MODS on DYNACO SCA-35..PICS!!

Greetings....I thought I would describe as my first post my renaissance of this DYNACO SCA-35 on EBAY I bought for 65 bucks a few years ago.:thmbsp: They sell for ALOT more these days even in sub-standard condition which,fortunately, this particular unit was not. Other than replacing the power caps it worked. Hooked it up via a VARIAC and slowly turned up the voltage all the way when the transformer smoked and blew up!!...NO...just kidding...heh hehe..a tube afficianodo's nightmare....The unit went all the way to full and realized the caps were truly gone. Sound output was not what it should be. Pretty way down. Used the VTVM and found that the voltages were good. 385 volts on the red hot output line... Filaments too were steady. I took out the 2 MALLORY electrolytic power caps and replaced them with my home brew SUPER Joule reservoir of 20, 47uf/450volts caps in seperate banks of 5 for all power taps of the amp. 21.00$ for all these tiny power babies at NEWARK electronics btw.(SEE PICS)
The Super Joule Reservoir, or simply LAKE MARCONI,was mounted on a breadboard and neatly hot-glued onto it and then the entire assembled platform mounted to the chassis with 4 standoffs screwed and tightened into place.
The amplifier still has the original DYNACO labeled output tubes. The tubes measured fine and sound top of the line so there isnt any need to replace them although I am REALLY tempted to buy some exotic juicy replacement tubes....but i can wait...

I didnt bother to change the resistors. I kinda like ALLEN-BRADLEY CARBON FILM and these were really quiet so they have all earned a stay of execution.

CLARITY-CAPS, RELS,JENSENS, MUSE M AND INFINITYCAPS were used throughout.

The original plastic-crappy coupling caps were super tiny compared to their replacements and there was no real room on the chassis to place the CLARITY CAPS down to the circuit board. I used the original large gauge leads of the caps without stripping them and soldered above the circuit board. I neglected to check the dimensions of the cap when I purchased them a while back!. Pretty to look at..
I canned the solid core colored input wire and replaced with 27 gauge-19 strand-silver teflon wire.
All return grounds usually strapped to the power caps via the chassis were individually run to a terminal block which was connected to the front leg screw of the amps power transformer. (See pic below!) Dressing up the beast. The unit is unusually quiet even with gain turned up all the way.
I just ordered a 24 stepped attenuator-DALE RESISTORS 1%- to replace the plastic crappy slider volume pot of time long gone. This will further resolve sound and music nuance. I may just do that for the 3 remaining -treble,bass and balance...but im too lazy.

I replaced the tiny metal front knobs with BIG BLACK RADIO DIALS of yesteryear because it looks great and M U S C U L A R..Never liked the tiny silver knobs for these units- they look wimpy and not emblematic of such a manly amplifier reborn. The original knobs made the amp look like a real weeny.
All switches and pots were tuner cleaned and De-Oxit used as preservative. The MAIN SELECTOR switch was an abomination. Really dirty contacts. Small layer of yellowish film throughout circuit board I need to clean a tad more. Maybe cigarettes??
Since the power coupling caps are too close to the output tubes I corrected this with the placement of a tiny 9 volt CPU fan that vanquishes all heat surrounding the components. You cant even hear the fan...

Sound: Crushing dynamics are effortlessly reproduced. Frequency response is SUPER WIDE. Lots of detail and image placement. You can feel the fluttering of a kettle drum at full WHALLOP!! I have POLK MONITOR ONES with the silver coil Dome tweeter,SEAS MIDRANGE and Passive radiator. Once I turned this amp on I was listening for days. Everything sounds great...ROCK,24 BIT VINYL DUBS,CLASSICAL,TECHNO,DISCO....cannon shots......
Now, I incorporated a cathode bias to this unit EFB circuit with an lm337 chip that sets the bias. Truly , rock bottom bass with no smoosh.defined...can hear the skin on some recordings. There really is no comparison after listening to the unit with cathode bias install. Using a meter, .270 volts is the optimal setting.

How about New Modifications for the SCA35?

Big mod!!

Hey..thanks for the info..NEVER SAW IT BEFORE...This is a big mod upgrade and im already on DIGIKEY ordering the parts. TYoo bad you need a matched quad set.......Once again thanks...

Great updates!

I've given thought to a SCA-35 as my first tube amp and your information is greatly appreciated.

I used Dave's EFB circuit on my ST-35 rebuild last winter and it sounds wonderful.

Liking these mods (even the knobs).

Something about approaching practical considerations toward achieve a better performing unit with a sense of fearless ambition I find inspiring.

Nicely done.

This is a fun restore. Thanks for posting. I was listening to a record on mine as I read your post, and realise it's time for me to upgrade the caps on my phono stage.

My only minor disagreement would be over the knobs. The originals are nice and solid and look good to me. However, you could probably sell yours on to someone who's got a beat up unit.

its an easy amp to work with…tons of mods u can do. a great amp…...

These little Dynacos keep finding me....updated with Dave Gillespie EFB power supply and driver boards. The amp now accepts 6U8A driver tubes, and sounds pretty good.

I need to replace Volume and Balance pots, and as I understand it, the balance control is going to be the real PITA to find.

Love lake Marconi!

real nice job with the new boards. neat work….I can admire the handiwork of crafting and soldering on new boards from scratch. Just replacing the caps and wiring on mine and power supply was kinda arduous. Tell me more about the new drivers. why u changed them and how they sound….

thanks…..seemed appropriate…..

LGSUTEKH said:

real nice job with the new boards. neat work….I can admire the handiwork of crafting and soldering on new boards from scratch. Just replacing the caps and wiring on mine and power supply was kinda arduous. Tell me more about the new drivers. why u changed them and how they sound….

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I've only got about 5 hours on it, and it sounds fine. The 6u8 is more plentiful than the original 7199's

cc43fan said:

I've only got about 5 hours on it, and it sounds fine. The 6u8 is more plentiful than the original 7199's

Click to expand...

I had an AK'er do the same mods for me over a year ago and I have had no problems and it sounds great. I think the PT runs cooler now too.

I recently bought an SCA-35 on epay that was carefully modified by a previous owner in the 1990s. The tone controls were removed, the selector switch was wired with silver wire and Elna Cerafine caps were installed in place of the originals. My supply is 100uF to 6H choke to 100uF to 3k resistor to two 47uF in parallel. The EL84 B+ is taken off of the second 100uF cap and the front end tubes off of the pair of 47uF caps.

Since the unit had already been rebuilt and rewired, I decided to continue with the mods. I couldn't connect the unit to my system until the speaker jacks were replaced since my speaker wires couldn't accomodate the tiny screw strips the SCA-35 comes with. I thought that since I wasn't going to use the phono stage I could replace the phono inputs with some high quality Switchcraft jacks. Removing the phono stage left considerable room for a PS inductor and anything else I wanted to add like a negative supply transformer or more caps.

I was able to install the speaker binding posts and the RCA jacks with a minimum of filing as they fit nicely into the original spaces. I rewired the power cord with a new cord, grommet, and strain relief; installing a grounded three conductor cable is the first thing I do to any unit that reaches my work bench. I connected the input jacks directly to the volume control with Kimber cable since I will only use a CD input and want the cleanest input signal. I left the beautifully wired input jacks and switch in case any subsequent owner needs them.

I am about to have Front Panel Express make a new aluminum panel to replace the two high level boards. The panel will have 6 holes for the 9 pin sockets and provisions for mounting the 4 coupling caps to the top of the panel on standoffs. I will be able to point to point wire any circuit I want using the two available 9 pin front end tubes. For instance, I could replicate the ST-35 circuit with a 12AX7 and a 6CG7 or I could go with the "El Cheapo" circuit if I add the negative supply transformer in the space I left next to the inductor.

The Dynaco SCA-35 is a versatile platform to build any number of interesting circuits. I am also considering a choke input supply to lower the B+ so I can increase the plate current of the output tubes towards class A. Five or ten watts is plenty for my speakers.

Nice work.....bet it sounds ...er....B E E FY...