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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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It is currently 22 Jan 2021, 08:21

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 Post subject: SAE MK31 Service Tip
PostPosted: 31 Dec 2016, 03:40 
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Joined: 08 Aug 2009, 03:11
Posts: 2229
Location: Chilliwack, BC
A pesky thing about these and others is the "SAE 402" chip. It's a CA3086 and there is absolutely no replacement, other than discrete.

Fortunately as the specs show, they're basically a quintet of 2N3904's. The arrangement of the chip pins are as shown in the schematic below (the SAE schematic doesn't tell the whole story, you have to follow the traces).

You can put the transistors on a little perfboard (make them all equal height!) and mount it transistor-side against the heatsink with some thermal goop. Then three wires is all that's needed to a 14-pin DIP header to the pins indicated on the schematic.

The old chip is in physical contact with the heatsink. So you'll have to bend the three used header pins over, trim the rest, then insulate with epoxy, lest it shorts out.

Depending on the hfe of the matched-up 3904's, you may need to make the bias pot bigger... eith with a 500R unit, or just cut a trace and add a 150 ohm resistor. This will allow you to obtain the base-to-base recommended bias of 2V1.

The output transistors don't cross well, but ON Semi MJ11021G and MJ11022G are the drop-in replacement darlingtons.

(I'm putting these service tips here as it's the only forum I go to anymore with a SS section and my own forum is completely cloaked from search engines - and this info needs to get out there, as it doesn't exist!)


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

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