2a3 parallel driven 6sn7 schem

[jer3my]

Hi,
I'm a first time tube amp builder wannabe. i've got a pair of EH 2a3 tubes and a pair of tungsol 6SN7 tubes laying around.
Can somebody please post a good and simple schematic for me to follow please? probably parallel feeded. A bit of explanation of the schem would help me much also. Cheers!

 

[Richard Ellis]

Jeremy?
Welcome, your set of tubes is probably the best set you can start out with, take a look at www.angela.com.
Check out the "2001 edition" 2a3 build, very simple & straightforward.
A pair of 6SN7s' give you great flexibility to drive a pair of 2A3s'.
Granted the schemo is for 6SL7 but the schemo can be tweaked for 6SN7. Unfortunately the schemo uses dirt cheap OPTs' and should be upgraded to--- One Electron, Electraprint, Edcor, or if you dare Plitron output transformers.
______________________________________Rick.........

 

[jer3my]

any other

any other schems I should consider as a choice?

 

[kmtang]

You better pick the best Output Transformer for your 2A3. Otherwise it will be a waste of the beauty from the 2A3.

I am running it with Lundahl's LL1623 60mA SE OPT. Sounds great.


Johnny

 

[pageboy]

On the Angela 2A3 circuit, from a poor ignorant noob, what would it take to use a 6SN7 in the circuit, and what is wrong with a 6SL7?

 

[jer3my]

Can somebody post the resistor values on the angela schematic for 6SN7 as the driver?
cheers

 

[Richard Ellis]

A very simple 6SN7, 2A3

Found an even simpler 2A3 amp, by combining these two schematics we can make a very easy to build amp.
First off, thanks to Alessandro Galavottis very straightforward design for 1/2 of a single 6SN7 for a driver, going to a 2A3.
We can use his design for the front end & the angela design for the 2A3. Still working on number crunching for a merged schematic as I'd like to use the easily available Hammond transformers.
_____________________________________Rick.........

 

[jer3my]

The tubes in my possesion are a pair of EH 2a3, a pair of Tungsol 6SN7
and an EH 5U4GB. Will use all of them. parallel 6SN7 will make more output power?

 

[Richard Ellis]

As to my previous post we unfortunately can't 'tweak the 6SL7 circuit to accept a 6SN7 as it would require a full redesign.
Yes 2A3s' in parallel will increase the power output (not the 6SN7s') , but we need to take into consideration if the 6SN7 can drive the increased load, furthur you would need to get two more 2A3s'! And you can't mismatch brands.
Just search "SE 2A3 tube amp"..there are lots of these out there.....especially the 6SN7 variety.
The Angela and Galavottis can be put together...using the Angelas tube rectifyed PS, & the Galavottis amp circuit.the only caveat is dropping the PS from 350VDC to 324VDC....

 

[pageboy]

Thanks for the help.

What is so bad about a 6SL7 in the first place?

 

[Richard Ellis]

Its' not "bad" , just different, the 6SL7 is a high mu (Amplification factor), and can hold a lower anode voltage (Low power).
The 6SN7 is a lower mu (20 vs. 70) and can manage some 450 volts. You have to take into consideration these values when designing your amp. Like tearing thru anode curves like these.........Part of designing an amp circuit.
___________________________________________Rick............

 

[pageboy]

That explains some of why the circuit needs to be redesigned. But what is the point of wanting to use a 6SN7 over a 6SL7? I'm guessing some subjective thoughts on the sound of high mu tubes or something? (You're talking to a very green noob here.)

 

[SHiFTY]

You could always use the driver stage design from a typical 6SN7-300B circuit at slightly reduced voltage. Simply replace the 300B with a 2A3 and lower the voltage as reqd.

I have done this, the gain is a little high but otherwise it works fine.

 

[Richard Ellis]

Check this out.....................www.6SN7.com
______________________________________________Rick........

 

[Miles Prower]

That explains some of why the circuit needs to be redesigned. But what is the point of wanting to use a 6SN7 over a 6SL7? I'm guessing some subjective thoughts on the sound of high mu tubes or something? (You're talking to a very green noob here.)

The 6SL7 was deliberately designed to have a very high Rp, both static and dynamic. It will easily operate with plate currents south of the 1.0mA mark, and its Pd(max)= 1000mW. That allows for large plate load resistors for high voltage gains without requiring high DC rail voltages.

Unforch, that makes for a VT that can drive Hi-Z, low C loads only. That doesn't describe the grid circuit of a 2A3 by any means. Trying to drive that with a 6SL7 would mean severe slew limiting since there won't be enough current to charge Ci + Cmiller + Cstray at the high end. When that 2A3 tries pulling grid current, a 6SL7 driver stage will just roll over and die. Those are some severe sonic compromises right there.

The 6SN7 isn't so limited, and it can pull quite a bit more current (6SN7GTB: Pd(max)= 5.0W) at the expense of less voltage amplification. It'll stand a much better chance of driving a 2A3. Although, I would use it as a cathode follower grid driver (active pull-up, passive pull-down -- which is what you need) as opposed to a grounded cathode (passive pull-up; active pull-down) for an even lower Z at the 2A3 grid, and better current sourcing capability. Whenever you see complaints about disappointing sonic performance from a DHT, look for inadequate grid drive.

 

[pageboy]

Now that helps there. I appreciated the 6SN7 page, but this actually answers the question. Thanks.

 

[jer3my]

Thank you for that last explanation. Really helps understanding.