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RossD's 2A3 SET
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Author:  RossD [ 11 Oct 2013, 07:28 ]
Post subject:  Re: RossD's 2A3 SET

Well, if people are suggestion 6SL7, then how does this schematic look to everyone?

I'd like to use what I have already but since I had to buy the 2a3 triodes already, what's the harm in buying one more tube. :D :headphones: (If you note, the schematic says 2a3 or 45 and I actually do have a single 45 triode. I just don't have a pair. :( ) I don't know if building the amp to handle both the 2a3 and the 45 tube is the right move. No one makes new 45s, I only have one, and good used ones go for more than my pair of 2a3s cost me.

My last two stereo tube amps (6aq5 and 6k6) where mostly DIY endeavors, but I'd like to reach a little further into the audiophile side of things with this 2a3 amp.

I looked at the 5z3 tubes that I have and they are closer to the size of the 2a3s so I'm going to go with one of those for rectification.
The first filter cap appears to be 1.5uF. Seems like it's off by a decimal point, or is it not an electrolytic? I'll probably get a choke that is rated for 150mA, which is the PT's rating.

I think I'd still like the volume pot after the 6SL7. Any suggestions on that front?

I already have two 50 ohm 5 watt hum balance pots (and since the 2.5v windings don't have a CT), so I will be using those.

Thanks for the link, but I've got my power transformer already.

Author:  mwhouston [ 11 Oct 2013, 08:50 ]
Post subject:  Re: RossD's 2A3 SET

Shared Rk and bypass cap? A little Decware? Nice simple schematic. Can you really drop a 2A3 or 45 in with no mods? I like that.

Author:  RossD [ 11 Oct 2013, 09:42 ]
Post subject:  Re: RossD's 2A3 SET

I wouldn't share the Rk and bypass cap. Each filament has it's own winding without a center tap on the power transformer and I'd use the 50 ohm hum pot similar to the one in the first schematic.

Author:  Suncalc [ 11 Oct 2013, 13:41 ]
Post subject:  Re: RossD's 2A3 SET

Ok, a few more points. Channel separation is important. So sharing cathode circuits is really out of the question. I think things will go better if you approach this is a somewhat more methodical manner. You have some nice Edcor outputs, so use them. This fixes your load impedance at 5kΩ, not a problem. Second, upon further investigation, the EH-Gold grid 2A3s are a Pd=15w tube. This means that the first schematic is too hot for your assets; you'll need to adjust the bias. Consider the following output load line plot.
Load Line 2.jpg

The red line represents the plate dissipation power limit for the tube you have. This is a 5kΩ load with a 1200Ω cathode resistor and a 50Ω hum pot across the heaters. The quiescent plate dissipation is 14.3W (about 5% under max). It has a 3.9W full swing output, a full swing distortion of 3.5%, and an incremental distortion of 0.9%/W. This is a good solution for your output stage given the parts you have on hand.

The 5V4 rectifier you have on hand is relatively low loss so the ≈360v B+ requirement should be easy to meet. In this configuration, the 2A3 will live with up to 0.5MΩ of grid circuit resistance and the Miller input capacitance is only about 62µµf. As such, I would use a 10kΩ grid stopper and a 250kΩ potentiometer volume control for the grid resistance (or just a 470kΩ resistor if you agree with Mark's assessment that power amps shouldn't have a volume control). Your high end frequency response should then be fine. A simple 6SL7 with about a 220kΩ load, properly biased, should complete the design. If you like, I could provide you a schematic.

Author:  RossD [ 11 Oct 2013, 14:49 ]
Post subject:  Re: RossD's 2A3 SET

A schematic with a volume pot would be great Matt. Thanks a bunch. I would like to have a quiet ps this go around. I am guessing i should use a choke.

Author:  Suncalc [ 11 Oct 2013, 23:54 ]
Post subject:  Re: RossD's 2A3 SET

Ross, I've run into a snag.

The inclusion of the volume control (between the 6SL7 driver and the 2A3) is causing undue distortion in the driver stage due to loading constraints. So I'm going to recommend that you either forgo a volume control all together (using driver volume to control the amp), or I'll put the volume control up front (in which case I highly recommend using a switched attenuator rather than a traditional potentiometer). Which would you prefer to see on the schematic?

Author:  RossD [ 12 Oct 2013, 08:03 ]
Post subject:  Re: RossD's 2A3 SET

Would there be an advantage to using a seperate tube preamp that has a volume pot? If so i would go for no volume control in the power amp, but if not I guess I would go with a stepped attenuator. I would go with your thoughts on this. Thanks Matt for you time and effort.

Author:  Suncalc [ 12 Oct 2013, 14:49 ]
Post subject:  Re: RossD's 2A3 SET

RossD wrote:
Would there be an advantage to using a separate tube preamp that has a volume pot?
Well, up until recently I would have said no. However, after using my 4S Universal in front of a couple of different amps and doing some tube rolling, I have changed my opinion and now say you should give it a try. Even if you do, a stepped attenuator up front really can't hurt anything. Just replace the grid resistor in the driver stage with the attenuator.

Now here is my recommendation for your amp. This is a basic two stage (driver/power) solution. Po is 3.9W peak into the output transformer. Input sensitivity for full output is 1.26v peak. Distortion is 4.0% at peak with an incremental distortion of 1.0%/W (essentially all second harmonic). Here is the schematic:
2A3 Amp Schematic.jpg

For those of you who would like to see the design particulars, here are the load lines for the driver and power stages:
6SL7 Load Line.jpg

2A3 Load Line.jpg

As for the power supply, this should be a simple design. I'll run some numbers on the transformer and rectifier and give you some recommendations. If you could supply the DC resistances of the high voltage secondary winding and the primary winding, that would help in the overall PS design.

As always, questions, comments, or ranting are more than welcome. Let's hear what people think.

Author:  mwhouston [ 12 Oct 2013, 17:53 ]
Post subject:  Re: RossD's 2A3 SET

Two things: firstly I like the design sans pot and secondly secondly ( and I have made this comment before) most systems sound better with a preamp whether they need it or not. This last comment was originally made by a reviewer in Stereophile just so few months back. I always agreed with the statement but it was really driven home very recently when I put a 4SUTea box preamp, with jFET buffer stage, in from of my Chip and Tube amp, Number 9.

Number 9 really shouldn't need a preamp with the tube (6N3) on the PCB being the preamp or at least buffer. For quite a while I ran it without a preamp (but I was not happy with the sound) until I tried it with the single stage tube (12AU7) preamp. Big difference and a lot better. Just a few days back I took out Number 9 and put back the Silver Dragon (300B designed by Matt). The 300B is a lot better.

So I would always recommend a preamp and especially in this case where this amp is really only a power amp. A preamp is a must!

Matt can we really swap the 2A3 for a 45 with no changes to values?

Author:  RossD [ 12 Oct 2013, 18:07 ]
Post subject:  Re: RossD's 2A3 SET

I like the idea of the 4s preamp into the 6sl7-2a3 amp.

Just so I am on the same page with everyone, I measured the resistance of the power transformer windings with my dmm straight across the leads, nothing special.
I measured the resistance of the 120v-0 as 2.6 ohm, then as 2.9 and again as 2.6 ohms.
The 320v-320v measured three times as: 107.8, 108.4, 107.8 ohms.

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