Being curious to see whether the 19ish-dB gain it produced was truly necessary, I decided to experiment by changing the configuration of my Aikido preamp from line stage to cathode follower.
I had thought I'd read how to do this at some point, but I was unable to google-fu any particular details when it came to it. Looking over Broskie's original ACF manual indicated it'd be relatively simple conversion, so I offer the process up here in the hope that it helps anyone in the future.
More importantly, it'd be much appreciated if anyone could confirm that what I've done here is actually correct and not a magic smoke generator.
Comparing the two schematics shows the ACF substituting the triodes, cathode resistors and grid stopper from the gain stage of the Aikido for a capacitor and two 1 megaohm resistors.
My assumption is the ACF resistors R2 & R5 are required to reference the signal AC to half B+, but admit I don't know to what end. Also a mystery to me is the purpose of the input cap. DC blocking? The cap currently in use is 0.1uF, with the manual specifying any value up to 1uF can be utilised.
Adjusting the Aikido schematic to match the ACF, but allowing for the least amount of tinkering with the PCB, the cathode resistors were retained in anticipation of their resistance making negligible difference in conjunction with the new megaohm resistors being fitted.
Because my Aikido board is the 9 pin version I was able to insert the new resistors directly into the tube sockets, connecting the plate pins to their respective cathode. The resistor leads need to be about 1mm thick to fit securely.
The grid stopper resistor was (unnecessarily?) desoldered from its pad on the input side, and the new capacitor soldered directly to the grid leak resistor at one end with the other lead placed into the top triodes grid pin socket.
Initial listening suggests that although the ACF configuration is fine for most listening situations there is now little headroom for particularly quiet material, while the usable adjustment range on my Khozmo attenuator has reduced quite a bit.
Ideally a measure of gain may be my preference but not the full amount the 6N6P tubes I've been using provide. Around 6dB would likely be plenty. My amp is a DIY Aleph J - 20dB gain, 1.5V input sensitivity.
The other noticeable change is that the noise floor seems audibly lower, probably to no surprise.
I had thought I'd read how to do this at some point, but I was unable to google-fu any particular details when it came to it. Looking over Broskie's original ACF manual indicated it'd be relatively simple conversion, so I offer the process up here in the hope that it helps anyone in the future.
More importantly, it'd be much appreciated if anyone could confirm that what I've done here is actually correct and not a magic smoke generator.
Comparing the two schematics shows the ACF substituting the triodes, cathode resistors and grid stopper from the gain stage of the Aikido for a capacitor and two 1 megaohm resistors.
My assumption is the ACF resistors R2 & R5 are required to reference the signal AC to half B+, but admit I don't know to what end. Also a mystery to me is the purpose of the input cap. DC blocking? The cap currently in use is 0.1uF, with the manual specifying any value up to 1uF can be utilised.
Adjusting the Aikido schematic to match the ACF, but allowing for the least amount of tinkering with the PCB, the cathode resistors were retained in anticipation of their resistance making negligible difference in conjunction with the new megaohm resistors being fitted.
Because my Aikido board is the 9 pin version I was able to insert the new resistors directly into the tube sockets, connecting the plate pins to their respective cathode. The resistor leads need to be about 1mm thick to fit securely.
The grid stopper resistor was (unnecessarily?) desoldered from its pad on the input side, and the new capacitor soldered directly to the grid leak resistor at one end with the other lead placed into the top triodes grid pin socket.
Initial listening suggests that although the ACF configuration is fine for most listening situations there is now little headroom for particularly quiet material, while the usable adjustment range on my Khozmo attenuator has reduced quite a bit.
Ideally a measure of gain may be my preference but not the full amount the 6N6P tubes I've been using provide. Around 6dB would likely be plenty. My amp is a DIY Aleph J - 20dB gain, 1.5V input sensitivity.
The other noticeable change is that the noise floor seems audibly lower, probably to no surprise.
Look back through Broskie's post of the past year or so. He shows a fairly simple mod to a standard Aikido line stage that keeps all the tubes and adds, if memory serves, 2 resistors and maybe 1 cap that reduces the gain. I think it was between the first and 2nd tube.
S.
S.
Broskie's site is so dense with information I'm a bit surprised I found it at all.
Thinking about it - while it's a good workaround for an existing build - having the circuit produce a sizeable first stage gain increase only to then significantly attenuate it doesn't seem to be an efficient way of configuring a low gain solution from scratch. Am I wrong?
I kinda hoped one could just slap a 1:2 transformer on the input, but some cursory reading suggests it's a far more complicated proposition than expected.
Heresy, but maybe a bolting a transistor gain stage like this Minimalist Discrete Hi-Fi Preamp to the front of the ACF is feasible?
I suppose those are questions better asked in the Analog Line Level forum.
Thinking about it - while it's a good workaround for an existing build - having the circuit produce a sizeable first stage gain increase only to then significantly attenuate it doesn't seem to be an efficient way of configuring a low gain solution from scratch. Am I wrong?
I kinda hoped one could just slap a 1:2 transformer on the input, but some cursory reading suggests it's a far more complicated proposition than expected.
Heresy, but maybe a bolting a transistor gain stage like this Minimalist Discrete Hi-Fi Preamp to the front of the ACF is feasible?
I suppose those are questions better asked in the Analog Line Level forum.
How long do tubes last while maintaining good sound both by an Aikido line stage and an ACF (Aikido Cathode Follower) ?
Some guys claim, normal line stages and cathode follower haven't the advantage concerning PSRR but the advantage concerning greater insensitivity to the effects of tube aging. This isn't mentioned under
https://www.tubecad.com/2004/blog0011.htm
unfortunately.
Where I can find papers about that ?
Under
https://www.diyaudio.com/community/...ow-long-will-tubes-last-in-good-sound.404519/
I ask a similar question.
Thank you very much for any advice.
Some guys claim, normal line stages and cathode follower haven't the advantage concerning PSRR but the advantage concerning greater insensitivity to the effects of tube aging. This isn't mentioned under
https://www.tubecad.com/2004/blog0011.htm
unfortunately.
Where I can find papers about that ?
Under
https://www.diyaudio.com/community/...ow-long-will-tubes-last-in-good-sound.404519/
I ask a similar question.
Thank you very much for any advice.
I think predicting tube life is difficult. Having said that, meeting the following helps extend life:
1. You are well under a tube's dissipation limit
2. You are well below maximum plate voltage.
3. You are not pushing the maximum voltage differential specification between heater and cathode.
Also, when used as cathode followers, some of the guitar amp guys find JJ tubes less prone to failure than new production russian tubes.
In my experience and also in the view of John Broskie, new production 12AX7s don't last long (go noisy) even when run way under their max dissipation. Broskie's Aikido phono kit is now supplied (when available) with all 12AT7s where previously it had 12AX7s and 12AT7s (four of each).
In the end, it's more or less a lottery. That's the "magic" of tubes.
S.
1. You are well under a tube's dissipation limit
2. You are well below maximum plate voltage.
3. You are not pushing the maximum voltage differential specification between heater and cathode.
Also, when used as cathode followers, some of the guitar amp guys find JJ tubes less prone to failure than new production russian tubes.
In my experience and also in the view of John Broskie, new production 12AX7s don't last long (go noisy) even when run way under their max dissipation. Broskie's Aikido phono kit is now supplied (when available) with all 12AT7s where previously it had 12AX7s and 12AT7s (four of each).
In the end, it's more or less a lottery. That's the "magic" of tubes.
S.