Dynaco SCA-35 kit amp.

[retailer]

On an impulse I hit 'make an offer' on a 60's Dynaco SCA-35 amplifier kit, partially assembled, the offer was immediately accepted, which was a bit of a shock, now a few days down the track, waiting for it to arrive and not knowing much about it I'm having a case of buyer remorse, just wondering if any forum members have had experience with a SCA-35 are they a half decent amp. ?

[Herald1360]

No direct experience but a quick trawl turns up some positive reviews.

It's a ~15 Wpc integrated amp/preamp with EL84 p-p ul outputs so it should give a pretty good account of itself.

Asking prices for good ones seem to be north of £400

[Robert Gribnau]

I don't have experiences with this amplifier. I take the liberty to react anyway.

Does it come with the valves? If not, the 7199 became quite expensive. But alternatives for the 7199 are possible.

Maybe this thread is interesting, if you didn't find it already:

https://audiokarma.org/forums/index....sca-35.999693/

[GrimJosef]

I don't know the SCA-35 first hand, but I have one of Dynakit's ST-70 amps and it's very nice indeed.

That said, to get 17.5W out of a pair of 6BQ5s/EL84s they will need to be worked hard. I see from the SCA-35 instructions, which include a circuit and measured voltages, that the cathodes run at +16V with the anodes at +360V and the screen grids at +365V. Not every type of this valve will survive those voltages for long.

The bulk of Dynakit's products were sold in the USA so it's also worth checking the primary voltage of the mains transformer. My ST-70's is 115V and if I run it from a typical step-down transformer, which halves the UK's mains to 120V, then the mains trasnsformer buzzes. It really does need to be run at its nominal voltage and no more.

Cheers,

GJ

[Robert Gribnau]

In the SCA-35 positive feedback is being applied from the triode section to the pentode section of the 7199, like described on pages 4 and 5 of the attachment.

This arrangement makes it possible to achieve an enormous amount of gain in the pentode section, which in turn makes 20 dB (and sometimes even 23 dB depending on the particular 7199 being used) of negative feedback from the secondary of the output transformer to the pentode section of the 7199 possible. See:

https://audiokarma.org/forums/index....#post-15945646

[Ted Kendall]

It's a valve Dynaco, so undoubtedly a good design of its sort. The transistor power amplifiers are a less good bet...

[Valvepower]

Hi,

I felt it worth a post as I have a Dynaco ST35 power amp, which also uses 6BQ5s/EL84 in the power amp. The ST35 uses the mongrel 12DW7 (7247) half ECC83 and half ECC82 triode, whereas looking at the SCA35 manual that uses 7199 pentode/triode as in the SC70.

Dynaco output transformers are very good, well the ones I’ve encountered in the SC35 and 70 are.

I remember reading the PEC modules can be a bit flaky though.

As GJ said they push the 6BQ5s/EL84 bottles hard.

Pics of the SCA35 attached for perusal and comparison.

Terry

[Robert Gribnau]

I think you've made a typo since your pictures are showing a ST-35 instead of a SCA-35.

[Gabe001]

I imagine it comes with some sort of cover, given the exposed PCB connections?

[GrimJosef]

The ST-70 certainly comes with a cover, and I run mine with it in place. But these days some people like to see the valves. If you look for images on Google you'll find plenty of the amp running with the cover removed.

Cheers,

GJ

[Radio Wrangler]

The ST-70 was a David Hafler design, I recall reading. Any idea who was behind the others?

David

[retailer]

Thanks for all of the replies. I believe the amp, an unfinished kit comes complete including valves, the mains tx is the 110V US version, I have a few possible mains tx's that may be an ok substitute - if not permanently then temporary until I rewind the 110V tx for 240v.
Thanks for the Audiokarma links not only do they look like a very worthwhile read, a bit of a search turns up many posts related to improvements, so maybe I shouldn't be in a rush to complete the amp until I've looked into all of the mods, it will probably be easier to implement any of them before construction starts.

[Jan Zodiac]

My cousin has an SCA-35 and had it for years. I think he bought it around 20 years ago. I restored it for him, and I remember being pretty impressed by actually squeezing 17,5 Watts per channel from the EL84's.
I don't know about the phono input because he only uses it with the matching tuner and an SACD player. Large horn speakers with a 12" coax in each. I find this little amp to be very well sounding.

I had a pair of Dynaco/Dynakit mk.3's and a pair of mk.4's (half ST-70) over the years. None of them were better than just decent with the standard driver circuit. I replaced them with long tail splitters, and that helped. No doubt that the output transformers are good, but with the standard circuitry this will not be noticed at all. I didn't find the SCA-35 "dull" or "flat" like the larger models though.

Regards,

Jan

[joebog1]

Origional Dynaco transformers are VERY good indeed. Some of the best in fact.
Its been said already, that 17.5 watts from 6BQ5,s is a bit like racing a draught horse on derby day.

I dont think you wasted the money. Build it EXACTLY as described and sell it on as
" perfectly origional ", untouched and non modified. I will almost put money on a private jet arriving from Japan to collect it.


Joe

[retailer]

I've had a bit more of a look at the Audiokarma threads - there is a lot to take in with regard to modifications - for me one of the plus points of the Dynaco is that the power and pre- amps are built on pcb's, so if you want to go all out on mods it is possible to fit diy/aftermarket pcb's and retain the originals making it easy to returned the amp back to 'unmolested' condition.
That said the one modification/improvement that looks to have universal approval is to change the cathode bias to fixed bias, the attached pdf explains it, I have renamed it so it better reflects the contents of the document, I hope the document owners doesn't mind.
I'm guessing other forum members are like me in that they like pictures, so I'll post some of the parts once it arrives.

[Valvepower]

Hello,

Oops sorry for the Pics of the SCA35 typo as it should have been ST35.

The ST35 has a cover.

Personally, I’d build it as-is and not modify anything in the first instance. Maybe after you have got it going you could tweak it, that's if you want to keep it though.

I’ve got to be honest I’m not personally sold on the mod to the cathode circuit using a regulator. I’d separate out all 4 cathodes with a standard resistor/capacitor network per valve - but that's me!

There is some folklore of a collaboration between David Hafler and Arthur Radford on output transformer design.

Terry.

[qualityten]

Unless you already have a very good valve amplifier, or paid silly money for it, you'll not regret buying this amp. Especially as it is a partially unassembled kit and therefore has not already been badly or wrongly assembled or modified. Mine came to me seriously hacked about and has been a lot of work to restore properly.

As others have indicated, the output transformers are highly prized in the USA (and elsewhere).

Dave Gillespie is one of the most respected members of Audiokarma and has taken the trouble to study the SCA-35 extensively. Here is the thread: https://audiokarma.org/forums/index....sca-35.999693/. Not that you need to delve into that yet. Just build it exactly as per the manual, taking care with the wiring layout, and you'll enjoy good music!

p.s. this little report was what started me on my own Dynaco journey: https://www.tnt-audio.com/ampli/dynaco-sca35_e.html

[joebog1]

Seems you cant go wrong, retailer. The audiokarma discussion is excellent reading. Takes a little while however .

Joe

[retailer]

The Dynaco Kit arrived late yesterday, it seems to be complete, sans the original box, I've included a few pictures. I was surprised by the size of the transformers - the online pictures were a bit deceptive (no fault of the seller) and with an output of 17W per channel I was expecting something bigger. All of the transformers have the same EI75 laminations, the laminations on the power transformer are quite chunky at around 0.66mm thick, there is no internal or external shielding which is also a surprise. I haven't measured the output transformer lams but they look to be at least half as thick so I'm assuming 0.35mm, I measured the primary inductance of one of the output transformers at 90H.

The mains transformer is the US 110V version, the temporary replacement I had in mind is way too big, I have a few options - while I can rewind the existing I'm not keen on that, I'd rather keep that original and intact, I have enough EI75 laminations to wind a complete new transformer or I can wind a new transformer using a slightly bigger lamination size that I have on hand, it will fit if turned on it's side - at present this looks like the better option to me as I did see quite a few comments on other forums that the SCA35 mains transformer runs quite hot, a slightly bigger core will help here and I can also incorporate an internal shielding layer as well as an external copper strap, the rest of the amp will be stock for now, I'm not if I will ultimately keep it.

[qualityten]

Thanks for the pictures. Those output transformers are a good size for EL84s in push pull. I guess the decision of whether to keep this amp or not will be informed by your listening impressions once you get it assembled. From your comments about the transformers, it sounds as if you have experience of other valve amplifiers.