Hi all, I have Sun Audio 2A3 which really has too much gain. The schematic shows the 6SN7GT being used in 2 stages. To reduce gain could we just bypass half this input tube? Drive pin 1 instead of pin 4...
Probably other changes required but you get the idea...
Appreciate your thoughts.
Cheers
Probably other changes required but you get the idea...
Appreciate your thoughts.
Cheers
Attachments
Try removing the bypass cap on the first cathode resistor for each channel.
This is not an amp where it's easy to bypass the first stage since it's directly coupled.
This is not an amp where it's easy to bypass the first stage since it's directly coupled.
You can't simply bypass the second triode stage because you will have lots of distortion, poor hf response and too little power output, the first stage only have 70V at plate it simply won't drive correctly the power stage and it will distort a lot, you will need to put a fixed resistor atenuattor at the input or to put a global negative feedback, if you don't want to redesign or modify a great part of the amplifier.
Also the first stage is dc coupled to the second you can't work a lot with that if you plan to don't touch it a lot.
Also the first stage is dc coupled to the second you can't work a lot with that if you plan to don't touch it a lot.
You will get no sound because triode 2 will be biased into cut-off. As audiowize in post #2 stated, the two triodes are directly coupled, and as such triode 2 depends on the DC anode voltage from triode 1 to bias correctly with its high value bias resistor.
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My mistake and just deleted my post when I saw your reply. But you still can disconnect pin 1 from 5 and connect to 4, then change Rk to 1k8 and plate load to 51k, then see if you have enough gain.
HK
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Without bypass on first stage you will have -3dB (around) less of gain .
Don't touch the volume pot.
This is the simplest way, otherwise you have to change the working point of the two stages.
Walter
Don't touch the volume pot.
This is the simplest way, otherwise you have to change the working point of the two stages.
Walter
That is also a good observation you will reduce a tad the overall output, and it will also reduce the total distortion of the amplifier.
Why would that be helpful? The 10K value will not lower the gain at all unless source impedance is really high. If source impedance is really high, a 10K pot is a terrible choice.
Going to a linear pot will make the problem worse (the linear pot gets louder more quickly, we don't want this!).
Thanks, as per drawing ?
Can you please explain the reasoning for removing the cap so I can maybe learn some more about tube amps...
Cheers Dave
100k goes between input and top end of pot. That will reduce gain by 6dB.
The removal of the cap reduces the gain because of the negative feedback caused by the cathode resistor.
Rather than explaining it can I point you at this,
How to design valve guitar amplifiers
http://www.valvewizard.co.uk/Common_Gain_Stage.pdf section 1.18
The two books are both very good and there's also loads of stuff on the web. The guitar one is a bit more interesting as it show how to create distortion.
The removal of the cap reduces the gain because of the negative feedback caused by the cathode resistor.
Rather than explaining it can I point you at this,
How to design valve guitar amplifiers
http://www.valvewizard.co.uk/Common_Gain_Stage.pdf section 1.18
The two books are both very good and there's also loads of stuff on the web. The guitar one is a bit more interesting as it show how to create distortion.
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rewire to an SRPP, or an aikido firsts stage using equal cathode resistors say 470 ohms,
you can do this without too much pain...
you can do this without too much pain...
using the other half of the 6sn7 is a quick and easy way to do it....and it is reversible too...you can always go back the the SUN topology if ever..