SunAudio 2A3 to 300B

[eddylws]

Need to a expert here.

I have just bought a set of SunAudio 2A3 AMP, due to my speaker is not allow me to drive on a low voltage AMP I try to modify it to drive in 300B.

I did a compare with the SV-300BE & SV-2A3 schematics, and I found the different is filament voltage is 5V and the 300B runs on 3k5, 2A3 runs on 2k5.

It take me about 2 day to hold up the Filament voltage, and after that I hold up with the 3.5k cable from the OTP to the plate PIN.
After a double check on all the cable and connection, I turn on the amp and check on the B+ and Plate voltage both went up to 400v (the original voltage on the Schematics was 320V).

How can I turn it back to 320V?

Is anyone done this before?

Thanks

Eddy Lim

[eddylws]

This is the 2A3 Schematics.

[5150ed]

Hi eddylws

I am not an expert but I am using this schematic on my 300B amp.

The first thing I want to tell you is you do not have to follow the original schematic exactly,
The plate 400V voltage is just fine with 300B, 320V in the original schematic is too low for 300B.

What u got to do is replace the 750R 2A3 cathode to 1K or 880R depend on the operating point you want. My favorite one is 60ma with voltage at plate is 360V, with this operating point I use 1K cathode resistor. 70ma with voltage at plate is 420V (also use 1K but increase plate voltage)will give you a little more watt but sound a little more stress to my ear.

Then you need to change the 2.7K resistors (I suggest at least 5W not 2W) until the voltage after the resister is around 280 to 300V. Mine use around 15-27 K very far from 2.7K.

If you want to reduce the B+ voltage, I suggest you find a high quality oil capacitors like 2-8mf to replace the 47uf input cap, the less uf of the input cap will reduce the B+ Voltage but you got to try yourself which value (2mf, 3mf, 5mf, 8mf) give you the B+ you want.
I recommend you use a so so quality one first then when you know the exactly value you want, buy the best quality one.

It got to trial error sooooo... many times to get this done coz once you got the right voltage for 300B, the voltage on 6SN7 is change.

And after you finished tuning your amp, please let's it burn in again for 50-80 hours before you judge it.

[kmaier]

Hi Eddy,

Suggest you go back and check your work. The only differences between the 2A3 and 300B versions are the filament drive, output transformer tap and the cathode resistor on the output tube. If your voltage went up, I would suspect you are not drawing any current thru the 300B. You can easily check this by measuring the voltage across the cathode resistor, would suspect it should be around 50-60 volts.

If you're not getting voltage across the cathode resistor, check to make sure you have B+ at the plate on the 300B (check at the tube socket)... if you do have voltage, suspect your filament supply and check for proper lighting of the 300B and a solid 5.0 volts on the filament at the tube socket.

On another note, your changes will not result in any significant power increase. A 2A3 SET is good for around 4.0 - 4.5 watts and the voltages and biasing used in the 300B version of the Sun amp will result in about 6.0 watts, no more. That results in an increase of 1.75dB, which isn't much from a SPL view.

Regards, KM

[Akita]

2A3 opt is design to work with about 3.5watt, it may not be able to handle up to 8watt. So...beware of your OPT tempreture.

[Tubes4e4]

Hi folks,

as it happened I had some closer look at the SV-2A3 driver circuit and because some things didn´t work out on paper, I even breadboarded it.

Here are my notes (in short):

1) The 2k7 resistor in the RC smoothing chain cannot be right: A drop of 50V across it means 18,5mA running through it, but by computing the anode currents of the two 6SN7 sections you will get only about 6..6,5mA in sum. A value of about 7k5 would be more likely.

2) It is not a good idea to have DC-coupling (just the input/volume pot) at the front of an internally DC-coupled cascaded stage, since even a most tiny DC part of the input signal will pull the stages out of their op points grossly, especially the 2nd stage. An input cap would prevent this.

3) The cathode bypass cap at the input stage is superfluous, if not contraproductive. Due to the very high Ra/Rk ratio, there is only very very few gain change when using or omitting it.

4) The total gain of the cascaded, DC coupled input/driver stages is about 220 times, or 47dB. For the +/- 45 Vpp swing needed to drive the 2A3 it will give a less than practical (at least nowadays) input sensitivity of about 0,14Vrms/0,4Vpp.

5) The second stage has very arkward operation conditions for the 6SN7 section used there. Eak is only about 135V, and the anode has to swing 90Vpp with less than 3mA running ... in a linear manner. A quick glance at the 6SN7 curves shows, that the 6SN7 is running into the nonlinear cut off area of its plate curves already, especially at the anode "high" swing part of the cycle, resulting in severe, nonsymmetrical distortion.

Especially point 5) I found so irritating that I made an exact plot (see attached below) and even breadboarded and scoped the driver stage. Guess what, at the needed 90Vpp output swing the scope absolutely confirms what the plot below suggests on paper: A sine curve is visibly non-symmetrically rounded at the "upper" crests. Experience shows that easily visible (at the scope) sine wave distortion means a THD very well in excess of 5%. Go figure.

Some might argue that the resulting driver distortion might be even intentional, to cancel out (even harmonics) parts of the power stage distortion. But considering the very different transfer functions and loading situations of 6SN7 and 2A3 I have my doubts such an effect really will take place to a noticable ammount in this circuit.

All in all, I am disappointed by the very obvious - and verified - technical shortcommings of the circuit as shown.

Regarding the original poster question: If this driver cannot even deliver the swing to drive 2A3 w/o severe distortion, don´t expect it to be able to deliver the much wider swing needed for 300B.

Regards,

Tom Schlangen

[Tubes4e4]

P.S.:

To clarify, the plot shows the extrema (the two vertical bars at about 90V and 180V) the 6sn7 anode would be _required_ to swing on its load line in a linear manner, but not its actual swing with a symmetrical input signal.

In any case, the reason for the distortion observed/verified at the scope is obvious.

Regards,

Tom

[kmaier]

Tom,

All points well taken. These schematics have been floating around for years and the calculated voltages on the input/driver simply don't fly..... and I'm guessing the completed amps themselves are probably using different values from the ones shown.

Eddy's request was to convert it from the 2A3 version to the 300B version, hence only output stage changes (which are minimal) and the filament change... also pretty minimal. I gave him some obvious checks based on his B+ voltage climbing, but also the simple fact that he won't gain enough (power) to actually compensate for his speakers.

As I'm currently working on a 300B SET design, I've managed an adaption of my earlier design using a similar topology to drive a 45 or 2A3 using a 5814A input/driver. With a 400 volt B+ supply and cathode bias, 6 watts is the maximum... and gain linearity starts to suffer as you get around 5 watts.

I'm leaning towards a transformer driver setup as the P-P voltage drive for a high-voltage 300B output stage are literally at 200V, and the fact that the 300B becomes more difficult to drive as you push the grid towards zero.

I think his (Eddy's) focus and expense would be better spent looking for more efficient speakers to compliment his SET amplifier. Of course... just my $0.02.

Regards, KM

[rtsang]

Thanks Tom, for explaining the whole short coming of this circuit.

I had a SV300BE but I found the amplifier sounded very bright. I thought that it was due to the low operating point of the 300B (268V, 52mA, plate dissipation of 14W) and DC filaments. So I reconfigured the amp to run a 2A3 with ac heating and changes the cathode bypass cap of the 2A3 to a 47u film. Still sounded bright and way too much gain. So I removed the cathode bypass cap of the 2nd stage. As Tom has predicted, not much loss in gain, maybe 2-3dB, but still bright and even more distorted. Finally sold the amp in return for a Yamamoto 2A3 amp.

In another occasion, I heard the amp reconfigured for a 45 with grid choke, still to bright and rough edged for me.

Now I understand why the amp sound so bad, it was the distort of the 2nd stage.

Another reason that I sold the amp was that the power transformer was way too hot. Here in HK the AC is 220V, not sure if the amp would be less hot if it was run in Japan with 100V AC. But I think the power transformer was too lean for me to gut out the whole driver circuit and built something else using the chassis and the transformers.

Also the chassis was to small to allow many big components.

Raymond.

[eddylws]

Hi Every One,

Thanks rtsang, kmaier, Tubes4e4, Akita, 5150ed for sharing, Really appreciate that.

Hi All,

I am not sure what i have did wrong cause the high voltage after the retified tube was 410V instread of 340V and after the High voltage capasitor it still stay at 410V, same as the plate voltage.

Could be the retified faulty?

I am really worry about the transformer will blow. hmm...

Thanks Eddy Lim