Custom design - clipping a bit splatty/gated

[joelap]

Hi, just completed a custom preamp design last night and gave it a run through to see how my design translated to the real world.  It was designed to be a clean preamp which it does well, but when I really crank the controls and hit the strings hard, or boost the front end, the clipping sounds a bit gated.  The design consists of two 12AX7 tubes, as such:

V1B:
Rp=124k (62k and 62k in series, bootstrap connection between the two)
Rk=5k6, 50uf capacitor on a switch for -4dB attenuation for active pickups
Rin=typical 1M,68k input resistors
DC coupled to V1A cathode follower.
Rk=100k
Cbootstrap=33uf electrolytic, positive side on the 62k junction
Voltage off of the filter cap going to this stage before Rp: about 260V (I wanted more, but only had 2k2/2W resistors on hand)

-Typical Fender Ladder-style tonestack -
1MA volume control, output to V2Bgrid

V2B:
Rp=100k
Rk=5k6 // 50uf in series with 10kA as variable resistor switching/in out for adding extra gain to circuit
DC coupled to V2A as cathode follower.
Rk=47k
Cout=.1uf
Voltage off of the filter cap going to this stage before Rp: about 285V (I wanted more, but only had 2k2/2W resistors on hand)

This is my first time using a bootstrap in an amp, so I am tempted to attribute the gated sound to that, even though it is unlikely since it is the first stage in the signal and is likely not clipping.  Otherwise, I do recognize there is a lot of bass in the signal... I’d like to get a bit more gain out of it, debating getting rid of the bootstrap in favor of two separate gain stages, but then I would have to feed the tone stack off of the plate of the second stage, and there would be additional losses associated with that though it would probably still outdo the bootstrap stage.  Plus, the clean sound is exactly what I was going for… just trying to make it a bit louder and get rid of the gated sound when driven a bit.  There’s no room for an additional 12AX7 (built into a 1U rack enclosure) so that option is out too. 

Looking for any ideas or suggestions.  On a side note, this was my first project I had designed every aspect of, from the circuit to the layout to the arrangement of the chassis, etc. and it worked the first time I flipped the switch.  I can’t even get kits or pedals to work the first time I turn them on. 

Thanks,

Joe

[FYL]

Too much gain = blocking
Too high cathode R's = asym cutoff, poor useful range
Too high cathode C's = farting
Too high bootstrap C's = blocking

Try to use 47n for bootstrap caps, not 33µ
Lower Ck to 4µ7 for the first stage, 22µ or lower for the second one
Lower Rk on V2b to 1K5 or so in order to avoid cutoff
Play with Rk on V1b, try from 1k5 to 4k7

[joelap]

Thanks for the clarification.

If I lower to 47n for the bootstrap, would the bootstrap effect be extending through the entire range of frequencies, as this is primarily to be used as a bass preamp?  Admittedly, I don't have the calculations memorized and my notes aren't in front of me.  I do recall that being the reason I stuck the 33uf in there.  Actually, I know I could have gone lower on that capacitors value, but I remember making the realization that I didn't have a film cap large enough to pass full frequency, and had to make due with a spare filter cap.

I'll experiment with the values of the cathode resistors/bypass capacitors as well. 

[phsyconoodler]

Bass like a wider frequency range,so it will work nicely.It sounds trebly with guitar but bassy with a bass.It passes MORE not limiting like a larger cap.

[FYL]

If I lower to 47n for the bootstrap, would the bootstrap effect be extending through the entire range of frequencies, as this is primarily to be used as a bass preamp?

The bootstrap cap sees a lowish source Z from the cathode follower, so 47n is fine and dandy even with a drop-tuned 6-string.

[joelap]

Thanks everyone.... sounds great and no blocking distortion with .1uf in the bootstrap position.

Now... to get rid of that 60cycle hum I keep hearing.  Time for a humdinger.