Image

Here's the VTA wiring diagram, showing one lead going to point 20, and the wire going to point 19 abandoned:

Image

There is now a 2.2k 2w resistor between the two lugs that were connected to the stock board. The instruction manual sets this out at steps 4, 6, and 11 (e).

Am I correct in understanding that I should (1) remove and discard the wire connecting one cap can “output” lug to pin 19 on my board; (2) leave intact the wire leading from the other “output” lug to pin 20 on my board; (3) bridge the two lugs (do I put a 2.2k 2w resistor between them, or just connect them with no resistor?); (4) run the (former) pin 20 wire to the B+ connection on the VTA board, and (5) leave intact and attached every other wire connected to the cap can and the caps under the chassis?

This issue has me scratching my head. And one other thing: it looks like my NFB wires are going directly from the output transformers to the driver board (pins 12 and 13), without linking up to the speaker terminals as in the stock and VTA layout. Does that makes sense? I guess I would just move them over from the stock board to the new one?


Otherwise, I think I’m good. I figure I’ll need to add two test points for the additional two bias pots that I lack, since Chris removed the preamp sockets, but otherwise simply follow the VTA instructions and move the wires over from the old board to the new one, abandoning the OEM bias pots entirely. Any other issues I should worry about, in terms of wiring the new board to my modified amp?

Also, I'm leaning toward getting the VTA high-gain version, even though my Altec 846B (modified to work like Model 19) speakers are very efficient (about 99 dB), simply because I can adjust the gain using the Sonos controller I have connected to it, which outputs about 1.7V max. It's easier to turn it down than up, if I have the low-gain version and it doesn't play loud enough. Does that make any sense?

Thank you for your time!
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Postby dcgillespie » Fri Apr 15, 2011 4:53 pm

To mimic the VTA installation pic in your amplifer, you would:

1. Replace the resistor feeding the Weber can from the tie wrapped cap where the OPTs connect in your amplifier with a straight wire.

2. Connect a 2.2K 2W resistor between the two positive lugs of the Weber 50/50 cap.

3. Take the B+ to the VTA board from the 50/50 cap opposite the terminal where power is connected to it in #1 above.

Any other wiring associated with the positive terminals of the Weber cap would be removed.

No changes would be needed with the NFB wiring, since both the VTA and the original design take the FB from the 16 Ohm tap. The reason it is coming straight from the output transformer lead is because your amp has been modified to have only two output posts. Since one is the common, and the other is wired to the 8 Ohm lead (it looks like anyway), there is no terminal for the 16 Ohm lead to connect to. But the FB comes from the 16 Ohm tap in either case, so the 16 Ohm lead is simply extended back to the board.

Dave
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