Attic find AMP

[secant]

Hello,
First post ever here. Attic find amp and I am wondering if its worth cleaning up or pulling the transformers....they are UTC LS-35's and quite heavy. Id have to buy all the tubes to determine if it works.

Pics attached. I think this amp is homegrown.

any responses appreciated!

Thank you,
Brian

[Zen Mod]

naah ........

useless drek

send that to me , for proper disposal

you need to pay just shipping , disposal as favor

[c2cthomas]

Originally Posted by Zen Mod

naah ........

useless drek

send that to me , for proper disposal

you need to pay just shipping , disposal as favor

Shipment to me would be much less expensive!!!

[kevinkr]

Do you have the power supply unit? This is clearly a stereo amp with an outboard supply. It's also missing an electrolytic capacitor. (Sockets for FP style caps are very uncommon.) Looks like it might be a Williamson based on the number of coupling caps, but that is a swag.

[secant]

Hi there.....Thank you.....I dont have a power supply, just the amp......could one be bought or made?
Thank you again,
Brian

[jrenkin]

Where are you located? Maybe another member is near enough to look it over. Not enough info to advise without seeing it better.

[Eli Duttman]

I agree with Kevin about it being a Williamson setup. 2X 6CG7s per channel matches up with the coupling cap. count. That UTC O/P "iron" is exactly the sort of high quality stuff needed to make Williamson style circuitry work well. Also, notice the pairs (1/channel) of test points.

Even without the builder's schematic, a rebuild should not be especially difficult. Get a meter out and start measuring resistance values.

I think a satisfactory PSU could be built around a comparatively inexpensive AN-4TK360 toroidal trafo. The prices of Cree's 2nd generation high PIV Schottky diodes are favorable compared with their 1st gen. offerings.

For O/P tubes, 3 Saratov, Russia, made variants will work well: the "reissue" GEC KT77, the ElectroHarmonix (EH) 6CA7, and the "reissue" Mullard EL34. In the small signal dept., NOS 6GU7s or current production EH 6FQ7/6CG7 stock will do the job.

[dchisholm]

A number of places on the 'net tell you the LS-35's are high-quality output transformers, intended for P-P EL-34's in the 25 - 35 watt range, with taps for several speaker impedances. Sounds like a Dynaco Stereo 70, but your amp is NOT a Stereo 70 clone. The front end tubes are wrong - 6CG7 is a dual triode (a lot like a 12AU7), and there's an unidentified socket on the outboard side of the 6CG7 sockets. (Possibly a home for an 6AU6 or EF86?)

The amp appears to have been built with high-quality components and some attention to detail in the construction. Before I broke it down to salvage the parts I'd spend an hour or two reverse-engineering the circuit, then try to locate documentation.

You may have a copy of the (illegible) circuit shown on pg 19 of the UTC 1963 Transformer Catalog reproduced at < http://www.scribd.com/doc/64202971/U...r-Catalog-1963 >. I don't know if a diligent search can uncover a better image.

The LS-35 specs look similar to the Dynaco A-420 or A-470 specs shown at < http://www.triodeel.com/images/dynaxfmr.pdf >, so your amp's schematic may look a lot like the "improved" Mullard design shown at < http://diyaudioprojects.com/Schemati...-Schematic.htm >.

• The version at < http://www.drtube.com/schematics/ai/ai800.gif > has a different design for the output tubes' cathode circuits.
• Or a variant at < http://web.tiscali.it/audiofanatic3/...ullard_PP1.jpg >, with parts list at < http://web.tiscali.it/audiofanatic3/...ullard_PP2.jpg >.)
• Or the one at < http://diyaudioprojects.com/Schemati...ynaco-A420.png >
• And another at < http://digilander.libero.it/giunchif...EL34schema.jpg > (which doesn't use the ultralinear screen taps on the output transformer)
• And then there is the variant used in Marantz' well-regarded model 8B, with a triode-strapped 6BH6 as the input stage. See < http://zilla.li/Resources/PDF/Marant...ice_Manual.pdf > (schematic, pg 9). A downloadable schematic alone is at < http://ampslab.com/vintage_marantz8b.htm >.

The document at < http://www.sowter.co.uk/schematics/Mullard%205-20.pdf > seems to be a pretty complete description and analysis of the original Mullard design, in case your amp is a near-clone of it. Another version is at < http://www.r-type.org/static/5-20.htm > (get a look at those big paper-in-oil capacitors!). There is a recent, very thorough, analysis and modification project at < http://www.lundahl.se/pdfs/claus_byr...fier_30wpp.pdf >. That article plus your amp could probably keep some hobbyist happily at work for a month or two.

As a possible alternative genesis, one place to find a copy of the Williamson Ultralinear article is < http://www.pearl-hifi.com/06_Lit_Arc...Williamson.pdf >, or an earlier version at < http://www.aikenamps.com/UL.pdf > - though, as already mentioned, there must be dozens of variants of that design floating around. The page at < > includes early links dealing with "Ultralinear" amplifiers in both the technical and popular literature.

Dale

[secant]

Wow guys! Thanks for all the valuable info! And special thanks to Dale and Eli for the extensive info. Doesn't seem as simple a plug in and go but I think I'll give resurrection a shot!
Thank you again,
Brian

[dchisholm]

Originally Posted by dchisholm

. . . The page at < > includes early links dealing with "Ultralinear" amplifiers in both the technical and popular literature.

Oops . . . missed pasting a link. "I just washed my hand, and can't do a THING with it!"

• The page at < ultra-linear > includes early links dealing with "Ultralinear" amplifiers in both the technical and popular literature, and considers more recent analyses at < ultra-linear >.

"secant", if you're so inclined there's enough pieces there, and enough information floating around, to keep you busy with experiments for decades. If you can identify the source of the actual circuit it probably has value to certain people as a piece of history or nostalgia - but that's a marketing problem, and not my area of expertise.


Dale