An Utter Novice's Simple 2A3 Build

Discussion in 'Tube Gear' started by jmathers, Feb 2, 2018.

  1. jmathers

    jmathers Junior Member

    For right now, this is just an optimistic placeholder in the hope that it will get me started working on building one of these amps. I haven't accomplished much so far but will detail a little of my initial explorations.

    I was able to try some weeks back a wonderful 45 amp, built by Redboy and now owned by Olson, on my newly acquired and cobbled together Altec 2-ways. See this post for details on the speakers:

    My regular amps are a set of AES Sixpacs, which I absolutely love - except for the expense of re-tubing with twelve EL34 tubes and the fact that with the higher efficiency Altec 2-ways now, I have virtually no volume leeway on my preamp. As I type this, playing music through Roon and my iFi dac, the preamp is set to the 3rd detent (7 o'clock) and I'm at about 75dB volume level!
    So a little less power would be welcome here. And it wouldn't hurt to have something with a little more triode magic.

    After reading @marantzfan 's thread on his build I got interested in perhaps trying to do the same.

    So far, I've talked to Jack Elliano over at Electra-Print about output transformers. I'm thinking at this point something like @marantzfan 's OPTs. I'm thinking 3K partial silver at 60ma to 12ohms. Sound right for the 2A3s?

    I've also dabbled a little bit at trying to understand the schematic. I made a list of caps to buy but am unsure of exactly when to substitute a can cap for an electrolytic, etc. Here's my list:

    Power Supply:

    20uF@500v - ASC can cap
    (2) 50uF@500v - ASC can caps
    100uF@100v - electrolytic?

    Tube Circuit:

    (2) 40uF@450v - ASC can cap
    (2) 100uF@25v - electrolytic?
    (2) .22uF@630v - coupling caps/paper in oil/fancy
    (2) 100uF@100v - Cathode Resistor Bypass Caps. @marantzfan used Sprague Atoms.

    So, the goal is to get working as soon as possible. I'll revisit as I get parts and try to understand this crazy tube amplification thing.
    Last edited: Feb 10, 2018
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  2. TubeHiFiNut

    TubeHiFiNut Administrator Staff Member

    Welcome to the SET Revolution. ;)
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  3. mhardy6647

    mhardy6647 Señor Member

    Are two of those capacitors in "the tube circuit" cathode resistor bypass capacitors?
    If so, the gurus tend to intone gravely that they shouldn't be electrolytics. I am slowly but steadily moving towards converting the cathode resistor bypass caps in my "Simple 2A3" to film capacitors. Yes they are huge. That's a challenge.

    There is also -- differing opinion -- as to how big those bypass caps have to be.

    I'll stop there, since I don't even actually know if the amp in question uses cathode bias :)

    Derp, you're asking about the Simple 2A3 schematic, yes? :)
    If so, then I might actually be right.
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  4. mhardy6647

    mhardy6647 Señor Member

    Here're the underpinnings of "mine" FWIW (also, not at all coincidentally, crafted by Redboy):

    [​IMG]Simple 2A3 after the fire 08Jun17 by Mark Hardy, on Flickr

    that bypass cap (electrolytic) circled in yellow was popped when the photo was taken. Those two electrolytics have, in the interim, been replaced with a generic (NTE, gasp) pair of 105 degree rated 100 uF electrolytics (and moved them a bit away from those cathode resistors)... but I have a nice, big honkin' pair of Electronic Concepts film caps to try to find space for in their place.

    This being said, I have been thinking quite seriously about using smaller value coupling caps in their stead. How small? 30 uF,? 50 uF? The cutoff frequency will still be very low (at least relative to the bandwidth of this amp).

    EDIT: For the record, I've re-done some of the wiring on this amp in the meantime, too.
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  5. opa1

    opa1 Moderator Staff Member

    For the 20/2-50uf(100)/40uf caps you can use ASC cans. @marantzfan used the 20 and 2-50uf's. They do take up a lot of area on the amp. PIO coupling (0.22), no brainer. Non-electrolytic bypass caps Would be quite large in the 100uf valve and expensive. Maybe @Redboy has a solution for them.

    EDIT: The 100uf ASC cans should be 2- 50uf.
    Last edited: Feb 2, 2018
  6. Redboy

    Redboy Knobophobe

    I’m planning to build my next 2A3 amp(s) with 50uf caps in that position, most likely the ASC poly in oil types.

    They’re positively enormous compared to the little 100uf/100v electrolytics, though.
  7. jmathers

    jmathers Junior Member

    Yes, the JE Labs Simple 2A3. I thought that's the 2A3 amp you have, is it not? And it's good information you mention since I'd like to avoid any sort of meltdown.:eek:
  8. Redboy

    Redboy Knobophobe

  9. Celt

    Celt Peanut Head

    That's one you lick the terminals to see if its still holding a charge, right?
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  10. opa1

    opa1 Moderator Staff Member


    I'm guessing you are using this schematic. There is a simple 45/2A3 schematic. It only uses one driver 6SL7 tube.
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  11. Thermionics

    Thermionics Post Whore In Training

    You may be opening a can of worms here with regard to advice...

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  12. jmathers

    jmathers Junior Member

    Yes, that's it. I did not know though there was a 45/2A3 schematic. Maybe I should take a look at that since I was interested initially in running both tubes. Hmmm.:chin
  13. opa1

    opa1 Moderator Staff Member

    The schematic posted is a good working amp. I'd stick with it. I was just making a point that that the schematic posted and the simple 45/2a3 schematic are different amps.
    Last edited: Feb 3, 2018
  14. jmathers

    jmathers Junior Member

    I've updated what I think the consensus is on caps thus far in the first posting. If I got something wrong let me know.

    Some things I forgot to mention in the first post.

    From @marantzfan 's thread I did look at Online Metals a few weeks back. I chose a 14"x 14" 6061T6 aluminum plate .125 thick for the main top plate. It came in at $50 since shipping ($18) was a little more than half the cost of the actual plate ($34). I think I'll search around locally and see if I can do better than that. Seems like a lot for a piece of aluminum.

    I also picked up a couple of 6SL7 tubes at a local ham radio show. And, yes, I searched for 2A3 and 5AR4 tubes too but couldn't find any at a decent price. I'm assuming that if I take Redboy's/PakProtector's advice and rewire the tube sockets for 6SN7, 6EM7 use that I will also be able to use 6SL7s. Is that right?

    Thanks for all the advice - even the advice that it may be a mistake to get a bunch of conflicting advice.:)
    Last edited: Feb 3, 2018
  15. marantzfan

    marantzfan Administrator Staff Member

    I keep meaning to chime in here but this last few days has been a little hectic. Now today I'm going to be running around quite a bit, but I promise to add some comments tomorrow that hopefully you might find helpful.

    I'm excited to see you embark on your first amp build. This is a VERY good one to choose IMHO. :)
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  16. paulbottlehead

    paulbottlehead Active Member

    Do not order a 3K:12 ohm transformer, any winder is going to be able to make you a 2.5K:8 Ohm transformer gapped for 60mA. This is an incredibly common transformer design.

    The cathode resistor and bypass cap on the first stage can be replaced by an HLMP-6000 diode with the silver stripe facing ground. Frankly, I would just use the resistor and no bypass cap to lower the gain of the amp a bit.

    The cap across the 68K/2W resistor is way bigger than it needs to be. 1uF would work just as well and save money/space.

    If this is your first build, I'd recommend going ultra budget on all the parts to get something working and get some practice. I've almost always found that the second iteration of something I build is a gazillion times better than the first!
    opa1 likes this.
  17. jmathers

    jmathers Junior Member

    I realize this I think. Jack said he can make me whatever I want. Isn't there an advantage to going a little higher on the primary? Or at least no real disadvantage? That was my understanding. Also my speakers are 16 ohm and I suggested to him that I may want to move to an 8 ohm speaker in the future. Jack recommended a 12 ohm secondary. Again, I was looking for flexibility here.

    I like your idea of going ultra budget since no one has suggested this. However I think it wise to to spend some decent $$$ on quality of the OPTs since these often determine the final sound/voicing of the amp.

    Thanks for the suggestions. I'll have to research your diode and 68K resistor cap ideas too.
  18. paulbottlehead

    paulbottlehead Active Member

    For a given operating point, a higher step-down ratio will give you less power, higher damping, and less distortion. If you increase your operating voltage and decrease the current a little bit, then a 3K transformer will become more optimal.

    If you have 16 Ohm speakers, get a 2.5K:16 transformer with a tap at 8 Ohms. Whatever winder you go with may want to build you something with two tapped 4 Ohm windings that would get you more possible outputs.
  19. marantzfan

    marantzfan Administrator Staff Member

    A couple of my thoughts on this coming off just building one.

    14" by 14" for your plate is a good size and gave me plenty of room to work without feeling like things were cramped. If you opt to use more oil cans then I did, you may want to widen it a bit, maybe something like 14"x16", to give you the room.

    The first two caps in the PS are where I used oil caps, one 20uf and two 50uf in parallel for the 100uf cap called out on the schematic.

    I don't have near as much time as I used to, so my goal was to shoot the moon the first time as I only wanted to build it once.

    I really am impressed by the Electraprint Partial Silver OPT's. I can recommend them without hesitation. The other area where I splurged on my build was on the coupling caps where I used Jupiter Copper foil.

    Again, I don't feel as if it was money wasted but @paulbottlehead makes an excellent point about trying something less expensive first so that you can see just how much (or little) of an improvement boutique caps can offer.
    opa1 likes this.
  20. jmathers

    jmathers Junior Member

    Thanks for the info in general, especially on where you used can caps. I'll likely copy that directly - I ordered a top plate in 14" x 14" size.

    While I appreciate Paul's advice, I'm of the same sort of mind when it comes to building this only once. I'm up for other amp projects but this one is filling a special and important need for me. Parts will be critical.

    Next up: I need to figure out the remaining capacitor choices. What kind of cathode resistor bypass caps did you use? Looks like Sprague's? Electrolytics?
  21. marantzfan

    marantzfan Administrator Staff Member

    Yep, and I made a point to give a bit of room between the resistors and capacitors so they don’t get cooked.

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