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 Post subject: Re: RossD's 2A3 SET
PostPosted: 12 Oct 2013, 18:26 
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RossD wrote:
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.

The 4SU preamps and all the 4S preamp only work well into a high impedance the pot. 250K is the suggested value and Alps Blue Velvets are my choice. Once again I would suggest the 12AU7 in the preamp. I found the 12AX7 too aggressive in sound and can clip. But there are lots of tubes you can try with the 4SU.

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 Post subject: Re: RossD's 2A3 SET
PostPosted: 12 Oct 2013, 18:29 
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Suncalc wrote:
Matt can we really swap the 2A3 for a 45 with no changes to values?
Nope. Not unless you'll appreciate the in-tube fireworks as your 45 tubes go to tube heaven. :eek:

The type 45 tube is kind of a younger sibling to the 2A3. They are similar, but the 2A3 has a max plate dissipation of 15W and the type 45 has a max plate dissipation of only 10 watts. In this design you would have to significantly increase the bias resistors to get the dissipation down. And then the operating point would not be very good. It might be possible to slip a 2A3 into a 45 design, but it also would not sound very good. As far as I'm concerned, the two tubes are far enough apart in characteristics that they really require dedicated designs.

Just my opinion on the matter.

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 Post subject: Re: RossD's 2A3 SET
PostPosted: 12 Oct 2013, 18:54 
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Suncalc wrote:
Suncalc wrote:
Matt can we really swap the 2A3 for a 45 with no changes to values?
Nope. Not unless you'll appreciate the in-tube fireworks as your 45 tubes go to tube heaven. :eek:

The type 45 tube is kind of a younger sibling to the 2A3. They are similar, but the 2A3 has a max plate dissipation of 15W and the type 45 has a max plate dissipation of only 10 watts. In this design you would have to significantly increase the bias resistors to get the dissipation down. And then the operating point would not be very good. It might be possible to slip a 2A3 into a 45 design, but it also would not sound very good. As far as I'm concerned, the two tubes are far enough apart in characteristics that they really require dedicated designs.

Just my opinion on the matter.

That's all I need to know. Thanks.

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 Post subject: Re: RossD's 2A3 SET
PostPosted: 12 Oct 2013, 19:57 
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Ok, here is my first PS put. I have used the 5V4 rectifier that Ross said he liked. The good news is that you can hit the required B+ with this rectifier and transformer. The bad news is that it only gives 4V of margin. In essence, any more than 40Ω of resistance in the filter and the B+ falls behind. Here is my spreadsheet. It's the 371v number at the bottom that gives me pause.
Attachment:
5V4 Rectifier.jpg

My strong recommendation is that you go with a 5AR4/GZ34 rectifier. You'll pick up an additional 14v or so which will give you some margin to actually hit the required B+ cleanly. The GZ34 is available both old stock and new build. The current production JJs are excellent.

Ross, let me know and I finish the power supply design.


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 Post subject: Re: RossD's 2A3 SET
PostPosted: 12 Oct 2013, 21:10 
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Location: Darboy, WI, USA
5AR4 it is then. I was hoping to use the big ST shape of the 5z3 but that can wait for a different project.

Thanks again Matt for all the help. If you are ever in Wisconsin, I definitely owe you a beer.

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 Post subject: Re: RossD's 2A3 SET
PostPosted: 12 Oct 2013, 21:16 
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Location: Darboy, WI, USA
Mark,
I tried a 12au7, a 12at7, and a handful of new and old 12ax7 and I settled on the 12au7, too.

Any suggestion on brands for 12au7, 6sl7, and 5ar4?
-JJ electric for the 5ar4.

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Ross in Wisconsin


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 Post subject: Re: RossD's 2A3 SET
PostPosted: 12 Oct 2013, 23:13 
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RossD wrote:
Mark,
I tried a 12au7, a 12at7, and a handful of new and old 12ax7 and I settled on the 12au7, too.

Any suggestion on brands for 12au7, 6sl7, and 5ar4?
-JJ electric for the 5ar4.

I'm not a 6SL7 user though that is about to change. Nor can I comment about the 5AR4 but I have used a number of 12AU7s. Current Gold Lions and some old Ozzie tubes-Miniwatt. I was a big fan of Golden Dragons but can't get them easily any more. I'm told JJs are very good.

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 Post subject: Re: RossD's 2A3 SET
PostPosted: 13 Oct 2013, 20:31 
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Location: Darboy, WI, USA
mwhouston wrote:
I'm not a 6SL7 user though that is about to change.


Are you saying you are going to build this with me? :beerchug:
Just don't show me up. :up:

I know you will. You do good work. I will probably copy your layout!

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Ross in Wisconsin


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 Post subject: Re: RossD's 2A3 SET
PostPosted: 13 Oct 2013, 21:58 
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RossD wrote:
mwhouston wrote:
I'm not a 6SL7 user though that is about to change.


Are you saying you are going to build this with me? :beerchug:
Just don't show me up. :up:

I know you will. You do good work. I will probably copy your layout!

No I'm not going to build one but will require a 6SL7 for my 211 build. That's what I meant.

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Projects: "Carbon” - MM phono preamp | 180W RAW Class D amp | "Illusion" - 6SN7 SRPP preamp in a plastic jiffy box | “Salt Cellar” - Mono 807 triode integrated amp |


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 Post subject: Re: RossD's 2A3 SET
PostPosted: 21 Oct 2013, 07:25 
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Location: Darboy, WI, USA
Matt,
Any luck with the design of the power supply?

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Ross in Wisconsin


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