SCA 35 Iron

I have a good set of original Dynaco SCA-35 iron, and I would like to make something out of it. I did do the "Clone" of the 35, and I find it unsatisfying as the output appears to be quite low as compared to the original 35. Lest you think there is something wrong with the transformers, the donor had been moused, flooded and more such that not much more than than transformers could be salvaged - and those with effort. I also feel brave enough to take on more than just following a recipe and assembling stuff-made-by-others.

I would like to keep this as simple as possible, of course.

I have reasonable tooling, I own a small lathe and mill (Unimat), and I have a LOT of experience soldering, troubleshooting and building kits.

I am wide open to suggestions. Amp only. I am in possession of multiple pre-amps, so no need for an integrated amp.


I also have a set of Fisher 500 iron - and would like to do something with this as well. I am told that Fisher use voltage doublers to make B+, so this may be a bit more problematic. Suggestions?

On the Fisher Iron, I purchased these at the Kutztown Radio Show for $3 at the Buy-it-Now table. They check out and I have ohmed them and checked them - all good. They have been sitting on my junk-shelf for 6 years now - time to do something!  But, an illustration of the many opportunities available at the largest vintage radio show in the US by a considerable margin - happening September 16/17 - less than a month from now. Look up DVHRC Kutztown in your browser for details.

One last note: I have a plethora of 7199 tubes - pure blind luck, but I found a brown paper bag of tubes at a flea-market, which contained a bunch of NIB RCA-branded 7199s. So, I have no worries along those lines.

Thanks in advance!!

As regards to "A" : you say "output appears to be quite low".
Are B+ correct ? What power do you get before clipping ?

peterh wrote:As regards to "A" :  you say "output appears to be quite low".
Are B+ correct ?  What power do you get before clipping ?

Yes, B+ is correct.That was my first check. All voltages and bias are correct per the instructions.

I try not to drive the amp to clipping.

My intuition suggests that some of my signal path is a bit long and may account for it. This is by comparison to my stock ST35, and not by much.

As it happens, I have a pair of ST35 boards that I rebuilt years ago as spares - very late iteration glass boards, so worth the effort. So I always have that option.

I love the ST35 and think it can make for a cheap but still excellent amplifier.  While I've built a pair of MkIVs and rebuilt a ST70, the ST35 was my first total scratch build with my own custom layout.  It worked out really well and I was encouraged to keep trying and playing with different parts and further refining how best to assemble an amplifer.  I ended up rebuilding it 3 times!



This was my first attempt.  Using a Hammond box from a previous preamp project I was able to keep the total cost just under $400!  Design goals were to separate the power supply from the audio circuitry and essentially mount everything inside of a box so that nothing stood out on top like most tube amps.  I also incorporated a different driver circuit using a 12AX7/5750 for voltage gain and a 12AU7 for phase splitter.  I also included Dave Gillespie's EFB mod for more power and lower distortion.

While this worked very well and sounded great I eventually found troubleshooting, measuring the various parts of the circuit were not optimal.  I had part failures that had me tearing almost the whole amp apart to deal with.  Back to the drawing board.



My 2nd version went back to a more traditional style build but with a twist.  Just prior to this I started experimenting with Clarity Cap TC 700V film power supply caps.  I found them to improve the sound of my amp by removing a hither to unnoticed haze to the music.  Since the caps are large mounting them became a bit of a challenge and required a taller box.  I also decided to try a 6CG7 for phase splitter.  My amp sounded great like this but over time I grew to dislike the large box.  It was relegated to back up amp anyway after building my ST120.



About a year ago my older brother asked me to build him a nice little tube amp.  He wanted to get back into spinning vinyl and felt like a tube amp was a natural progression.  He had certain design ideas he wanted me to add such as all controls and connections on top.  He was going to build a nice wood box to essentially drop the amp into.  Being budget conscious he also wanted as cheap as possible but still good quality.  So I decided to rip mine apart and rebuild it.  I used a small hammond box of only 12x10x2".  This left out the fancy clarity caps but those are more for my projects anyway.  The circuit otherwise is essentially the same although I went back to 12AU7 for phase splitter.  (Either tube worked well)  I also turned it into an integrated amp by adding 2 inputs and a volume control.  Once I worked out my usual mistakes it sounded as good as ever and in fact was dead quiet when not playing anything.  (Guess I learned a few things over the years.)  I kinda miss it now!

I hope this gives you some ideas for your project.  It's a great little amp to play with.

Impressive!

First Stop - look into Hammond boxes! Compact is good, too.

Would you care to share schematics of your last effort?

Peter,
Why not just build an ST-35 amplifier...you already have the iron & circuit boards. I think the ST-35 in it's stock form out performs the original SCA-35 by a big margin. All you need is a chassis and a few minor components. Add a cover and you will have a nice vintage unit. PS: I have a number of the ST-35 covers in Black....Looks great on the stainless steel chassis.

Kevin @ Dynakit

[quote="DynakitParts"]Peter,
        Why not just build an ST-35 amplifier...you already have the iron & circuit boards. I think the ST-35 in it's stock form out performs the original SCA-35 by a big margin.

That is always an option - and not a bad one.

Now, in the same batch of miscellaneous Dynaco junque from which I acquired the ST35 boards, I also acquire a set of glass SCA35 boards - that I have not rebuilt yet. Disregarding the phono pre-amp board, and considering that I have ample 7199 tubes, would there be anything against/for using either driver board? Conceivably, I could add the phono-pre board at a later date...

That would make taking the simple and immediate expedient much more tempting....

Peter,
I do not believe the SCA-35 line stage boards will work correctly without the SCA-35 phono boards..Although they are dimensionaly the same as the (ST-35) PC-13 boards. I would not encourage anyone to design an amplifier or preamp around a out of production tube like the 7199...even if they had a barrel full of these tubes. I would sell all of these tubes while they still have some value which would easily fund this project and more.

The SCA-35 in it's original design was a marginal performer. Another option would be to contact our Forum friend at Tubes Nirvana. He has a number of updated SCA designs that can utilize your transformers.

Kevin

Peter W. wrote:Impressive!  

First Stop - look into Hammond boxes! Compact is good, too.

Would you care to share schematics of your last effort?

Best I can do is give you links.

Driver circuit here: http://www.diytube.com/st35/ST35REVE.pdf

EFB and power supply here: http://tronola.com/ST35-EFB_Installation.pdf

I have a drawing of the layout but have to check it for errors before I'll post it.

Driver circuit here: http://www.diytube.com/st35/ST35REVE.pdf

EFB and power supply here: http://tronola.com/ST35-EFB_Installation.pdf


Many thanks!

I am familiar with the Parks Audio board. Not so much with the next one.

Peter W. wrote:

DynakitParts wrote:Peter,
        Why not just build an ST-35 amplifier...you already have the iron & circuit boards. I think the ST-35 in it's stock form out performs the original SCA-35 by a big margin.

That is always an option - and not a bad one.

Now, in the same batch of miscellaneous Dynaco junque from which I acquired the ST35 boards, I also acquire a set of glass SCA35 boards - that I have not rebuilt yet. Disregarding the phono pre-amp board, and considering that I have ample 7199 tubes, would there be anything against/for using either driver board? Conceivably, I could add the phono-pre board at a later date...

That would make taking the simple and immediate expedient much more tempting....

The reason that the SCA-35 used a 7199 instead of a 12DW7 was to compensate for the insertion loss of the tone controls, approx 17 to 20 db as I recall. So really no point in using 7199s, you will only have excess gain that will go to waste.

FWIW, several years ago I built what I called an ST-35 integrated: SCA-35 with tone controls deleted and changed to ST-35 boards. Plus changed the phono board to PAS specs. Made a very nice sounding amp. Changing to the ST-35 circuit and no tone controls was clear improvement, and changing the phono board was an even bigger one. Always felt that the phono board in the SCA-35 was its greatest weakness.

Good luck with your project!