Alternative take on the Radiotron SE2A3

Ok, it's time to get building my second amp (or fourth, depending on my perspective). Before I start posting "progress", it might help to share a little about what this thing is and the reasons for doing it.

I currently own a version of the Radiotron SE2A3 posted on the JE Labs blog: https://jelabsarch.blogspot.com.au/2012/08/radiotron-se2a3_5.html#! I really enjoy this amp into my little GPA 604 monitors. Still, I've been wondering whether using the same tubes in a different topology, with different layout and components would sound different? Plans have been in the making, for years, to find out. I won't bore you with the design objectives, but I will say two things: safety and reliability are paramount and I'd like the amp to be a platform for future experimentation... I've had to compromise on elegance and compactness.

Credit where it is due: the initial design was drawn by Jeff Medwin; he draws a nice schema, with values, voltages, currents and calculation checks clearly marked; very nice. I've taken his design, recalculated the cathode bypass Cs and 2A3 load, added some adjustments, and designed the PS. The design is a 6C6 directly coupled to the 2A3, which runs at my fave I/V ratio and low dissipation: 40mA, 250V with a slightly higher than normal load (4K2). The PS is a simple Pi filter designed for low impedance at audio frequencies, fast and clean recovery, and low enough ripple. About the same power as a 45, but hopefully what there is will be good.

I'm no engineer and probably know just enough to be dangerous. I think of amps as being modulated power supplies and current loops and this one has been designed and will be laid out with that in mind. Still, that needs to be balanced with other design objective and my skill level in mind...

This build is a wacky thought experiment that I would have done differently, or not at all, if I'd known how far I'd fall down the rabbit hole... But hey, that's the fun, right? I'm not for a moment suggesting others should build an amp like this.

So, here's hoping I have something to show for the effort.
 
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The chassis are in the shop awaiting bead-blasting, so it is a good time to refresh my memory of layout and start building up the internals. The amp is designed to be modular: signal sub-chassis, PS sub-chassis, and connector mounting plates. I'm still trying to remember how this thing was meant to be laid out; my drawings are partial and scratchy.

Old_layout_drawing.jpg


A quick check confirms that while everything will fit there is not a lot of room to spare on the signal sub-chassis.

Signal_layout_top.jpg

Signal_layout_side.jpg

This part is a balancing act of good grounding practice and keeping current loops tidy while allowing enough space for fun and provide sufficient ventilation. I need to drill some smaller component mounting posts and then I'll be ready to smell solder. Hopefully things will get interesting as I build up the signal module layer by layer.
 

dowto1000

Junior Member
What a surprise to see. Thanks for the kind words. I don't know , or recall who you are, but I have A COMMENT OR TWO TO MAKE.

Q1) After receiving a schematic from me, why did you not stay in touch so we could jointly discuss changes you were going to initiate? To use your own words " I'm no engineer and probably know just enough to be dangerous."

Q2) Eliminating a cap couple between the amplifier's two active audio stages, is a good ( no...great!! ) thing to do, but why did you stay with the original Radiotron's Pentode 6C6 input tube, when high mu Triodes exist, and sound better ( KISS ) than any multiple element pentode driver from the get go ??? All the better-received, well regarded Loftin-White ( two stage DC SET ) amps I know of, over the last several decades, use a high mu triode driver tube ( Isamu Asano, Nobu Shishido, Craig Uthus / Moth, Don Garber / Fi , Dennis Fraker / Serious Stereo ).

Q3) Are you building the amp as two monoblocks?? That is always the best sounding way to hear 604s, or VOTTs, Valencias, etc etc etc.

Q4) Why are you building multiple sub chassis ?? That will be sub optimal. The goal in a SET - type amp is to have as short a wire path as possible, throughout the amp. This can only be done optimally with monblocks and a LOT of very well thought-out layout.

Since you have obviously corresponded with me before, why don't you continue to do so, and we can look over your proposed build. I am sure I have experience I could relay that will help you, and I would be pleased to assist !!

I get to hear 604 MLTLs, and such an amp as I discuss, this Oct 5-7th, at Denver's RMAF audio show, open to all. You should plan to attend, as you will hear the best amps and best 604 speaker ever made, partnered together. Its really fun to listen to, mind boggling in a way, and has a bottom-end to die for.

Dowto1000
 
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Hi Jeff,

Let's make this succinct.

a1) Creative differences.

a2) to a4) I'm not interested in an absolutist, follow the dots approach to "helping". I'd like to experiment with different design goals and techniques.

I can read your suggestions on innumerable other threads if the inclination strikes.

Edit: sorry, that came off a bit snarky - just don't want to get into circular exchange when your suggestions/opinions are already posted extensively.

Regards.
 
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Redboy

Knobophobe
I'm still trying to remember how this thing was meant to be laid out; my drawings are partial and scratchy.
Hey, I could have said that about any one of my projects! I have a feeling I'm going to like this one.

Love the subchassis and the progress thus far. What output transformers are you using? I don't recognize that one, offhand.
 
Thanks for the kind words all!

What output transformers are you using? I don't recognize that one, offhand.

They are J&K Audio Design L3 custom wound on NanoCrystalline C cores a few years ago. From memory, the winders had to work closely with the core suppliers to ensure correct and consistent crystal size - a lot went in the bin. They are technically competent transformers... hopefully they'll help deliver the type of sound I'm after. My doubts are they'll be a little "tilted up" in response, but apparently the cores and winding geometry have addressed that. The J&K guys thought they'd be the best option... let's see. :)

Edit: they have big honkin' M5 bolts protruding 20mm from the base. Solid, but a pain to work or mount around. Later casings have female mounts.

Okay, time to get a drillin' those posts.
 
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Well, the weekend's here and it's time to get down to building.

Picked up my enclosure parts from the blaster yesterday. Not what I was expecting, but in the name of progress, I'll acept it and proceed with the build. Here are some pieces drying out after a clean and hose down:

Chassis_hose_down.jpg

The enclosures are 304 stainless: laser cut 2mm for the base and main enclosure and 1.6mm for the cage. I wanted a glass bead blast finish, but the blaster did not seem confident and suggested low-pressure garnet blasting, which leaves a matt frosted finish with obvious but not rough texture to the touch.

I wanted to try "non-magnetic" stainless as a learning experience. What did I learn? How I went about it was expensive; unjustifiably expensive. Stainless seems like an almost ideal material, until you factor in its working and finishing. Peened and abrasive finishes will scratch (the more texture the greater the likelihood). Stainless, unless prepared well, does not take paint well. Given that how I build probably is not greatly affected by a mildly magnetised chassis, next time I'll go back to quality steel with powder coat. This stuff is pretty "at the margins" in any case.

Finally, they are big suckers for a 2W amp. That is what happens when you leave extra space for easier access and and retaining flexibility. Despite measuring and planning carefully, I was surprised how physically large they are when I first saw them.

Now, time to heat up the station and do some real damage. :o
 

marantzfan

Administrator
Staff member
Very cool. I’m anxious to see how it all goes together.

It sure looks like you’ve spent a great deal of time and effort planning out your build down to the smallest details.

This should be really fun to watch.
 
Thanks for the kind words and encouragement. Yes, probably too much time...

Short day of soldering - I'm a bit rusty and using an unfamiliar iron... running 50 degrees hotter than usual has made quite a difference! I'm slowly getting back into the flow (dreadful pun, sorry).

Learning for the day:

Despite extensive planning, it is easy to overlook something important (and obvious!). Today I realised/remembered that:
  • 18awg solid core is stiff. I was intending to solder 2x 18awg slid core wires to the delicate leg of a Vishay 1280G trim pot. Reality check time: ain't happening; need to find an alternative.
  • Did I plan enough space for the local PS caps? Maybe, just.
One simply can't control for everything - just like life! :cool: Include flexibility and tolerance into your planning and put your (my!) inner perfectionist on the shelf - it's more fun that way.

Joint prep matters. I cleaned oxidised crap off even shiny gold connectors, not to mention component leads. Never compromise prep - spend (much) more time on prep than actual soldering.

My brain works differently when building and it flows into other areas of life. Wonderful. Make the time to experiment, try new things and, of course, build.

I enjoy this stuff. Worked for two and half hours without thinking about a break (bad!)... my attention was focussed on the task at hand... though I still made some mistakes.
Okay, enough talk:

day_1a.jpg day_1b.jpg

Tomorrow I put in some caps and build up the next level. But for now, I'm off to have a beer or three.

Edit: A beer or... well, I lost count.

Cheers all.
 
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A frustrating day to today - more annoyed at myself than anything else. I set a target, rushed and did not think through the layout or build order sufficiently. And it's lookin' a little messy... but I guess that's what happens when using big caps and attempting a direct layout.

Learning for the day:
  • Setting daily targets turns the build into a (rushed) task - too much like work. I enjoy the process more when I take my time and think it through.
  • Don't faff and fuss. The Yamamoto socket pins are a bit of pain to solder to, but all seemed well (and probably was) until I saw that solder had not cleanly covered the whole joint on a few pins, although everything else about the joint was fine. So I revisited them... and made a bit of a mess. There are plenty of other examples of faffage throughout the day. Think, do, test, and be done.
Here's where I'm up to:day_2a.jpg
day_2b.jpg




Thanks for looking. Hopefully I'll be able to make a bit of progress through the week and Easter.

Cheers.
 
Time for a brief post-Easter update. I didn't have as much time as I'd have liked to work on the amp. And, to be honest, I feel like I'm struggling with this build. My concentration and care is not what it could be, I've been reworking more than I'd like, and I can see a few areas that could be significantly improved. But hey, it's a learning experience right? :)

The 6C6 input stage has been a bit tricky - I could have planned the cathode circuit/layout a bit better. The main R, variable R and 10 Ohm measurement R string is messy and I'll be in a world of hurt if the pot needs replacing...

Still, it is edging forward and it's looking like a pretty nice build that will hopefully sound decent. Stage current loops are tidy enough (if not as short as I could have done) and I've been able use component-to-component connections more than I'd planned, which is good.

Anyhoo, photos of my efforts:

Easter_1.jpg Profile shot, before adding the capacitor platforms. Fiddly 6C6 cathode circuit in the foreground. Those Vishay 1280G pots are nice, but I'm constantly afraid I'm going to damage the pins. That ambitious vertical mounting is so their adjustment screw is accessible through the 3.5mm holes machined into the sub-chassis... and the chassis.

Easter_2.jpg
Now with the capacitor platforms in place.

This module is almost ready to go into a chassis... just a couple of wires to add. I will build the other module first though - this one has the layout I need to apply to the other channel.

Oh, if you see that I've missed something, please let me know. I suspect I have... :o

Cheers.
 
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dowto1000

Junior Member
Hi Shoshin ,

You concluded

" Oh, if you see that I've missed something, please let me know. I suspect I have... :o "


OK.

As a general comment, your choice of wiring looks like it is too high in loss .

The green wire coming off the red cap is what I am looking at. You should solder the cap's lead out closer to film cap's body, so you will not have to rely on the cap's PUny sized leads, and use either good quality, wideband larger wire, or, maybe double-up the green wire you have on hand.

It is a 2A3 power amp, and inadequate wiring causes a loss of dynamics, and dynamic contrasting of the music, through the amplifier.

Always wire your 2A3 amp with the term " peak instantaneous " going through your mind. Also, the term " transfer efficiency" comes into play.

Warning : Be aware however, that too much wire can roll off highs, so you have to learn to employ wideband wire, and use different gauges for different parts of the 2A3 amp.

The effects of optimized wiring inside a 2A3 amp, and on to the high efficiency driver's terminals, is truly - much fun to hear.

I liked seeing your three-dimensional wiring. Good. Wires generally ( there are exceptions ) should float in air and cross at 90 degrees to each other. One should NEVER ever bundle wires together inside a amp or preamp, that is to avoid doing.

Ask questions. In the below-deck photos of my " 46 to JJ 2A3 " thread, I was using either single or doubled-up Mil Spec m22759/11 wire, in 12, 14 16 and 20 gauges, in different amp locations.

http://www.hifihaven.org/index.php?threads/from-a-type-46-set-to-a-jj-2a3-set-in-one-weekend.3448/

Have fun. Peak Instantaneous !! Transfer efficiency. That will rattle some of the boys' cages. But, it works !!

Dowto1000
 

paulbottlehead

Active Member
Hi Shoshin ,
Also, the term " transfer efficiency" comes into play.
No, it doesn't. We've been over this before on a different forum. High transfer efficiency is TERRIBLE for an audio circuit, as distortion will increase dramatically as transfer efficiency increases (again, this is a defined term, you don't get to make up your own definition for something that is already defined).
 
Jeff, thanks for your suggestions. I don't see your proposed theory of the wiring being "high in loss" holding up to technical assessment. I'd like to give it a go someday, but not today.

Some guiding principles for the build are:
  • Good signal grounding technique using decently-implemented multiple star. There are a lot of sub-points to this; I re-read my grounding summaries and some articles before each build - take nothing for granted when it comes to grounding.
  • Calc the circuit and use appropriate values; try to understand the inherent compromises.
  • Power supply matters: low enough impedance, low enough ripple, quick and tidy recovery, and no weird resonances. This can be a contentious area - I'm playing with some Thorsten Loesch (sp?) ideas.
  • Keep current loops direct, (ideally) as short as practicable, and appropriately wired. The cathode matters.
  • Minimise dissimilar conductors. Component-to-component where possible and desired; otherwise, use quality wire of sufficient size.
  • Minimise parasitic capacitive coupling - between components themselves and the chassis.
The first two points are fundamentals. The 3rd point is an attempt to achieve a relatively dynamic SET amp. The final 2 - 3 points are largely about voicing.

Will this make an audible difference? Let's find out, though I won't be getting all reductionist and explaining what I think is the impact of each point. Should make for a relatively quiet amp (AC filaments aside!) at least.

paulbottlehead, I'm guessing you are aka CB and PB on other forums? Nice to see a familiar face, so to speak.

Edits: cleaned up some typos and added some brief thoughts about each point.

Cheers.
 
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dowto1000

Junior Member
Jeff, thanks for your suggestions. I don't see your proposed theory of the wiring being "high in loss" holding up to technical assessment. I'd like to give it a go someday, but not today.

Some of my guiding principles for the build are:
  • Good signal grounding technique using decently-implemented multiple star. There are a lot of sub-points to this; I re-read my grounding summaries and some articles before each build - take nothing for granted when it comes to grounding.
  • Calc the circuit use appropriate values; try to understand the inherent compromises.
  • Power supply matters: low enough impedance, low enough ripple, quick and tidy recovery, and weird resonances.
  • Keep current loops direct, (ideally) as short as practicable, and appropriately wired. The cathode matters.
  • Minimise dissimilar conductors. Component-to-component where possible and desired; otherwise, use quality wire of sufficient size.
  • Minimise parasitic capacitive coupling - between components themselves and the chassis.
The last 2-3 points are largely about voicing.

Will this make an audible difference? Let's find out, though I won't be getting all reductionist and explaining what I think is the impact of each point. Should make for a relatively quite amp (AC filaments aside!) at least.

paulbottlehead, I'm guessing you also answer to CB and PB as well? Nice to see a familiar face, so to speak.

Cheers.


Soshin,

Bravo - AC heating of a 2A3 filament, IMHO, IME, is the very best way to implement it, sonically speaking, performance wise.

I see you are reading "all the books" from your post's above list. These books do lead you astray, I am sorry to tell you.

Having an amp that is " quiet" should not be your design goal.

That will be a "dead-sounding" amp, they always are, when uber-quiet is used as the design philosophy. Boring ... to hear. Emotionally un-involving. Just plays a tiny little bit of the midrange's glory...that is all it might do !!

We really should be building, IMHO, an amp that will play dynamic and wide-band to the maximum, " get up and boggie with the music. " A " play-the-music " amp,..... and not a tuner - only playing a " little bit of the midrange ". These rare-to-find, well-designed 2A3 amps I describe, are fun, and long-term involving to use.

It takes good wiring, as I suggested, to do the instantaneous peak currents, etc. The EE's traditional formulas, to determine wire size ( AWG ) are largely worthless in an optimal 2A3 amp build, is also whats been found.

Fortunately, we have the 2A3 amps, with the perfomance, available for all to hear, that fully backs-up what has been posted. You ought to come hear it.

Dowto1000
 
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Soshin,

I see you are reading "all the books" from your post's above list. These books do lead you astray, I am sorry to tell you.

Having an amp that is " quiet" should not be your design goal.

That will be a "dead-sounding" amp, they always are, when uber-quiet is used as the design philosophy. Boring ... to hear. Emotionally un-involving. Just plays a tiny little bit of the midrange's glory...that is all it might do !!

Dowto1000

Jeff,

Not sure what you are trying to achieve with your post. You appear to have created your own context for my words "relatively quiet" then used it as an opportunity to repeat here as you do frequently.

I am open to suggestions and learning. Irrespective, the objectives for this build stand. With that in mind, I have no interest in getting into a circular, strawman-based discussion with you. I'd rather be listening to music, building an amp, digging around the garden, or, well, almost anything.

Let's respect that we have different design objectives and that my thoughts expressed in post #5 on this thread haven't changed. Thanks in advance for your understanding and future restraint.

Cheers.
 
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