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 NEW  Matt presents bias and operation data for the 6V6 tube in SE operation - 6V6 Single-Ended (SE) Ultra Linear (UL) Bias Optimization.

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PostPosted: 07 Jul 2015, 00:19 
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Joined: 08 Aug 2009, 03:11
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Location: Chilliwack, BC
When it comes to Sansui, or any vintage amplifier, recapping the electrolytics will solve a plethora of problems. However, there are times when the obvious isn't so obvious. I'd like to share some tips discovered via experience.

1) Bias wandering. On the driver boards, there's a TO-1 cased transistor with long leads attached to the heatsink for the driver transistors. Replace it, even if it tests good. You can use a 2N2222 no problem and just compression mount it with a piece of metal and a dab of heat sink goop.
This transistor is prone to failure at the most inopportune time, usually when things heat up.
Another failure mode of this transistor gives the same symptoms as motorboating... it will cause the outputs to lock hard to one of the supply rails, putting full V+ or V- to the speakers, tripping the protector relay to chatter, giving hundreds of watts of "machine gun" coming through the speakers.

2) Everything checks out, but protector still stays engaged. The PCB the output transistors are mounted on, sucks. Hairline cracks in the traces will drive one crazy. The best answer is to solder the thermistor leads "P" and "Q" straight to the thermistor.

3) Testing the bias. When doing the test, you have to have the test probes dangerously close to the spring retaining clip for the driver connector. Just temporarily solder some wires to test points "2" and "4". Set to 35mV at start, then re-adjust after 30 minutes.

4) A note on recapping the regulator PCB - I use Panasonic FC caps. Yes, FM or Nichicon PS are lower impedance, but also so low that you're popping things with surge currents. Remember that even todays worst cap is superior to yesterday's best cap.

5) Replacing the giant can caps. If you find you have to, you're in luck, they still make them! Digikey 493-8984-ND or Mouser 647-LNT2A223MSE. They aren't cheap. But even better news! If you're capable of dispensing with the screw terminals and using a smaller bracket, Digikey 338-1986-ND (no Mouser equivalent) out-specs the original by a good margin and is 1/4 the price!

I hope this information is of some help for someone.

Cheers!

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* Ratings are for transistors - tubes have guidelines*
Home: GeeK ZonE
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PostPosted: 08 Jul 2015, 12:59 
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Joined: 08 Aug 2009, 03:11
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One thing I forgot on the above:
6) The 1uF polarized electrolytic capacitor positions on the meter board (F-2496), the silkscreen is wrong! The polarity of those caps is reversed to the silkscreen.

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-= Gregg =-
* Ratings are for transistors - tubes have guidelines*
Home: GeeK ZonE
Work: Classic Valve Design


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PostPosted: 08 Jul 2015, 16:42 
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Joined: 04 Jun 2008, 20:59
Posts: 4156
Location: Arizona, USA
Hi Good info. I guess I have been fortunate that all my old SS gear is by Marantz. One power amp from 1969 and two receivers from 1976. All have worked perfectly and still do. The only failure in any was the AC power switch in the power amp. An apparently common problem and easy to replace. BTW the amp (a dual 30WPC) is in daily use to drive my 7 cubic foot subs (only 50HZ and down).

Good listening
Bruce

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PostPosted: 08 Jul 2015, 23:24 
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Hehehe, yep! Marantz = power switch replacement.
(and the faulty cap across them)

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* Ratings are for transistors - tubes have guidelines*
Home: GeeK ZonE
Work: Classic Valve Design


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