Archibald questions & bass mod

[benh]

Hi !

While I'm waiting for Tayda to drill, paint and decorate the enclosure, I'm looking at Archibald schematics and wondering about a few things :-)

I would be building this for my son who is primarily a bassist. Now, I couldn't find the schematics/component values out there for the "bass" version of the BK Butler Blue Tube, though I think I found every other variant.

First, I noticed Archibaled has C5 (10n) and R6 (10k)/R7(10k) seemingly forming a high pass filter at the input of V1A with an f3 around 796Hz. Did I miss something ? I feel this would be rather ... detrimental for bass, wouldn't it ? Ideally I clean blend path would be good for low freqs but that's more of a mod than I think I can do easily at this point without doing my own PCB.

That said, I looked at the schematics for the Blue Tube RT-903, and this has in that spot a 47nF cap, and about 110k of resistance to A- (100k + 10k), which would bring f3 to around 30.8Hz. I'm thus thinking of doing that same change. Any particular reason one would think this wouldn't do ? The circuit of the RT-903 is a bit different in that it has 100K before the 10K to A-, then another 10K to the tube grid, meaning 20K from A- to grid. But that also comes with different plate resistors, cathod resistors, etc... ie, all the values differ quite a bit around the tubes themselves.

Any opinion/suggestion before I solder things ? :-)

Another thing I am wondering is how necessary is the regulator circuit when feeding the pedal 12V ? Can't we just wire 12V more/less directly into the tube heaters in series ? This seems to be the go-to option for a number of other tube designs I've seen around (including feeding them a meek 9V)

Thanks !

Cheers,
Ben.

[benh]

So I ended up breadboarding it  :-) I don't have the tone stack(s) yet, but I have the power supply, op amps and tube (Russian 12AU7 from EHX). It's for my son who isn't around right now, I'm no guitar player so I haven't really "listened" to it, but I scoped it and found at least one interesting nit already...

(Note: this is based on the Archibald schematics, I haven't checked if they match the PCB 100% or not).

First, I'm feeding it a +/-500mV (1Vpp) 1kHz signal from my scope signal generator. I will play with different frequencies later. This cover hot pickups and active instruments to some extent (ideally I should see how it handles 2Vpp).

The main thing I noticed is that it always clips. The 10k resistor (R5) in series with the gain pot means that we always end up clipping somewhat, which diminishes the variety of tones it can produce I assume (yeah I know, I haven't actually listened to it yet :-)

By taking that resistor out (like the RT-901 Real Tube and some other TubeWorks schematics I found around), I take out that minimum gain, which allows a range of "clean" gain values or at least much subtler clipping, which along with the bias pot, allows some interesting games of asymetric "rounded" warm clipping that are less fuzzy (at least on the scope) than what I manage to get with that 10K in series.

I suspect in practice that means a bit more flexibilty of tone with a wider gamut of instrument with hotter pickups (or active). It's very fiddly though, on those low gain, it doesn't take much movements of the pot to drastically change the outcome.

I also noticed that a good chunk of the gain pot as no real effect, we are fully clipped. I'll play around see how it looks like with a 250KA one (once I get one, I'm out of them right now), it might get me an easier time playing with lower gainst without losing much in the upper end of the gain range.

Yes yes, as soon as I get hold of the kid, I'll try it for real :-)

Next, I'll play with various frequency ranges and see the effect of C5/R6/R7 on low frequencies.

[benh]

Another thing I did notice: I added the J201 buffer (using a $$$ "real" one from DigiKey), and it clips quite a bit, especially the negative half of the signal.

I modded the circuit by changing R18 from source to ground to instead go from source to V-, which seems to improve the situation.

Is there any reason why you have it going to ground and thus losing all that headroom ? Or is it a schematic mistake vs. PCB ?