New 2A3 SET Stereo

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Built this for my brother-in-law this week. It is based on Sun Audio SV-2A3 stereo minus all the pricey Tamura irons. The cascaded 6SN7 seems to give lots of gain/voltage swing for the 2A3. Very impressed with the bold yet delicate sound. Very easy to build- there is hardly any passive components in the circuit. It would make a great primer amp for beginners (the sound is not entry level at all).

[EWBrown]

Very nice! I've been considering using the cascaded 6SN7 or 6CG7 in place of my usual SRPP driver stage.

/ed B

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Thanks, Ed! I don't have the technical background to understand this but do you know why hi mu tubes (like 6SL7, 12AX7, etc.) are typically used in SRPP?

[Geek]

Mmmmm, 2A3..... my favorite DHT

[highflyin9]

Beautiful work! Very clean and attractive build.

[thehoj]

Built this for my brother-in-law this week. It is based on Sun Audio SV-2A3 stereo minus all the pricey Tamura irons. The cascaded 6SN7 seems to give lots of gain/voltage swing for the 2A3. Very impressed with the bold yet delicate sound. Very easy to build- there is hardly any passive components in the circuit. It would make a great primer amp for beginners (the sound is not entry level at all).

Wow, this looks like a really awesome simple project. And I'm surprised that 2A3's can be had for pretty cheap actually.

I think I've figured out what parts I will need to build this. I'm having trouble locating a 290-0-290 power transformer, and was also curious if you think a rating of 170mA would be enough for the amp as you've built it.
Can I ask what Power transformer you're using?

Also, just curious about the specs on the choke. Can you give any information on what choke you've used?

Oh also, are you using a 5U4G in there? Or is that a 5AR4?
If I used a 5AR4 instead of 5U4G I'd have more like a 20V drop instead of 40V.. So I could probably go a 280-0-280 power transformer instead.
I'd be interested to know what you're using for a PT though.

[EWBrown]

Thanks, Ed! I don't have the technical background to understand this but do you know why hi mu tubes (like 6SL7, 12AX7, etc.) are typically used in SRPP?

SRPP can use hi mu, (6SL7, 12AX7, 12AT7) or lower mu (6SN7, 6CG7, etc) tube, it all depends on the gain requirements and application.

Using a high mu tube in SRPP gives the high gain, and a lower output impedance, as compared to the standard resistance coupled gain stage.

Something that I have been considering is using a high gain SRPP driving a low-mu SRPP, which in turn, would directly drive a power tube's screen grid,
and just apply appropriate bias voltage to its control grid. This might work well with horizontal deflection tubes, which can tolerate fairly high plate voltages, but have lower (150 to 250 VDC) screen grid ratings. Use 300VDC plate supply for the gain stage, and 450-500 VDC plate supply for the screen-grid driver stage.

Just another, yet-untried, crazy SWAG idea....


/ed B

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Hammond 272hX 300-0-300, 150 mA
5-10 Hy choke
Both 5R4 and 5U4 work in the circuit. Remember that you don't have to get the volts exactly right. You can always fine tune the volts by changing the capacitance of the first filter cap. Go lower in value to get lower volts and vice versa.

Min

[DerekVa]

Very nicely done! Was the pair of small 2.5V heater transformers because the transformer you chose didn't have 2.5V taps, or was it to try and reduce system noise?

-D

[EWBrown]

The 2A3s in this circuit need to have their filament voltages isolated from each other, so that the "cathode" resistors will control the current flow through each tube.
Using separate small (2.5V, 3A) filament transformers is the most effective way to do this. Locating them close to their respective tubes also reduces/ eliminates any extraneous 60Hz AC hum.

A power transformer which also includes the two 2.5VAC filament windings would be expensive.

HTH

/ed B

[tmbg]

hammond 302AX is what I used for my 2A3 amp, it has 6.3, 5, and two 2.5 secondaries, and is about a hundred bux. Not the cheapest ever, but not TWO SMILEYS WITH DOLLAR SIGN EYES expensive

[Geek]

The Edcor XPWR183-120 I had designed also for a the 2A3. Not on their online catalogue anymore, but they can make it.

Datasheet: http://edcorusa.com/Content/Manuals/XPWR183_120.pdf