Fisher SA-16

for the DIY ST35, the Dynakit and every other PP EL84

Fisher SA-16

Postby kevco » Tue May 03, 2011 1:16 pm

I was looking for an inexpensive point to point wired EL84 amp to potentially recap/modify as A: I've learned enough through this forum and recapping my Fisher 400 to finally feel confident enough to take this on and...B: I've never heard an EL84 amp :( So I've been looking for a beater Fisher SA-16 for cheap to test my skills on. It's got nice big output transformers, indicating pretty good bandwidth. Apparently internet buzz has made this console amp a hot commodity, trading for up to $600. Long story short, I finally located one but it's already been recapped. I picked it up for $360 including a pair of Telefunken 12AX7's, a pair of Mullard 6CA4's and a brand new quad of 6p14EV's. I figured it's fair considering this thing's turnkey, ready to go....But, alas no resto opportunity for me. I don't even have it in my hand yet, as it's halfway between Washington State and me in Pennsylvania. I'll post some photos when I get it, but in the mean time does anybody see anything of note in this low res picture of the wiring? Anything glaring? The gentlemen I bought it from didn't do the work and knows less than I do about what's been done to it.
http://s302.photobucket.com/albums/nn11 ... er%20SA-16
It looks to my untrained eye that the quad cap is at least partially still wired in. I know that an uprated version is available. There's a downloadable .pdf of a slightly modified schemo here, it's the only one I've seen:
http://www.quadesl.com/refurb/refurb_fisherSA16.html
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Re: Fisher SA-16

Postby DerekVa » Wed May 04, 2011 12:07 am

kevco wrote:I was looking for an inexpensive point to point wired EL84 amp to potentially recap/modify as A: I've learned enough through this forum and recapping my Fisher 400 to finally feel confident enough to take this on and...B: I've never heard an EL84 amp :( So I've been looking for a beater Fisher SA-16 for cheap to test my skills on. It's got nice big output transformers, indicating pretty good bandwidth. Apparently internet buzz has made this console amp a hot commodity, trading for up to $600. Long story short, I finally located one but it's already been recapped. I picked it up for $360 including a pair of Telefunken 12AX7's, a pair of Mullard 6CA4's and a brand new quad of 6p14EV's. I figured it's fair considering this thing's turnkey, ready to go....But, alas no resto opportunity for me. I don't even have it in my hand yet, as it's halfway between Washington State and me in Pennsylvania. I'll post some photos when I get it, but in the mean time does anybody see anything of note in this low res picture of the wiring? Anything glaring? The gentlemen I bought it from didn't do the work and knows less than I do about what's been done to it.
http://s302.photobucket.com/albums/nn11 ... er%20SA-16
It looks to my untrained eye that the quad cap is at least partially still wired in. I know that an uprated version is available. There's a downloadable .pdf of a slightly modified schemo here, it's the only one I've seen:
http://www.quadesl.com/refurb/refurb_fisherSA16.html


First of all, nice acquisition!

Second - yes, it looks like the can cap is still in use in some form or another. IMHO, while replacing it with individual electrolytics under the chassis works just fine, it'll look nicer if you replace with an uprated can cap.

Third - Fisher drove its EL84 tubes hard. Even though the prior owner added 6p14p tubes, you may want to consider adding a bucking transformer to drop the line voltage down to 110V. Also make sure you test all the voltages when you first get this amp unpacked. Better safe than sorry.

-D

p.s. If you haven't already done so, you should check out the Fisherforum at AudioKarma. Tons of good information about Fisher there.
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Postby kevco » Wed May 04, 2011 6:53 am

The idea of a buck/boost makes sense, but I'm not so sure I'm going need a dedicated 110v line in my living room. Amps come, amps go...How about this?
http://www.dynakitparts.com/store/product.aspx?id=293
Will it work?
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Postby TerrySmith » Wed May 04, 2011 7:53 am

That's a really weird bias scheme, looks like it puts 10v positive on the grids.

The dynakit bias could work but not necessary.

IF it were my amp the changes I would make:
New, larger electrolytics.
Change the coupling caps to .1uf and grid resistors from 750k to 470k and connect to ground.
Totally get rid of the bias circuit and replace with 470r 3w on each cathode bypassed with a 220uf@35v cap. This way you don't need tightly matched output tubes.

This circuit is really similar to the ST-35 and a ton of other EL84 circuits.
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Postby DeathRex » Wed May 04, 2011 8:06 am

+1. The 6P14P-EV can take alot of B+ (500 max), so higher B+ is OK, but you need to limit the current so plate dissipation stays below 11-12 watts, maybe 25-30ma. Might have to lower the filaments too.
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Postby Ty_Bower » Wed May 04, 2011 11:45 am

kevco wrote:The idea of a buck/boost makes sense, but I'm not so sure I'm going need a dedicated 110v line in my living room.


There's no need to run a dedicated line. I built a nice little bucking autoformer out of an old dead computer UPS. From the outside, it looks like a big power strip. Inside, I rewired the transformer to get 117VAC off my 125VAC line.

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Postby Geek » Wed May 04, 2011 4:05 pm

Not even that complex.... a 120V:12.6V @ 3-5A filament transformer on the primary with its secondary out of phase with the main transformer's primary will do it.

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Postby EWBrown » Wed May 04, 2011 4:18 pm

I'm not that familiar with the SA-16, but I do have a SA-100 which is stereo, PP fixed bias 7189s, one 7247 per channel and 5AR4 rectifier. Price was free (dump swap table foundling). It runs the 7189s pretty "hot", and lesser kinds of EL84s (like J/J EL84s) would not last a long time in this unit.

It is quite happy with 6P14P-EVs, however.

/ed B in NC
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Postby Geek » Wed May 04, 2011 4:42 pm

I was going to suggest 7189's, but true ones (not overstamps) are dang pricey $)

Oh, that "EL84M" that's supposed to be a 7189 replacement? BWAHAHAHAHA! Yeah, I have pulled dozens out of 7189 guitar amps and sent them straight to the bin :/

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Postby kevco » Wed May 04, 2011 9:52 pm

Great! I love this forum! Will upload photos when it comes in and fire it up. Perhaps things will be more obvious then...Thanks to everybody so far and we'll see what I'm gonna do when it does what it's gonna do, which is hopefully...good! And after looking at these thoughts, I see something that maybe wasn't in my mind at first. I need to make a decision as to how far away from a Fisher SA-16's original circuit I want to go. That may not be apparent at first. My experience tells me that the market for antique amps wants "restored enough to be a safe operational unit", while an amp this old with anything other than modest recapping may actually lose quite a bit of value...This isn't something built on a Hammond chassis where you can have it any way you want it, this is a hard amp to find at a fair price. I know. Let's see how it sounds...
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Postby EWBrown » Thu May 05, 2011 7:04 pm

Like any other tubes, the EL84M/6P14P-EVs have a fairly wide range of quality "latitude". And Russian QA standards tent to be a bit more loose than vintage US and Eorupean NOS glassware. They might not take everything that a REAL 7189 can handle at the max ratings, but they are more robust than most of the J/J EL84s under "harsh" conditions. (B+ over 400VDC, or IK over 35 mA)

The SA-100 hasn't yet killed them off, but then I didn't put a lot of hours usage on it after re-tubing it.

The SA-100 isn't 100% "mint" as it is missing the top cage and bottom cover, and some previous owner did a wee bit of chassis metal-hacking in order to run a remote power switch to it. Otherwise it is electrically 100% original, I didn't recap or change anything in it.

At one time I was just going to strip it down and start over, but after seeing what these can fetch on e-bay =:o I decided to leave it be.

/ed B
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Postby DerekVa » Fri May 06, 2011 10:53 pm

EWBrown wrote:I'm not that familiar with the SA-16, but I do have a SA-100 which is stereo, PP fixed bias 7189s, one 7247 per channel and 5AR4 rectifier. Price was free (dump swap table foundling).

I hate you, Ed. (sick)

-D
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Postby EWBrown » Sat May 07, 2011 5:43 pm

The SA-100 was not alone, it also came with a PAS-2 with all four of the original Telefunken 12AX7s, and a very clean mono HH Scott FM tuner, which needed a couple of new tubes in the front end to be returned to normal operating condition.

There were also two really crappy home built speakers, which were basically junk, with cheap Rat Shack speakers, and the woodwork was not of very high quality, just roughly cut and unfinished plywood.

Some "kid" was more than happy to take the two speakers, and he told me, "you can keep that old tube junk"... :/

Little did he know...... In that case, his ignorance WAS bliss, though I was the winner fof that round [:) O:) (lol)

A find like this was a very uncommon, and special event, I lucked out maybe once every two or three years, at best (and I'm sure that I missed a LOT more, as I wasn''t there every weekend).

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Postby Geek » Sat May 07, 2011 6:01 pm

EWBrown wrote:The SA-100 was not alone, it also came with a PAS-2 with all four of the original Telefunken 12AX7s, and a very clean mono HH Scott FM tuner, which needed a couple of new tubes in the front end to be returned to normal operating condition.


I hate you too, Ed (sick)

(lol)

Cheers!
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Postby kevco » Mon May 09, 2011 11:18 am

Ok, here it is at last: http://s302.photobucket.com/albums/nn11 ... r%20SA-16/
Coupling caps are .047 @ 660v orange drops, the quad cap seems to be fully in circuit. The added electrolytic is 100uf @ 100v, I'm hoping somebody can tell me what it's purpose is. I listened to the amp last night for 3 hours and it sounds very good. It really drives my Klipsch Forte II's very well. Bass was very strong, perhaps too strong, I actually had to lower bass response on my pre well into the negative range for all listening. I'm shocked at how much bass these tiny power tubes can put out. Overall though a very musical and pleasant sounding amp. I did notice that after 3 hours that the output trans closer to the quad cap and rectifiers was running quite a bit hotter than the other one. I'm not sure what to think of that. There does seem to be more heat producing mass on that end of things and a couple of large resistors under that portion of the chassis, but the heat seemed to be emanating from the top of the core and the 6CA4's just don't throw heat like an octal rectifier. Any thoughts on this?
Edit: This afternoon I turned it on again and approx 5 seconds after power up, one of the power tubes sparked and produced a pop through it's speaker. It was momentary and I cautiously left it on, all seems normal and has not recurred. Maybe the Russian tubes don't like high voltage after all...
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