Dynaco SCA35

https://www.ebay.com/itm/Dynaco-Dyna-SCA-35-Stereo-Tube-Amplifier-Factory-Wired-SUPER-COOL/303032107484?hash=item468e1f01dc:g:wvoAAOSwGYhcGAQ2:rk:31&frcectupt=true

Note the red-plating 6BQ5 about photo #4.

I have a rather low snipe set for this beast, and over the years, I have accumulated all the various parts-and-pieces as it might need for repair, including boards, transformers and so forth. But, if anyone wishes to save me from myself, you are most welcome...

Should I get it, I do enjoy the challenge! And when restored, I would most likely recycle it to one of the grandkids. The oldest has my Revox system, the next in line has expressed an interest in tubes, and the SCA35 is a very nearly perfect (as in kid, pet and bulletproof) option.

I will not save you. Hope you get it

I just finished rebuilding one for a guy. All new caps on both board and in the power supply. The 95 ohm cathode resistor tested fine but when I inspected the bottom of it I found a very dark area and a crack running thru it. An important resistor that was close to the end of its life. I have a supply of 10 W 100 ohm wire wound resistors that works out great for these. Slightly higher resistance yields slightly lower output tube idle current and the 10 watt rating runs much cooler. Also enough room under their to mount without bumping into other stuff.

Gillespies EFB board fits very well for these, and you don't have to search for new can-caps.

peterh wrote:Gillespies EFB board fits very well for these, and you don't have to search for new can-caps.

Thank you!

I would not use new can-caps in any case. I would either install the separate board, or restuff the cans. I have a plumbing tubing cutter that does an excellent and very clean job of cutting the cans right where it is needed. Ridgid, of course.

Got it. OK with the price, shipping is a little rich, but such is life. I will post pictures of the process.

It arrived last night (9:00 pm via FedEx). Upon brief examination, it is 'all there' but for the two missing knobs - I have plenty of those. I will take a range of 'before' pictures on Saturday and post a link.

https://www.dropbox.com/sh/byzusvy367c3cfc/AAAp2A5qpD0Y1FfSogHk-xWIa?dl=0  

a) Bent sheet-metal - the fencing pliers took care of that.
b) Frozen controls - why izzit that individuals insist on using so-called 'lubricants' off-label. In this case, lithium grease that has turned to lithium concrete. The controls are soaking in Kroil as I type.
c) Just plain filthy - as the pictures will show. The bass control may be unsalvageable. I do have spares.
d) A mix of German (OEM Dynaco 12AX7s x 2), Japanese (3 x Raytheon 6BQ5) and US (RCA 7199 x 2, Zenith 6BQ5 x 1). They all test OK on the big Hickok.
e) Draws about 95 watts @ 118 VAC unloaded. Nameplate is 110 watts. After I clean it up and check the caps that may calm down a bit.
f) 5A standard fuse. Ouch!

Gonna be a challenge.
Gonna be fun.

Looks like fun! Enjoy!!

Did the straightening of the sheet metal leave a crease? It looks nice and true in the photos.

I see there are new circuit boards for both the phono and audio sections available on Ebay. They are from China but they do not look bad. Made from FR4 with thru plating of the holes in what looks like the correct locations. I have not tried any but wondered if you or others have?

WLT wrote:Did the straightening of the sheet metal leave a crease? It looks nice and true in the photos.

I see there are new circuit boards for both the phono and audio sections available on Ebay. They are from China but they do not look bad. Made from FR4 with thru plating of the holes in what looks like the correct locations. I have not tried any but wondered if you or others have?

I spend several years of my mis-spent youth as a machinist, and worked my way through school as an electrician. During that time, I learned a few very basic skills with sheet metal. No creases, and no visible marks. Fencing pliers allow me to reach around flanges so that I can grip the flats. Handy to avoid creasing (crimping).

I have a set of Chinese boards - as you suggest, they do not look bad, and after a few re-solders, they are actually pretty good. What will happen is that after the basic diagnosis, cleaning and electrical checks, I will either install the new boards or rebuild what is there. Either way, the incumbent boards will be rebuilt for 'the next one'. The Chinese boards came out of a donor SCA - that, somehow, had blown the power transformer and one of the output transformers. Go figure. But that will be the source of spare controls and such, if needed.

Update: Pictures tomorrow.

After washing the beast down in WD-40 into a trash-can properly lined with kitty-litter I could see actual components on the boards beneath the sludge and scum - as seen previously. Then a rinse in CRC Electrical solvent to get the last of the hardened lithium grease from the controls and where it had melted all over the bottom of the chassis. . I freed the frozen controls completely, but for the selector switch, using a combination of Liquid Wrench and Kroil. The selector switch is undergoing its "second Kroil" immersion, as I type.

Previously, I had checked all the connections, cleaned the tube sockets and tested the tubes. I applied power without the tubes and got a rock-steady 20 watts, with no discernible heat at the caps or transformers. 20 watts over all that is not-much, so I was not at all surprised that after an hour, I should be getting no significant heat signatures.

Meanwhile, the selector worked free enough for me to set it on the "Spare" position.

More cleaning with a stiff artist's brush and high-proof alcohol. Soaked the knobs in hot ammonia with dish soap, then a tooth brush. More can be done, but I will have to raid my wife's Cream-of-Tartar for that work. Replaced all four feet, did a very brief plate-current match of the output tubes, replaced the tubes and fired up the amp unconnected to anything, spare input shorted. Went to about 80 watts quiescent, rock steady for an hour, no more heat than one might expect anywhere.

Let it rest for 20 minutes. Added speakers and a signal (Dynaco AF6 tuner) - and Thar She Blows - music! Not a trace of hum, buzz or fuzz. Test-point voltage right were they should be. I shut it down after about 40 minutes due to other demands. Next weekend, I will run it for several hours not on the Iso-Variac, and do leak tests, and re-test the various test points. I will also replace the line cord with a polarized cord so that the switch will directly switch the HOT - not that it should matter - and replace the fuse with a 1.5A dual-element rather than the 5A !! conventional fuse in place.

It has already found a home - my son-in-law is interested. So, it must be properly set for polite society in that case, and I will also replace the outputs with truly matched pairs of Telefunkens that I have stored.

I am not so sure, at this point, of whether or not I will replace the boards. This was a 'Factory (Drexel student) kit, with all the various parts-and-pieces riveted instead of screwed. Drilling out all those rivets, what with the marl to worry about getting into the pots and switches ... maybe not.  

Again. Voltages are good, things are staying cool, no hum.

Same Link as above now includes additional photos.

Update on the SCA35: Now completed 15 hours of burn-in without any issues. I find the SCA a bit anemic as compered to the PAS/ST35 combination, but a sweet little amp none-the-less.
Zero leakage to ground. No untoward buzzes, crackles or hums.
All controls and switches working fine.
Driving a pair of Dynaco A25s in a small room, fed from an AR tuner and a Revox CD player and Revox turntable for testing purposes.

Dynaco-Telefunken 12AX7s in the phono-section - working fine.
RCA 7199s for the drivers.
2 x matched pairs of Telefunken per-blight EL84s on the outputs.

Note: I installed the rear headphone jack per the Manual - guys and gals, if you want a merciless test of how quiet a tube amp might be (or not), this is the way to do it. And, yes, for the record, the hum-pots do make a significant difference.

Peter W. wrote:Update on the SCA35: Now completed 15 hours of burn-in without any issues. I find the SCA a bit anemic as compered to the PAS/ST35 combination, but a sweet little amp none-the-less.
Zero leakage to ground. No untoward buzzes, crackles or hums.
All controls and switches working fine.
Driving a pair of Dynaco A25s in a small room, fed from an AR tuner and a Revox CD player and Revox turntable for testing purposes.

Dynaco-Telefunken 12AX7s in the phono-section - working fine.
RCA 7199s for the drivers.
2 x matched pairs of Telefunken per-blight EL84s on the outputs.

Note: I installed the rear headphone jack per the Manual - guys and gals, if you want a merciless test of how quiet a tube amp might be (or not), this is the way to do it. And, yes, for the record, the hum-pots do make a significant difference.

The powertubes really should be a matched quad in this amp as they all
share the same cathode resistor.

peterh wrote:
The powertubes really should be a matched quad in this amp as they all
share the same cathode resistor.

I suspect that by modern standards, they do meet the quad standard. Given that the plate current is within 5% across the board. But I have the two higher as a pair on one side, the two lower on the other. Note for the record, that when I test theses beasts for noise and stability, I use four deliberately wildly mismatched tubes whose sole virtue is that they are all at/above "minimum good" on my big Hickok. If an amp can survive these, anything approaching matched tubes is gravy.

Thank you for the great write up on your new acquisition. I recently bought a SCA35 and got it sounding really good just the other night. This amp now brings me true joy but I wish it could play louder sometimes. I'm using it for a near field system for my PC but I am missing my much needed headphone jack.  You mentioned a method for installing one. Can you describe this method or point me in the direction of the manual?

Thanks again and congratulations on your new find.

MarkeMark wrote:Thank you for the great write up on your new acquisition. I recently bought a SCA35 and got it sounding really good just the other night. This amp now brings me true joy but I wish it could play louder sometimes. I'm using it for a near field system for my PC but I am missing my much needed headphone jack.  You mentioned a method for installing one. Can you describe this method or point me in the direction of the manual?

Thanks again and congratulations on your new find.

This ( or similar) is what you need : https://www.ebay.com/itm/Kopfhorerverstarker-SM-Pro-Audio-XPH4/173497890720?hash=item286547b7a0:g:uNMAAOSwzFxbB-dv

Although dynaco in the manual describes a way of connecting an earphone outlet i would advice
against it. Two reasons, one importent and one that only costs money:
1/ if anything goes wrong, you ear might suffer permanent damage as the power available
is much above safe levels.

2/ burning tubes for earphone usage is expensive . Donr wrong cold damage the
output transformers. Use something cheap for that!

If you want a nice tube headphone amp then this one is pretty nice.

https://bottlehead.com/?product=crack-otl-headphone-amplifier-kit

Guys and gals - thank you for the concern after my hearing - and the cost of burning tubes for headphones. A few things:

a) I use high-impedance over-the-ear phones for this amp (and for testing in general).
b) The model for the SCA-35 also parallels the factory installation for my AR receiver, Dynaco PAT-5 biFet, and my HK Citation 17, the latter two of the three having the headphone jack driven by the outboard power amp.
c) As I would never use ear-buds under any conditions or circumstances by choice, I feel OK with the risks of my choices.
d) And I have sufficient tubes for my target amplifiers to last me and my children's children into the foreseeable future. This is a luxury behavior that I chose to undertake - those few times I actually use headphones.

Thanks again!