2a3 amp

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Bizzie
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#1 2a3 amp

Post by Bizzie »

In a fit of madness I have bought a pair of sovtek 2A3's of ebay (cos they were cheap) and now want a nice simple design to get me started.

I have a pair of multi tapped (5k/2.4k+ 4,8,16ohm) output xformer's that I bought off Mike a while ago and 2 or 3 mains xformers but they're probably around 100ma and 550ct and 6.3v 3amp any good?.

I have also bought a xformer with 2 x 2.5amp windings for the 2a3 heaters.

There seems to be loads of schematics out there but which one to build for a numpty like me :D
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Nick
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#2

Post by Nick »

Loads of options as you say. I like CCS loaded d3a as a driver, but 6sn7 works well. I am sure others with also have suggestions.
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Bizzie
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#3

Post by Bizzie »

What advantage does the ccs offer and would this make a more complicated build as I'd really like to start off as simple as possible and perhaps upgrade once its up and running.
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Nick
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#4

Post by Nick »

It makes the driver more linear. Yep, you can start with a resistor anode load and go from there.
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Dave the bass
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#5

Post by Dave the bass »

That Shishido/LW/Bluebell ECC83->DC coupled-> 2A3 is pretty simple and worth a go IMO.

A few fella's here have had a bash on it, it's 'rocker' of an amp.

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simon
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#6

Post by simon »

Good question Alec, I want to build another 2A3 amp... one day.

Shishido LW is a decent place to start, and I quite like the thought of Nick's CCS D3a - might have a tinker with that sometime.

But if you want to build from a proven circuit I don't know too many options. I'm sure there are plenty out there, and there have been a few on DIY Audio, but another option might be Gordon Rankin's Bugle which actually uses 45s rather than 2A3s but is easy to convert. That's what I did when I built it: http://www.atrj13.dsl.pipex.com/na-Bugle.htm

Your OPTs would be suitable for both 2A3 and 45 too, but the mains tx is a little small at 100mA. 2A3s generally run at 60mA each plus whatever for the driver - 200mA might be a bit more comfortable? You might just get away with 45s...
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pre65
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#7

Post by pre65 »

I like the CCS D3a idea myself.

Something similar was contemplated before 833a took over. :wink:

If Bizzie has several 100ma mains transformers perhaps monoblocks are in order ?
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Nick
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#8

Post by Nick »

Simon, I think you were there when I took that CCS d3a 2a3 amp to Steve's. Many years ago now though.
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Bizzie
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#9

Post by Bizzie »

simon wrote:Good question Alec, I want to build another 2A3 amp... one day.

Shishido LW is a decent place to start, and I quite like the thought of Nick's CCS D3a - might have a tinker with that sometime.

But if you want to build from a proven circuit I don't know too many options. I'm sure there are plenty out there, and there have been a few on DIY Audio, but another option might be Gordon Rankin's Bugle which actually uses 45s rather than 2A3s but is easy to convert. That's what I did when I built it: http://www.atrj13.dsl.pipex.com/na-Bugle.htm

Your OPTs would be suitable for both 2A3 and 45 too, but the mains tx is a little small at 100mA. 2A3s generally run at 60mA each plus whatever for the driver - 200mA might be a bit more comfortable? You might just get away with 45s...
I've just checked the mains xfomers and they are 80ma 275-0-275 and 6.3 3amp and I have 2 identical ones but I have only ordered 1 2,5v heater xformer for the 2a3's so mono blocks are a bit tricky unless I buy another one

I have a danbury mains xformer which has 6.3v @ 7A CT, 5v @ 3A and either 250v or 375 @ 350mA but doesn't have a centre tap on the HT and is also way over spec for this amp but could be used I suppose.
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#10

Post by Cressy Snr »

Bizzie wrote: II have only ordered 1 2,5v heater xformer for the 2a3's so mono blocks are a bit tricky unless I buy another one
The thing is you'd still need the two 2.5V transformers even if you used a stereo chassis. With the 2A3s being direct heated filaments you ideally need one filament transformer per valve.

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Bizzie
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#11

Post by Bizzie »

Ok but the transformer I've ordered has 2 x 2.5v @ 2.5A each so I thought that on winding for each valve is that OK or do I really need totally separate Xformers?
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#12

Post by Nick »

No, thats fine. 275-0-275 and a valve rectifiler, or SS with a dropper should give you 350, maybe drop a bit in the power supply, you want to aim for a B+ around 300-320v.
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#13

Post by Cressy Snr »

Bizzie wrote:Ok but the transformer I've ordered has 2 x 2.5v @ 2.5A each so I thought that one winding for each valve
Ahh...that's alright then :) I thought you had ordered a TX with only one 2.5V winding.
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Bizzie
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#14

Post by Bizzie »

I've had a look around and found another danbury mains xformer this one has 6.3v CT @3A and either 240 or 260v no CT @250mA I think this may be the one but I'll obviously need to use a SS full wave bridge so no soft start whats the thoughts on this will it be OK?
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#15

Post by Nick »

You could sue a hybrid SS + valve bridge if you have a rectifier heater supply. Otherwise just use some UF7007 and a CLC and away you go. Another RC to feed the driver stage. Driver stage of your choice and you are away.
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