Best CLEAN channel topology for an amp with NO NFB

[SnickSound]

I have possibly 3 triodes to play with (although that requires adding an extra tube so 1 would be even better).
The amp already has a dirty channel, and I want to add a clean channel.

I've thought about a simple Tweed style channel, just Volume + Tone. It works, but a bit uninspiring without having the amp turned way high. While the Tone control can help with balancing lows and highs, there is zero control over the mids which means some guitars will sound good and others just won't. But it works!

So now I'm considering a Vox style channel (hence adding an extra tube to get the Cathode Follower stage, aka the Top Boost circuit). While Vox is mostly known for its dirty sound, I do believe it's a fine clean tone as well. At the end of the day, very similar to a Plexi using a single input, just with a smaller coupling cap and a different tone stack voicing (51pf treble cap, 100k slope, 1M bass control) that I'm guessing would work better in a no negative feedback context.

I know the Blackface AB763 circuit just doesn't sound good without NFB, so that's out.

The other thing is this will be running a pair of KT66 power tubes, not the usual EL84 that a Vox would be mated too.

Thoughts?

[Leevi]

Vox does not have NFB. If you want a clean Vox build an AC50 with fixed bias. The preamp is like in the AC30 but is using ECC82 in the preamp. KT66 could be used in the power amp instaed of EL34.


/Leevi

[SnickSound]

Vox does not have NFB. If you want a clean Vox build an AC50 with fixed bias. The preamp is like in the AC30 but is using ECC82 in the preamp. KT66 could be used in the power amp instaed of EL34.


/Leevi

Thanks for including the schematic!

That is pretty much exactly what I had in mind when I mentioned Vox. Just not sure whether I'd go with the Brilliant or Normal side, although I could easily add a switch for it (biggest difference is the tiny 500pF coupling cap)

One challenge though is that if I include the Cut control it would act on both channels which I don't want. So I might go with a Mesa style Presence control (which is basically a "Cut" in the tone stack.

I'm basically expanding on an amp I built for a friend (adding the clean channel and a FET boost). He really loves the core tone of that amp so I don't want the Clean side to have any impact on the core tone.

[Leevi]

Just not sure whether I'd go with the Brilliant or Normal side, although I could easily add a switch for it (biggest difference is the tiny 500pF coupling cap)

Switch is recommended but note that the channels are using different plate and cathode resistors.


CUT is not needed.


/Leevi

[Leevi]

Here is a pretty good history of AC50 including schematics.


https://www.voxac50.org.uk/ac50_introduction.htm


/Leevi

[SnickSound]

Just not sure whether I'd go with the Brilliant or Normal side, although I could easily add a switch for it (biggest difference is the tiny 500pF coupling cap)

Switch is recommended but note that the channels are using different plate and cathode resistors.


CUT is not needed.


/Leevi

Oh good point, hadn't noticed the difference in plate and cathode R/Cs. I actually have to share the input stage with the dirty channel which would be much closer to the Brilliant side.

Will probably share the 22n coupling cap for both channels, but then have an extra inline 500p on a switch for the "Brilliant" mode. Or more likely I'll play with the circuit and see what "meshes" better since I won't have the option of blending channels like on the real AC50.

I just want a nice sparkly clean that won't turn to mud with a Les Paul (his weapon of choice).

Thanks for all the help.

I'll report on my findings. Might convert my "test bed" amp to that circuit this week to test it.

[SnickSound]

Played around a bit with the tone stack. It's impressive how implementing a Vox tone stack with no NFB instantly gives the amp a Voxy flavor! Especially when you start driving it a little.

As expected, a Fender Blackface tone stack did not sound very good without NFB. Although when I mistakenly put a 68nF mid cap instead of 47nF... it sounded better. Guessing because this bleeds off more lows (which are boosted in the power amp when NFB is disconnected). I did have to run the Bass control very low still.

An interesting discovery though is that the Tweed Bassman tone stack (56k slope, 220pF treble, fed from CF) sounds surprisingly good with no NFB. Probably because it naturally has more mids than a Blackface tone stack. It however didn't quite have as much personality as the Vox one. It was just a generic clean tone, but one that would work in pretty much any context.

Almost tempted to have a switch to go between two types of response. But you can't just switch into a Vox tone stack because of the 1M treble pot. But just going to 100k slope and 47pF treble cap while keeping the Fender pot values already gives it a bit of that early Beatles flavor. So definitely an option.

[Bieworm]

I don't know, but listen tobthe tone king imperial.. that clean channel is only 2 triodes before the PI and onto a cathode biased 6V6 power stage without NFB loop. That thing sounds incredible.

[SnickSound]

I don't know, but listen tobthe tone king imperial.. that clean channel is only 2 triodes before the PI and onto a cathode biased 6V6 power stage without NFB loop. That thing sounds incredible.

Interesting, looks like a typical Blackface topology except with a 22nF Mid cap and 10k mid resistor. So slightly more pronounced mid cut at a higher frequency. PI is paraphase but that shouldn't matter until it's overdriving.

[Bieworm]

I don't know, but listen tobthe tone king imperial.. that clean channel is only 2 triodes before the PI and onto a cathode biased 6V6 power stage without NFB loop. That thing sounds incredible.

Interesting, looks like a typical Blackface topology except with a 22nF Mid cap and 10k mid resistor. So slightly more pronounced mid cut at a higher frequency. PI is paraphase but that shouldn't matter until it's overdriving.

That is that 😃
But maybe something else to consider is building an 18W and keep the B+ on the higher side. It offers substantially more clean headroom. But the clean and complex tone that amp has is tremendous!!!