SCA-35 Dynaco

[bulgin]

Hehe, I'm still pinching myself.

Thanks for the kind words and link, guys.

bulgin

[wicked1]

I got a good deal on one of those a couple of years ago. Not quite that good, though! I ended up doing a complete rebuild on my sca-35, but kept it in the same chassis. It's still my favorite PP amp.

[wabun]

How to bypass all the tone & volume control and use it solely as power amp with an external preamp ? I try to solder RCA socket to the coupling cap ( 0.1uf ) at grid of 7199
but it turn out to have too much gain.. any good suggestion or experience on how to get it right ?

[jack_b]

The SCA-35's power amp boards have extra gain to compensate for losses in its passive tone controls. If you bypass or remove the tone controls, which is a very good idea for improved sonics, replace the power amp boards with the boards for a Dynaco ST-35. Those boards are Dynaco part number PC-13. The PC-13 boards use the same tube set as the boards for an SCA-35, and the mounting holes on the PC-13 boards exactly match the holes in the SCA-35 chassis. The circuits of the two boards are similar, but the PC-13 boards do not have the positive feedback used in the original SCA-35 circuit to squeeze out extra gain. You can get the PC-13 boards at dynakitparts.com There are improved power supplies available from various sources too; check out Triode Electronics for starters.

[ArtG]

With an SCA-35 you already have two of the most important components to build an outstanding tube amplifier, and those are the Z-565 output transformers! The SCA-35 was sort of a "mixed bag" featuring an excellent power output section, preceded by a cost compromised, rather poor line and phono stage.

The post above suggests replacing the PC-10 boards with PC-13 boards, which, using the other existing hardware, will turn the unit into a ST-35. This is a great idea. If you look around the 'net, you will find these boards available both as "pre-stuffed", or " bare boards".

A couple posts above, "Wicked1" shows a different approach which is also an excellent idea. His picture shows that he installed a DIY Tube ST-35 complete pc board in his chassis. This single board has the advantages of having a step-by-step manual available for free, some built-in power supply upgrades, and a wider choice of input and phase inverter tubes. With the original Dynaco PC-10 setup, you are pretty much stuck with using the 12DW7/7247 driver, which has it's drawbacks. The DIY board is available from Triode Electronics, again both "stuffed" and "unstuffed".

Dave Gillespie has devised an innovative EFB fixed bias supply for the output stage of both the SCA and ST-35 amplifier. Whatever route you take, I'd strongly suggest that you look into this low cost, easy modification, as it should make a marked improvement in the amp's output. Follow the link below.

http://www.tronola.com/A_New_Look_At...riend_Rev0.pdf

[Mr. Zenith]

As far as speakers go, I'll second the Klipsch recommendation. I bought a pair of RF-3's new several years ago, and have never regretted the purchase. At 98 dB efficiency, they'd be a great match for the SCA-35, and would also work well with low-power SE triode amps (let's face it: if you get addicted to this hobby, you're gonna want to at least try one!). Best of all, they're relatively cheap; I've seen them for as little as $300/pair on Craigslist. IMHO, they're money well spent!

[kstagger]

low power and high efficiency speakers go hand-in-hand. At your price point though, decisions get a little more difficult.

Altec speakers - some of the less popular models can still be picked up cheaply. Like the Model 15 utilizing a bent horn tweeter and a 12" RCA woofer. I bought a pair for $200 at a garage sale. The (in)famous VoT (Voice of the Theater) is still available if you're lucky.

UREI and JBL studio monitors are fairly efficient. With a 20W amplifier, my UREI 813As are LOUD and still quite detailed.

for smaller speakers, I like the KEF Uni-Q line. In my "upstairs" family system, I use a pair of Q60s which are 90dB efficient.

You can also try to score some old 12" or 15" coaxial drivers. These are usually quite pleasant sounding, though perhaps not the most modern in terms of design or crossovers. Back in the day, I once bought a pair of 12" Stephens Trusonics for $60. I then screwed together a Jensen Ultraflex cabinet and was quite happy with the sound with a pair of 3W 2A3 SE amps.

[revintage]

Hey Wicked1,
Where did you find the circuitboard in your picture!

[cedricb]

Originally Posted by revintage

Hey Wicked1,
Where did you find the circuitboard in your picture!

DIYTube ST35 Driver Board with Parts

[Ty_Bower]

Originally Posted by jack_b

...replace the power amp boards with the boards for a Dynaco ST-35. Those boards are Dynaco part number PC-13. The PC-13 boards use the same tube set as the boards for an SCA-35, and the mounting holes on the PC-13 boards exactly match the holes in the SCA-35 chassis. The circuits of the two boards are similar, but the PC-13 boards do not have the positive feedback used in the original SCA-35 circuit to squeeze out extra gain.

I don't think that part about the positive feedback is true. Take a look at the PC-13 circuit. R4 (150k) provides a positive feedback loop from the phase inverter to the cathode of the voltage amplifier.

The driver circuit of the SCA-35 has more gain than the ST-35 because it uses the pentode section of a 7199 tube (mu > 1500) as the voltage amplifier. The ST-35 uses the high mu triode section of a 7247 (mu = 100) for this duty.