I propose a very simple mod to his amp which provides about 10X increase in output power at 2% distortion, and only requires adding 3 resistors to the stock circuit.
I recently purchased this amp knowing it would need to be modified in order to put out power anywhere close to the spec of 7 watts. My stock amp only put out 0.45 watts at 2% distortion and 1 watt at 3% distortion.
Stephe at Skunkie Design has done a wonderful job of testing and modifying this amp. Her many videos are on YouTube. She ended up with a cascode front end and replaced the rectifier tube with a 5AR4. This mod is called the Skunkie Mod. Her design apparently performs well, and involves totally rewiring the 6SN7 circuitry, and many new parts, although they are not expensive.
I have designed and built several tube amplifiers and was interested in finding out why the stock design performs so poorly. By looking at the 6SN7 characteristic curves, it was obvious that the second 6SN7 stage was biased incorrectly. The plate voltage is too close to the power supply voltage, which results in not enough positive voltage swing to drive the 300B tube. Therefore, this one stage limits the power output.
The mod involves adding a 39K 1/2 watt resistor in parallel with the existing 33K cathode resistor in the second 6SN7 stage. This increases the current through that stage, which lowers the plate voltage (see characteristic curves). A 10uf 250V capacitor can also be added in pararallel with the new resistor to slightly increase the bass response.
A 10K 3 watt resistor is also added in parallel with the two 20K resistors in the power supply circuit to increase the voltage to the 6SN7 stages. Finally, an optional mod is to change the rectifier tube to a 5AR4. This increases the power supply voltages and provides a softer power up, which will increase the life of the tubes.
The final output power result is shown below. 5.1 watts at 2% distortion with the stock rectifier tube and 6.2 watts at 2% distortion with a 5AR4 rectifier tube. This is as good as, or better than, the Skunkie cascode mod.
I have made this modification and it sounds really good. I have not heard the Skunkie Mod, so I can’t compare to that. I’m using Wharfedale Lintons, which are about 89db sensitivity, and the modified A50 drives them to satisfactory levels in my 14’ by 12’ listening room. I was pleasantly surprised by this. My listening position is about 8’ from the speakers.
I also have a Willsenton R8 amplifier which I really like, but I actually prefer the A50 on a lot of music. It’s not as analytical as the R8. I use a DIY switcher to switch between the two amps.
I recently purchased this amp knowing it would need to be modified in order to put out power anywhere close to the spec of 7 watts. My stock amp only put out 0.45 watts at 2% distortion and 1 watt at 3% distortion.
Stephe at Skunkie Design has done a wonderful job of testing and modifying this amp. Her many videos are on YouTube. She ended up with a cascode front end and replaced the rectifier tube with a 5AR4. This mod is called the Skunkie Mod. Her design apparently performs well, and involves totally rewiring the 6SN7 circuitry, and many new parts, although they are not expensive.
I have designed and built several tube amplifiers and was interested in finding out why the stock design performs so poorly. By looking at the 6SN7 characteristic curves, it was obvious that the second 6SN7 stage was biased incorrectly. The plate voltage is too close to the power supply voltage, which results in not enough positive voltage swing to drive the 300B tube. Therefore, this one stage limits the power output.
The mod involves adding a 39K 1/2 watt resistor in parallel with the existing 33K cathode resistor in the second 6SN7 stage. This increases the current through that stage, which lowers the plate voltage (see characteristic curves). A 10uf 250V capacitor can also be added in pararallel with the new resistor to slightly increase the bass response.
A 10K 3 watt resistor is also added in parallel with the two 20K resistors in the power supply circuit to increase the voltage to the 6SN7 stages. Finally, an optional mod is to change the rectifier tube to a 5AR4. This increases the power supply voltages and provides a softer power up, which will increase the life of the tubes.
The final output power result is shown below. 5.1 watts at 2% distortion with the stock rectifier tube and 6.2 watts at 2% distortion with a 5AR4 rectifier tube. This is as good as, or better than, the Skunkie cascode mod.
I have made this modification and it sounds really good. I have not heard the Skunkie Mod, so I can’t compare to that. I’m using Wharfedale Lintons, which are about 89db sensitivity, and the modified A50 drives them to satisfactory levels in my 14’ by 12’ listening room. I was pleasantly surprised by this. My listening position is about 8’ from the speakers.
I also have a Willsenton R8 amplifier which I really like, but I actually prefer the A50 on a lot of music. It’s not as analytical as the R8. I use a DIY switcher to switch between the two amps.
Last edited:
Looks like another great mod for this amp! Did you do a freq response sweep? Just curious if that changed too.
Yes. The frequency response didn’t change except for a little better bass response.
I did notice something interesting. For 8 ohm output and 8 ohm load, the response is like you’ve posted before. However, for 4 ohm output and 4 ohm load, the high frequency response is degraded significantly 2db down at 10KHz and 4 db down at 15KHz. For 4 ohm output and 8 ohm load it’s a little better, but still worse than the 8 ohm output.
Your videos inspired me to try this mod.
I did notice something interesting. For 8 ohm output and 8 ohm load, the response is like you’ve posted before. However, for 4 ohm output and 4 ohm load, the high frequency response is degraded significantly 2db down at 10KHz and 4 db down at 15KHz. For 4 ohm output and 8 ohm load it’s a little better, but still worse than the 8 ohm output.
Your videos inspired me to try this mod.
Yeah, I agree for some people. My hearing is degraded above 8KHz.
This high frequency reduction with the 4 ohm output is the same with the stock amp and with the mod.
This high frequency reduction with the 4 ohm output is the same with the stock amp and with the mod.
I have tried both the 4 ohm and 8 ohm outputs with my Wharfedale Linton speakers. The sound is surprisingly different.
With the 8 ohm outputs the soundstage seems narrow and there is very little bass. The vocals and instruments are very crisp.
With the 4 ohm outputs the soundstage seems very wide and the bass is much improved. The highs seem a little rolled off, but the vocals and instruments are fuller sounding.
I greatly prefer the 4 ohm outputs. The Linton impedance drops to around 4 ohms around 100 Hz so that might explain the loss of bass with the 8 ohm outputs. Other popular speakers like the Klipsch RP-600M also have low impedance at lower frequencies, so I suspect they may sound better using the 4 ohm outputs.
With the 8 ohm outputs the soundstage seems narrow and there is very little bass. The vocals and instruments are very crisp.
With the 4 ohm outputs the soundstage seems very wide and the bass is much improved. The highs seem a little rolled off, but the vocals and instruments are fuller sounding.
I greatly prefer the 4 ohm outputs. The Linton impedance drops to around 4 ohms around 100 Hz so that might explain the loss of bass with the 8 ohm outputs. Other popular speakers like the Klipsch RP-600M also have low impedance at lower frequencies, so I suspect they may sound better using the 4 ohm outputs.
Just looking at the 'hi-voltage' warning top of page ... i got struck by lightning in the shower, i quite enjoyed it.
As i was working outside over a portable table, there developed a lightning storm outside, a strong male voice told me 'i would be struck by lightning' ... i believed he was an angel warning me.
Little did i realise it was actually a statement of fact.
I ran inside and locked the bathroom door, 'safe and sound' - i thought - Then i decide to have a shower as it was starting to get dark...as i am adjusting the hot water,, i then --- got struck by lightning --- indoors --- 😐 through the pipes or water tank where i thought it was safe...everything became slow motion --- lasting what seemed like several minutes --- its like i was shaking hands with it and it would not let go ---, amazing --- and it had an organic feel to it - far prefereable than basic mains --- if your going to get shocked - forget the mains --- go 'organic' i say ... 🤔
On deeper thought and introspection --- it clearly occurred to me loud as a bell --- that whatever God via His angels says something will happen, you can be assurred it will happen ... no matter where you try and run and hide ... 😄
As i was working outside over a portable table, there developed a lightning storm outside, a strong male voice told me 'i would be struck by lightning' ... i believed he was an angel warning me.
Little did i realise it was actually a statement of fact.
I ran inside and locked the bathroom door, 'safe and sound' - i thought - Then i decide to have a shower as it was starting to get dark...as i am adjusting the hot water,, i then --- got struck by lightning --- indoors --- 😐 through the pipes or water tank where i thought it was safe...everything became slow motion --- lasting what seemed like several minutes --- its like i was shaking hands with it and it would not let go ---, amazing --- and it had an organic feel to it - far prefereable than basic mains --- if your going to get shocked - forget the mains --- go 'organic' i say ... 🤔
On deeper thought and introspection --- it clearly occurred to me loud as a bell --- that whatever God via His angels says something will happen, you can be assurred it will happen ... no matter where you try and run and hide ... 😄
Last edited: