SCA-35 Project

Just recently acquired an SCA-35 in fair shape,looking forward to a rebuild. I need an amp for a small room and something for headphones,so I thought I would jump in. I'm not real happy with the current boards available,I want to be able to try different driver tubes without using adapter sockets,by simply wiring jumpers to the appropriate pin,and another thing,most boards have the sockets screwed right to the board,which is inconvenient,and it's not good considereing the heat that the output tubes emit,they need to be moved away from the board,and I think using ceramic sockets is the answer,and I wanted to use a single board for the mounting,and I want the option of using a completely different phono section. Below is an image of the power amp section of the amp. I've also made a couple of changes to the curcuit,I've added 1K gridstoppers to the output tubes,which I think will add to the stability of the amp. Will update as the project progresses. Constructive opinions are also welcome.[img][/img]

If you can do without tone controls, I would agree with Roy, and suggest the ST-35 circuit. I don't find a great deal of difference between 7199s and 6GH8As, and I doubt that 6AN8s, 6U8s etc. would be much different. However, if you want to keep the tone controls, then your circuit board looks like a winner.

The two biggest improvements that I have found for the SCA-35 are converting it to an 'ST-35 integrated' and Dave Gillespie's EFB/power supply board. I also changed the phono board circuitry to that of the PAS plus a few tweaks. James Lin wrote an article in Audio Xpress a few years ago on the SCA-35 which included changing the phono circuit to the classic RCA passive EQ circuit, another good option.

Will be watching for updates!

I may deside to stay with the original 7199 design, I have enough 7199's to last me, but I will incorporate the HF stability components into the board that way if I want to use the adapter socket, I am able to. Great recommendations so far. And I will move the coupling caps further from the output tubes.

I see no indication of vents for airflow around the tubes. They should be accompanied by taller chassis "feet" and strategically located vent holes in the bottom chassis plate. Greenlee punches are your friend, here.

You could also mount the coupling caps under the circuit board where they will be protected from IR radiated heat as well as convection heat. If they can fit in the available space, that is.

Finishing up the SCA35, sounds really nice. It has new boards, and new transformers. I have completely removed the tone controls,loudness and filter switches, I am only having one issue, I have a slight hum when the volume control is in the midpoint position,but dead quiet at 0 and full volume, I have read that this could be an impedance mismatch at the preamp/driver input. I read that someone suggested using a higher value volume control,currently using the 250k,any other suggestions. I have double checked all my ground connections and solder joints.

[img][/img]

I have also increased the NFB to the input stage to decrease the gain,this reduced the hum somewhat, I'm also wondering if I should increase it just a little more.

Wow!!! that is all can say about this amp, from top to bottom, one of the most musical amps I've ever listened to for the money.
New boards, components,transformers, and a pair of MTM loudspeakers, in a TL enclosure. You couldn't ask for a better setup. I'm listening to some RR and Dorian label recordings. Particularly organ and orchestral pieces that are outstanding in sound staging and
dynamics. This is truly an entry level tube amp for anyone wanting to dive into valve amplification. You may not want to let this one go!![img][/img]

tubes4hifi wrote:With my move finally finished (but still unpacking) I've finally got boards and a web page for SCA-35 upgrades here:

SCA35 mods

Super mods! The 35  is simply a fun amp to work on and listen to. BTW if I could suggest, having PC-10 on one board is nice, that way you have a shared ground plane and only have one ground connection to the power supply.

Here are the boards, I had made for the amp. There is also an updated power supply board that I also installed.

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Denny, could you describe your mtm t-line speakrs? I have a pair of ported mtms that I have ben toying with reboxing in t-lines.

ramon68 wrote:Denny, could you describe your mtm t-line speakrs? I have a pair of ported mtms that I have ben toying with reboxing in t-lines.

They are the Tri-Trix MTM's sold by Parts Express, Since I have all the tools,I ripped all the MDF myself. The speakers are relatively efficient,rated around 90db. I'm painting the cabinets now, I will post pics as soon as they are finished. I was curious as to how they sounded, so I installed the drivers,after I put the cabinets together.

Wonder if your hum could be from the open sockets ... maybe figure out a way to terminate them when they're not in use?

Was just reading an interesting article in Vacuum Tube Valley (Issue 7) about building a comparison pre that allowed tube substitutions on the fly ... might be some tips and tricks you could use there ...

http://www.jumpjet.info/Pioneering-Wireless/eMagazines/VTV/vtv.htm

sKiZo wrote:Wonder if your hum could be from the open sockets ... maybe figure out a way to terminate them when they're not in use?

Was just reading an interesting article in Vacuum Tube Valley (Issue 7) about building a comparison pre that allowed tube substitutions on the fly ... might be some tips and tricks you could use there ...

http://www.jumpjet.info/Pioneering-Wireless/eMagazines/VTV/vtv.htm

It was the bottom cover, I had left it off not realizing that there's no shielding of the curcuit or heater windings. Its all quiet under the hood now, and I was switching out the 12ax7's in the phono.

Ah, so THAT's why you put bottoms on these things ... other than to keep the mice from nesting ...

I got a piece of 2.5mm 3003 aluminum for my custom case ... I figure to just do a couple rows of holes maybe an inch in from each side and the back ... that should provide plenty of ventilation.

Been wondering if I should also cut inserts for the wood sides too? I've already got metal for the back, and plan to strap that, The front shouldn't be an issue, but it'd be easy enough to add a strip there too.



I got plenty metal to do it ... as is, I relocated the chokes to the sides and that would give me good ground for those too with a couple straps to the main panel ... Hummmm ... I mean ... hmmmmmmm ...

sKiZo wrote:Ah, so THAT's why you put bottoms on these things ... other than to keep the mice from nesting ...

I got a piece of 2.5mm 3003 aluminum for my custom case ... I figure to just do a couple rows of holes maybe an inch in from each side and the back ... that should provide plenty of ventilation.

Been wondering if I should also cut inserts for the wood sides too? I've already got metal for the back, and plan to strap that, The front shouldn't be an issue, but it'd be easy enough to add a strip there too.



I got plenty metal to do it ... as is, I relocated the chokes to the sides and that would give me good ground for those too with a couple straps to the main panel ... Hummmm ... I mean ... hmmmmmmm ...

Your amp is looking really nice, I love the wood-metal combo look, it's not only attractive but it keeps the cost way down. The preamp project I'm working on now, I will be doing most of the fabrication myself,except for front and rear panels,which will be done by Front Panel Express.

Who needs to spend a lot money on ARC or VTL stuff,when you can build attractive and good sounding equipment yourself.

Yup ... my sentiments exactly. Doing a custom chassis also allowed me to have a bit of fun with parts selection and placement that the standard chassis doesn't allow. Pretty happy with how it's turning out ... and another vote for FPE ... they did an outstanding job on the panel, and the design software they provide is excellent. I just wish they provided a few more standard templates you can just drag and drop into a design.



To save cost and give yourself some wiggle room, I suggest you have them cut the big stuff that will be exposed, and do the final fastener holes and such yourself - stuff where you don't need a perfectly clean and pretty edge. I ended up tapping a bunch of holes here in the shop, but it was just nice to be able to use the components to locate them exactly as needed.

Follow the adventure ...

https://dynacotubeaudio.forumotion.com/t1656-custom-chassis

You'll find a few "in hindsight I might not do that" and an occasional dOH! along with helpful hints from the forums that get incorporated into the plan. That's the fun part! So far, it's been easy fixes and tweaks, along with random fiddlybits that bother me now, but I'll never notice when it's done. Part of the charm of the DIY thing.

Hi Denny,

I have the same boards from Brown Audio Lab for Dynaco SCA-35 but I don't have instruction to connect the boards with other components. Can you send me a copy of the instructions if you still have one. Thanks.