Sorta homemade EL84 Amp

[corndog71]

So for over a year now I've been itching to get my hands on a classic Dynaco SCA-35 or ST35 EL84-based amplifier.  As always money is limited and I have a life so I never get around to chasing these down on ebay or anywhere else for that matter.  A few months ago I decided I was just going to build one myself.

Dynakitparts.com sells terrific Dynaco replicas and they even offer the ST35 but I wanted to try something different.  I did however pick up the needed transformers from them.

Diytube.com also has produced a modified ST35 design and even leaves available for free the full instructions and parts list.  The main deviations from the stock circuit are:

1. Replaces the 12DW7/7247 driver tube with a 12AX7 for the voltage amp stage and a 12AU7 stage for the phase splitter.  These are for both channels.
2. Splits the common cathode connection into 4 seperate cathode circuits.

3. Boosts capacitance in the power supply.

I was preparing earlier this year to get that kit but procrastinated as I'm wont to do. And then I found Dave Gillespie's recently published EFB modification.  Enhanced Fixed Bias does several things to improve the measured and audible performance of these old amps as well as reduce distortion and extend tube life.  His paper on it can be found here: http://www.tronola.com/html/dave_s_lab.html#NewLook

So I wanted an amalgamation of these 3 circuits but was highly intimidated by the thought of doing it myself.  Which lead to more procrastination.  

Then I discovered Audioregenesis created an EFB enhanced power supply PCB for the SCA-35.  I really wanted to get it but at the time Canada was going through some internal drama with their post office and nothing was getting shipped at the time.

So I kept obsessing over these multiple circuits and picking them apart in my head and on paper. After many weeks I figured out how to do it myself.  And the end result:

(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi576.photobucket.com%2Falbums%2Fss207%2Fcorndog642%2FDynaco%2520ST35%2F5b42abad.jpg&hash=30992de39abf47ff629572db0befc31ac4a18f17)
I actually took this box from a previously cloned version of the Foreplay 2 preamp I was never quite happy with.  Hence the holes in the front.

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Top view.
It was quite a challenge figuring out a whole new layout and ground configuration but I've been a good student of Bottlehead, Gary Dodd and other tube circuit designers and safely winged it.

(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi576.photobucket.com%2Falbums%2Fss207%2Fcorndog642%2FDynaco%2520ST35%2Fd5d68d7c.jpg&hash=dfe1ccd5c21c730701011a3980687745f38c8da2)

It sounds better than I could've imagined!  I hear incredible bass and detail as well as an impressive soundstage.  It's really quite amazing for providing only 17.5 watts per channel.  Of course when pushed it will eventually reveal its limitations but with relatively efficient speakers and mated with the Quickie it makes for a fantastic tube stereo!  The best part is that I was able to put it together for under $400!

[RayP]

Nice work. It really is an interesting part of the hobby where you take a circuit, get the parts, work out the layout, build it and eventually get to actually listen to it. There's a real sense of satisfaction. I remember over a decade ago hearing the late Gizmo say essentially that you never get real satisfaction from an amplifier until you build it yourself.

ray

[John Roman]

That does ring true Ray. Congrats on a very nice effort Rob!

[corndog71]

Thanks guys.  I may be a bit biased but this amp is quickly becoming one of my favorite amps ever.  Whoever still believes tube amps can't do bass needs to build one of these.  Its performance is simply astounding!

(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi576.photobucket.com%2Falbums%2Fss207%2Fcorndog642%2FDynaco%2520ST35%2F2ab5eee6.jpg&hash=02067eb80d7526c54f4110c2895b5b06c0b6f968)

[Dr. Toobz]

Awesome amp! The EL84 is one of my favorite tubes, and sounds really good in push-pull, as you've shown here. I've been tempted to build my own EL84 amp, though I will stay with single-ended, as I don't need more than a couple of watts with 99dB/W speakers.

I think push-pull amps often do bass very well, due to the better damping factor a lot of them have. I've never been all that impressed with SET bass handling. How much NFB does your amp use? (I don't even recall how much the stock Dynacos came configured with).

[Doc B.]

 

I've never been all that impressed with SET bass handling.

You have to use a speaker design that is optimized for the low damping factor of a typical ZFB SE amp. There aren't many speakers that are designed for that specific purpose.

[Dr. Toobz]

I've never been all that impressed with SET bass handling.

You have to use a speaker design that is optimized for the low damping factor of a typical ZFB SE amp. There aren't many speakers that are designed for that specific purpose.

Very good point! I think even my Klipsch speakers were probably designed for relatively high-NFB push-pull amps, considering what was out in the hi-fi world when they were released (1957). How I wish Bottlehead still offered some sort of complete speaker kit, like the Straight 8s, for those of us with no woodworking skills or facilities to speak of. :-)

Corndog71, which speakers are you using with the EL84 amp?

[corndog71]

Most of my speakers are AV123 X-Series although I also have a pair of Klipsch KG4.2 speakers.

My A/V receiver died on me last night so I put the new amp in its place driving my X-Statik open baffle speakers.

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[John Roman]

I'm a fan of open baffle speakers. I was surprised to see the slim size of the baffle on your X-Statik's. I've been tempted to experiment a smaller baffle. Seems there are very few OB types out there
John

[John Roman]

Corndog,
The photo of your amp has now made it to the ever changing landscape that is my desktop. Well done!
John

[Doc B.]

 How I wish Bottlehead still offered some sort of complete speaker kit, like the Straight 8s, for those of us with no woodworking skills or facilities to speak of. :-)
[/quote]

Well, I haven't completely ruled out the idea, though shipping is getting to be a killer. But maybe we will come up with something some day.

Speaking of the S8s, I am getting ready to sell my own last and best pair that are in really pretty kit cabinets and have Aurum Cantus G2si tweeters, with good sounding but homely external breadboard crossovers. I love them but we need to stay focused on what we are doing and these are taking up needed space at the office. Probably would have had to sell this speaker kit for about $1500 if we went forward with the ribbon tweeter version as we had hoped to instead of retiring the kit because of supply difficulties. Anybody wants to come by the office with $800, I'll help you load them up.

[John Roman]

The challenge of speaker design as you know is always a compromise. That said, for me the additional challenge of building sound cabinets that have a professional quality is almost an equal challenge. That seems to stem from the almost overwhelming importance put on "the looks" of a product in today's marketplace, WAF aside. I've recently undertaken the completion of a pair of open baffle speakers. I've upgraded the ribbon tweeters and some crossover components with the gracious assistance of Dick Olsher. Yet it is the cabinets that have me stalled, mostly because of my inherently anal nature to get them "just right". It seems as soon as I settle on a specific path I'm never satisfied. Ah well, maybe I'll get them done before the end of the year. This conundrum gives me a greater appreciation for how long a process it can be get a new product successfully to market.

[corndog71]

I recently had an issue with my amp.  A small bias pot somehow went bad and caused one of the resistors in the power supply to let the magic smoke out.
I had ordered spare parts but also wanted to try some of the new TC caps by Clarity Cap.  They're BIG, 700V film caps designed for tube amps.  I had to move the PT back a bit and figure out a new layout for the power supply circuitry.  It was a bit tricky considering the size of the caps but eventually I found a way to shoehorn them in.

(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi576.photobucket.com%2Falbums%2Fss207%2Fcorndog642%2FDynaco%2520ST35%2FIMG_6924.jpg&hash=019c880b4705fa5e15ef7c1ad443f3c7865e4ce0)

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I understand the ESR of these caps is only a fraction of an identical value electrolytic cap plus they'll never leak or have any of the associated long term problems with electrolytic caps.

I had a feeling they would improve the sound but not to the degree I'm hearing.  My initial reaction was that it didn't sound much different.  After an hour or so of warm up I sat down to listen again and then I noticed not only what was better but what was missing.  That is there was a bit of a glare to the sound that is no longer there.  I could hear the room in the recording.  The most palpable thing I noticed was that the sound just seemed effortless.  There was an ease to it that reminded me of much more expensive gear like Audio Research or BAT.

I was playing a playlist of songs from my ipad and had it on shuffle.  You know it sounds good when there's no desire to skip tracks.  Every song sounded better and more life-like.  If I build any more tube amps I will definitely find a way to incorporate these caps!  They're not cheap but they're not unreachable either.

http://www.partsconnexion.com/capacitor_film_clarity_tc.html

[4krow]

Once again, you guys are KILLING ME! I hope to one day make my work look as good! And from what I see, it looks to be definite that I will be using a metal chassis for my Quickie, 'Bride of Quickie'

[Paul Birkeland]

I understand the ESR of these caps is only a fraction of an identical value electrolytic cap plus they'll never leak or have any of the associated long term problems with electrolytic caps.

Oddly enough, the ESR is actually specified as being higher than what we would typically include in our kits, though I'd imagine the Clarity caps will handle ripple current a bit better (and they ought to sound better just based on materials).