TUBEDUDE wrote: ↑Fri Nov 05, 2021 1:04 pm
This might be of interest. The feedback loop in the first tube allows for adjustment for a variety of input levels. This circuit has a clean and quiet output with great tone. You can sub the 6SN7's for noval tubes if necessary.
Please forgive the slight derailment. Tubedude, you have had good results with running that midrange control plate-loaded? I would imagine it would be a bit lossy there, and maybe that was a design goal. I like having it all on its own like that, after another stage.
I've split the bass and treble controls with gain stages before, but to keep tube count under control, isolating the mids is worth a triode.
True, a cathode follower has a lower Z than an anode follower, but the mighty 6SN7 has a crushing output that is more than capable!
Just kidding, i initially did this by rewiring my hangar queen using the anode follower that was there. I could have experimented further with the other topologies but it performed flawlessly so I moved on. I'll try it and see if there's a different response.
Tube junkie that aspires to become a tri-state bidirectional buss driver.
As I recall, regardless of the resistor type, there’s an inherent noise generated by resistors in a circuit, determined by the resistor value and the current flowing through it.
The inability to operate that design with a bypassed input stage cathode is why I was thinking it just wasn’t an ideal choice for an input stage.
Hence if the goal is to avoid input stage overdrive and reduce the opportunities for noise to make its way into the signal path, my thinking is it may be better to use a regular common cathode 12AY7 or 12AU7 section as the input stage, and use that variable gain NFB circuit as the 2nd stage, perhaps tweaked a bit to reduce the gain.
JJH0906 wrote: ↑Fri Nov 05, 2021 11:29 am
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If anyone has comments on the architecture itself, good or bad, please comment!
What’s the rationale for the value of R6 being so high?
With regard to the tone stack, the mid cap C5 looks to be connected to the wrong end of the mid pot U4.
pdf64 wrote: ↑Fri Nov 05, 2021 8:25 pm
As I recall, regardless of the resistor type, there’s an inherent noise generated by resistors in a circuit, determined by the resistor value and the current flowing through it.
The inability to operate that design with a bypassed input stage cathode is why I was thinking it just wasn’t an ideal choice for an input stage.
Hence if the goal is to avoid input stage overdrive and reduce the opportunities for noise to make its way into the signal path, my thinking is it may be better to use a regular common cathode 12AY7 or 12AU7 section as the input stage, and use that variable gain NFB circuit as the 2nd stage, perhaps tweaked a bit to reduce the gain.
That's true, some noise sources, like
Johnson noise is ever-present. Aside from those, others can, and should be tamed. Metal films are quieter than carbon film and much quieter than carbon composition. Lower values and higher wattage reduce noise also.
I think you are making way too much of the unbypassed cathode. Breadboard it and see, it's quiet. The advantage of this stage is it can't be overloaded. You can set the sensitivity to accommodate any pedals or active pickups.
Tube junkie that aspires to become a tri-state bidirectional buss driver.
To move beyond the preamp stage, regardless of what type of print tube is used, how much does that affect the design of the output stage? Having some trouble getting my head around that, as well.
Also, I intend on putting a parallel effect loop in between the preamp and output stage, so I'm not sure what sort of effect that would have on the output stage, either.
Just need sufficient signal level to fully drive the power amp, ie at least to full power, perhaps a little more, to a mild overdrive sweet spot.
Parallel loops require mixing back in with the dry signal. If done passively, another gain stage may be needed, if done with an active mixer, it definitely will be.
The idea is for me to add ambient effects with a parallel effects loop but be able to get food volume from the amp with as little distortion as possible. All of that will be coming from effects on the front end of the amp.
For me, I'm looking at this as a glorified PA system!
You have to design the loop to have enough wet gain for the effects you are using. Old-school rack-mount gear will output at 0dB or even +4, but most pedals will be -10 or -20. The dry signal can be at unity, but you can add or decrease dry gain by changing the relative values of the resistors that feed the "0" virtual earth node. I've used O'Connor's loop from TUT 1, which is similar to this nice one from bluesfendermanblues:
