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-   -   heathkit sa2 - new question (http://www.diyaudio.com/forums/tubes-valves/85885-heathkit-sa2-new-question.html)

-=Zepplock=- 4th September 2006 03:32 AM

heathkit sa2 - new question
 
I have old heathkit sa2.
Schematics here: http://www.freeinfosociety.com/elect...eathkitsa2.gif

I'd like to rebuild it.
But I figured the only valueable parts are traffos. So is there any good circuat I should follow to use same traffos/tubes and build new amp?

Thank you.

-=Zepplock=- 4th September 2006 07:20 PM

anyone?

It should use EL84 as output tubes and 6AN8 as gain?
My speakers are 91db.

Brian Beck 4th September 2006 08:55 PM

That link didn't work for me.

-=Zepplock=- 5th September 2006 02:58 AM

It works. I double checked.

Brian Beck 5th September 2006 12:17 PM

It says "Forbidden" when I try it. Maybe you have permission that others don't? That may be why you haven't had responses.

Sherman 5th September 2006 12:19 PM

Quote:

Originally posted by Brian Beck
It says "Forbidden" when I try it...

Same here- "Forbidden".

Robert McLean 5th September 2006 12:34 PM

how to get there ...
 
I had the same problem, so here is what I did. I went to the root url, ie www.freeinfosociety.com, and I got in. Then click on the topics there. Eventually I got to

http://www.freeinfosociety.com/elect...ew.php?id=2350

which is the schematic in question.

Basically an EL84 PP UL output, 6AN8 pentode direct coupled to 6AN8triode phase splitter. Nothing wrong in my opinion with the circuit as is, but there should be lots of other UL EL84 circuits out there that could use the same OP transformer and power supply.

One thing I notice is that both Left and Right output stages share the same cathode resistor. Perhaps theoretically this is OK if the 50uF capacitor is "perfect", but what about in practice ? For the price of 1 more resistor and capacitor I think I would give each channel its own.

Robert McLean 5th September 2006 12:54 PM

UL EL84 amps
 
Here are the names of some other EL84 amps using PP UL outputs, you may want to search on the web for these, that is where I found them some time ago

Heathkit UA-1
Sherwood S-5000
Acrosound using TO-310 and T0-320 transformers
Leak Stereo20
Leak TL/12

Brian Beck 5th September 2006 04:29 PM

As Robert says, this is a classic configuration: a basic pentode gain stage followed by triode splitter circuit, driving ultralinear PP EL84s, with global feedback. The shared output tube cathode resistor was also seen in Dyna's EL84 amps, the SCA-35 and ST-35, although in those cases the resistor was 95 ohms and the bypass cap was bigger at 100uF. There’s nothing criminal about combining the four cathode currents, but channel separation and sound quality depends on the quality of the 100uF (or 50uF) cap. There was purported to be some advantage in sharing current through just one resistor from bias averaging, although as I recall Dyna suggested buying matched quads of EL84s anyway.

You have several choices here, just clean up the existing design (new caps and tubes, etc.) and enjoy a classic tube sound, or you can go all the way to the other extreme and salvage only the iron for something different. Unless you really want a project, the first option is not a bad way to go. Many people like the sound of the Dyna ST-35 which used triodes only in the gain stages in the power amp (it used a 12DW7 which is like a half 12AX7 plus half a 12AU7 in one envelop). You could do worse than emulating that since you’ve already got EL84s connected to UL OPTs. You could also look at SY’s Red Light District amp thread here for ideas, although that was a pentode design.


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