My first SE 2A3 amp, impressions

[dfung]

Dfung, could you describe how to bypas the volume control? I use a preamp so I don't need a cheap pot in the way of the signal.
Thanks,
David

Hi David,
In reply to your question. It's pretty simple to bypass the volume control. Just select one pair of RCA input. You can see that the stock wires from the RCA inputs inside the amp are connected to the input selector pot. Just cut the wires from the selector pot. From the picture below, you can see the driver board, where there are two driver tubes (6SN7) base (each looks like a circle). From each circle, you can see the small word "in". This is where you connect the two RCA input wires to each point at the "in". The red RCA wire should be on the left "in", the black on the right "in". I suggest you change to pure silver wires. In this configuration you can still use the other two pairs of RCA connecting to the selector pot and so still use the volume control for these two pairs of RCA. Hope this helps. I have a video shown above for the amp in action with the Hashimoto trans and choke. The video doesn't do it justice though.

 

[TerribleT]

Hi dfung,
Nice videos. Could you share your impressions on the sonic changes of your amp after changing to the Hashimoto chokes? I think the original chokes are 15H and the Hashimotos are rated 25H. Did this change alter the B+ voltages much?
Regards,
David

 

[dfung]

Hi dfung,
Nice videos. Could you share your impressions on the sonic changes of your amp after changing to the Hashimoto chokes? I think the original chokes are 15H and the Hashimotos are rated 25H. Did this change alter the B+ voltages much?
Regards,
David

After changing to the Hashimoto output trans the sound is really better from top to bottom after a couple of days running in. Every thing is clearer and has better separation. At first, I thought the choke won't affect the sound. I was wrong. After replacing with the Hashimoto choke, it even sounds better. Now I hear better dynamic contrast of sound in high and low. Better separation. Better tonal contrast. So, even more musical. A piano note sounds more like a piano note. I had the Cary 805C ($9,000). This Bowei now sounds better than the Cary. It also beats my previous Audio Note Kit One 300B with all Blackgate caps comprehensively. This is the one amp I won't sell but will keep it for life considering its low cost and wonderful sound. My horn speakers already replaced with the Lowther PM2A drive units.

 

[dfung]

One more video for my Bowei 2A3 with Hashimoto output trans and Hashimoto choke installed.

 

[dfung]

Another video for my Bowei 2A3 with Hashimoto output trans and choke:

 

[dfung]

 

[TerribleT]

My goodness!! The Hashimoto choke is huge. It's almost as big as the power transformer. What other mods did you do to the power supply? Did you replace the electrolytic power supply filter capacitors with better caps?
Regards,
David

 

[dfung]

My goodness!! The Hashimoto choke is huge. It's almost as big as the power transformer. What other mods did you do to the power supply? Did you replace the electrolytic power supply filter capacitors with better caps?
Regards,
David

I didn't change the electrolytic power supply filter caps. I replaced the two signal caps to Mundorf Silver/Gold/Oil caps. All silver signal wires.

 

[TerribleT]

Hi dfung,
Thanks for all your information. At the moment, my amp is on the bench for some modifications. I will be increasing C2, the final filter cap in the power supply, from 47uf to 100uf. I'll also bypass the selector switch and volume control and install a voltage divider network to the input to maintain the 50k or 100k input impedance. I have a question concerning how you've done the bypassing. I am a little confused. You said to disconnect one of the inputs at the switch. This would disconnect that input from the pot's input connection. But, I believe the connection to the wiper is still connected to the grid of the tube. Like I said, a little confused. Could you play your amp through your bypassed input and vary the pot to hear if there is any changes in sound. Thanks.
David

 

[dfung]

Hi dfung,
Thanks for all your information. At the moment, my amp is on the bench for some modifications. I will be increasing C2, the final filter cap in the power supply, from 47uf to 100uf. I'll also bypass the selector switch and volume control and install a voltage divider network to the input to maintain the 50k or 100k input impedance. I have a question concerning how you've done the bypassing. I am a little confused. You said to disconnect one of the inputs at the switch. This would disconnect that input from the pot's input connection. But, I believe the connection to the wiper is still connected to the grid of the tube. Like I said, a little confused. Could you play your amp through your bypassed input and vary the pot to hear if there is any changes in sound. Thanks.
David

There is definitely a change in sound. I did that as you said. You can hear the difference in house. There is loss of definition and musicality when I switched to the selector-controlled inputs. But I am not sure the difference is so obvious via (1) a recording, and (2) the youtube. I can record two videos when I have the time. Thanks.

 

[TerribleT]

Hello,
I think the pot is not completely bypassed. This The selector is ahead of the pot so any changes in the selector would not affect down stream, which is the pot. The pot has three connections, the "in" for signal input, the "wiper" or output goes to the grid and "ground". When you cut the line in at the selector switch, you are only disconnecting the "in" connection and is now floating. The "wiper" and "ground" are still connected to the grid and gound. So when you wire the input directly to pin 4 (input grid), the grid is still grounded through the pot which now acts like variable resistor. This will change the input impedance depending on the pots' position, which will vary from 50k ohm to 0 ohms. This will distort the amp depending on the wiper position. I hope this makes sense. This is difficult for me to explain since I am just figuring out and learning this stuff. Let me know what you think.
Regards,
David

 

[dfung]

Hello,
I think the pot is not completely bypassed. This The selector is ahead of the pot so any changes in the selector would not affect down stream, which is the pot. The pot has three connections, the "in" for signal input, the "wiper" or output goes to the grid and "ground". When you cut the line in at the selector switch, you are only disconnecting the "in" connection and is now floating. The "wiper" and "ground" are still connected to the grid and gound. So when you wire the input directly to pin 4 (input grid), the grid is still grounded through the pot which now acts like variable resistor. This will change the input impedance depending on the pots' position, which will vary from 50k ohm to 0 ohms. This will distort the amp depending on the wiper position. I hope this makes sense. This is difficult for me to explain since I am just figuring out and learning this stuff. Let me know what you think.
Regards,
David

I kind of agree with you. If you want to completely bypass the pot, just disconnect the other wires on the pot. I did the way it is now because I still want to keep the other two pairs of RCA inputs. The sound is already phenomenally good now.

 

[TerribleT]

If you disconnect all wires at the pot and only have a direct connection from your preamp output to the grid of your amp's 1st stage, you still need a resistor across the grid to ground, 50k or 100k ohms. This would be your Amp's input impedance.

My Bowei amp is too sensitive in my system. I can only turn the volume control on my preamp to the 9 0'clock position before it's too loud. So I need to add a voltage divider network to my amp's input.
Regards,
David

 

[dfung]

If you disconnect all wires at the pot and only have a direct connection from your preamp output to the grid of your amp's 1st stage, you still need a resistor across the grid to ground, 50k or 100k ohms. This would be your Amp's input impedance.

My Bowei amp is too sensitive in my system. I can only turn the volume control on my preamp to the 9 0'clock position before it's too loud. So I need to add a voltage divider network to my amp's input.
Regards,
David

Yes, I also did use a 100K resistor when I removed the volume pot on my Audio Note Kit One 300B. But with the Bowei 2A3 since I disconnected only one pair of input wires from the selector pot, I didn't have to use a 50k/100K resistor. What speakers are you using?

 

[TerribleT]

The pot value of my Bowei is 50k ohms. In your setup, the input that was disconnected at the selector switch was then wired directly from the RCA jack to pin 4 (labled in). The pot's output (the wiper) is still connected to pin 4 .The common remains connected to ground. So there is resistance from your grid to ground. This resistance works as the 100k resistor that you had installed in your Kit One amp. The wiper to ground can vary from 0 ohms to 50k ohms. When you turn the volume control, you vary the input impedance of your amp from 0 (bad) to 50k ohms (good). The pot is still affecting your amp's performance.

I'm currently using my Altec 604e speakers.

 

[dfung]

The pot value of my Bowei is 50k ohms. In your setup, the input that was disconnected at the selector switch was then wired directly from the RCA jack to pin 4 (labled in). The pot's output (the wiper) is still connected to pin 4 .The common remains connected to ground. So there is resistance from your grid to ground. This resistance works as the 100k resistor that you had installed in your Kit One amp. The wiper to ground can vary from 0 ohms to 50k ohms. When you turn the volume control, you vary the input impedance of your amp from 0 (bad) to 50k ohms (good). The pot is still affecting your amp's performance.

I'm currently using my Altec 604e speakers.View attachment 1011794

Nice setup. What's the frequency response of the Altec?

 

[TerribleT]

The spec sheets say the driver has a 20Hz to 22k Hz response and its pressure sensitivity is101db at 1 watt. Don't know the actual FR in the Altec 612 utility box but it sounds great. It's a little light in the bass, but that may be because of my amp.

 

[dfung]

Dfung, could you describe how to bypass the volume control? I use a preamp so I don't need a cheap pot in the way of the signal.
Thanks,
David

Hi David,
I finally bypassed the volume control and selector pots. I removed the volume pot from the small circuit board and installed four 47K ohm resistors on that board. I did that with my Audio Note Kit One 300B before, converting it to a power amp. The amp sounds much better now. see the pictures.

 

[dfung]

Dfung, could you describe how to bypas the volume control? I use a preamp so I don't need a cheap pot in the way of the signal.
Thanks,
David

Last year, I got the info from Audio Note to bypass the volume pot. See the picture below:

 

[sheltie dave]

I hate to say this, but if you believe an Altec 604E has a useable frequency response of 20 to 20k, you have purchased a lot of bridges in you life, T.