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RossD's 2A3 SET
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Author:  RossD [ 10 Oct 2013, 13:58 ]
Post subject:  RossD's 2A3 SET

Hello all,
I've been slowly purchasing components for a 2A3 single ended stereo amp. I probably should have picked a schematic first, but what the hell.

I've got a matched pair of 2A3 Electro Harmonix Gold Grid triodes, a couple old stock 6sn7gt(b) and four 76 triode tubes. I don't know if they are at all good. I don't have a tube tester. I should be able to pick up tested NOS 76's for $20 a pop, I hope. They make new 6sn7, so I'm less concerned about that.

I've purchased an Edcor XPWR067-100/120. Specs as follows: 320v-0-320v @150 mA; 6.3v @3A; 5v @ 3A; and two 2.5v @ 3A

For output, I have two Edcor GXSE10-8-5. Specs: 40hz-18khz, 5k, 8 ohm, 10 watt. I also have two 15 watt Classic Tone model 40-18031 that have 5k and 8k primaries and 4-8-16 secondaries that I have considered for this project. The core on the '15 watt' guitar amp OT is smaller than the '10 watt' hifi amp OT. Which I understand, but...something to consider. I have the suspicion I will end up buying larger OTs as an upgrade in the future while using the Edcors for now.

For rectification, I have a bunch of old stuff: 80, 83, 5U4GB, 5V4G, 5Y3G, 5Y3GT, 5Z3. I'd like to stick with a larger sized tube for aesthetics. Maybe the 5V4G for the ST shape.

Some of you already guessed which schematic I've had my eye on so I'll share with the rest of you:
Attachment:
JELsc-ST2A3DX.jpg


I do have a concern though. My output transformers both have 5k primaries and schematic shows 2k5 or 3k5. Any suggestions on this front? The 15w guitar amp OTs have 4-8-16ohm on the secondary side so I could get the 2.5k with the use of 16 ohm secondary and 8 ohm speakers, right? I think I'd rather run the 2a3's at 5k, use the Edcors and adjust the cathode resistor.

Another concern is the direct coupling between the 76 and 6sn7. Should I be concerned? I would not like it if a 76 failed and took out a good 6sn7 or vise versa. This is one of the reasons I was turned off by the Loftin-White schematic; direct coupling and the risk of killing relatively expensive tubes, the 2A3-EHs (or so the internet stories say!?!? :| )

I have another Edcor power transformer slated for a 'yet to be designed' preamp. XPWR-136-120 Specs: 250v-0-250v @60mA and 3.15v-0-3.15v @2A. Would I need a preamp to drive the above schematic? I'm expecting: no. I would like a volume pot somewhere, though. Where should I put one? After the 6sn7 or after the 76? What would I change to get that in there?

Any comments? Any schematics you think I should consider? I would prefer not to use any miniature tubes to keep the big tube vibe going.

Author:  azazello [ 10 Oct 2013, 15:40 ]
Post subject:  Re: RossD's 2A3 SET

I don't like more of 2 stages SE. In my SE 300B and SE 2A3 RCA I used 1 stage preamplifier - E180F Phillips/amazing tube/ for driving 300B, /less elements - better sound,IMHO, High End is back proportional to the number of composite parts....my moto/
I think, potentiometer will be in input, and with value about 10 kohm, I use 2.2 kohm....If the meddle point is down / in 100kohm/, upper part is very big resistor for small currents of MF and HF and You will lose them, sound will be with less HF....but You can try....With 10 kohm the picture of sound is more interesting....
About core.....more iron, better sound....Guitar output transformer is small, because quality of sound is not very important, not like in HiEnd, I use 25 w OT for 3 - 6 W SE.

Author:  RossD [ 10 Oct 2013, 15:52 ]
Post subject:  Re: RossD's 2A3 SET

Thanks azazello for the reply. I'm trying to use up some of the other tubes I already have for this amp. I've never heard of the E180F before. It looks like it might be the same as a 6J9P and it looks like I don't have that either. I'm trying to stay with bigger tubes.

Oops, I missed the volume pot on the schematic. I've been looking at so many 2a3 schematics lately...

Author:  mwhouston [ 10 Oct 2013, 15:56 ]
Post subject:  Re: RossD's 2A3 SET

I Iike two stage power amps also but I think the schematic is quite ok. If the pot can go after the second stage it would be a lot better. Better still power amps IMHO should not have pots. The only pot should be on the output of your tube preamp.

I have never heard of a "76". All the others I know of.

Author:  RossD [ 10 Oct 2013, 16:15 ]
Post subject:  Re: RossD's 2A3 SET

So you don't see a problem with the direct coupling of the 76 and 6sn7?

I know there has been discussion of the placement of the volume pot, and I'd tend to agree with your statement.

My initial intention for the preamp power transformer was to build Matt's 4S with a 12au7 for a stand alone preamp.

Author:  mwhouston [ 10 Oct 2013, 17:53 ]
Post subject:  Re: RossD's 2A3 SET

RossD wrote:
So you don't see a problem with the direct coupling of the 76 and 6sn7?

I know there has been discussion of the placement of the volume pot, and I'd tend to agree with your statement.

My initial intention for the preamp power transformer was to build Matt's 4S with a 12au7 for a stand alone preamp.

Agree with the preamp not sure you can swap 76 for 6SL7.

Author:  Isophon [ 10 Oct 2013, 19:15 ]
Post subject:  Re: RossD's 2A3 SET

Hi,

For the OPT I would use the Edcor 5K even if the impedance is a bit higher you will simply have a bit less output power but your damping will increase and your overall distortion will go lower.

2 stage power amp would be sufficient to drive your 2A3, just use a 6N1P-EV biased at 9-10mA and you are up to some great music.

Enjoy building your amp, it's a great hobby !

Iso

Author:  Suncalc [ 10 Oct 2013, 19:18 ]
Post subject:  Re: RossD's 2A3 SET

A few simple comments. First, I don't believe the voltages on the schematic are quite accurate, but that's picking nits. I put together a quick load line on the power stage shown below:
Attachment:
Screen shot 2013-10-10 at 5.03.24 PM.jpg
It shows that with the component values shown, 348v at the plate to ground (as close as I could get), and a 5kΩ load line, you're looking at -55v at the cathode, about 3.6W output, full swing distortion of 2.2%, with an incremental distortion of 0.61%/W. However, the plate dissipation is 17.6W. Older 2A3s were only rated for 15W at the plate. This circuit is definitely running hot for the older tubes. As for new tubes, the JJs are 40W monsters. I don't know about yours.

As for the driver, the 6SN7 needs significant drive to make +/- 55 volts cleanly, this is the reason for the 76 driver. But it's an odd combination from my perspective. The 6SN7 is a much lower noise figure tube than the old 76 design. I would think that you'd want the lowest noise figure tube first in the chain to minimize the overall amplifier noise figure. Personally, I would go with a single stage 6SL7 driver and be done with it. With the 350V B+, the 6SL7 will drive this configuration with ease. In any reasonable configuration the stage gain should be between 45 and 55 meaning that something just over a volt would get you to full output.

Just how I would approach it. Keep us updated on how it goes.

Author:  azazello [ 10 Oct 2013, 19:43 ]
Post subject:  Re: RossD's 2A3 SET

Potentiometer between triodes, /preamp. and 300B/ reflect to freeq. band, changing output impedance of previouse stage, IMO....
......."Miller capacitance can kill the frequency response rather easily, so there are a few things to consider when designing guitar amplifiers. If you desire to minimize the effect of the Miller capacitance on frequency response, you can do the following things:

Reduce the output impedance of the previous stage. This can be accomplished by lowering the value of the plate load resistor, using a tube with a lower internal plate resistance, or lowering the value of any series or shunt attenuation resistors. Obviously, all of these things will affect the gain of the stages, so this must be taken into account as well."
this is from:http://www.aikenamps.com/MillerCapacitance.html

Author:  Geek [ 10 Oct 2013, 21:31 ]
Post subject:  Re: RossD's 2A3 SET

http://edcorusa.com/p/822/xpwr183_120

I had specifically designed specifically for 2A3 w/2 6S*7 drivers.

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