Tweet
OK, just to play devil's advocate here...
Portable- This suggests to me say 2x10" or 3x10", light-ish alnico speakers, Eminence 102 (6oz magnets) are my preferred alnico 10"s. As previously mentioned, a 2x10" should have one speaker arranged vertically above the other, to reduce acoustic feedback and improve on-stage monitoring.
About 30W - How are you meauring this 30W? 30W AVERAGE is about as much as you will get from any "harp" amp as yet concieved. Power output of a harp amp is mostly dependent on B+ voltage. For a 2x10" I'd suggest 440-470v B+, if going 3x10" maybe aim for the higher figure...up to 500v (like a brown bandmaster). The higher B+ PTs will weigh more and you'll end up with a 5F6A style weight. 2x6L6 & 2x12AX7 preamp tubes will only need a BFDR style PT & chassis which makes for a lighter, more portable amp.
Lo gain preamp - of all the preamps I have tried, I prefer something along the lines of the 6G? (rather than 6G?A) Fenders. Relevant also to your tone stack point. Maybe make the middle control a tweakable value with a 25K pot rather than a fixed resistor. If you want to be able to play with differing mu tubes then try a cathode follower tone stack (5E7/5F6A). Using a singe 12A?7 for both preamp and tone stack recovery stages (like a tolex Fender) will limit your choice of Lo-mu tubes.
Gain & master - only really relevant if you go the CF tone stack route, I've tried many circuits and can only recommend the Marshall 2 input style, mount the master on a Push-Pull SPST with the switch disconnecting the ground reference, with the pot pushed in and the knob fully up the master is now COMPLETELY out of the circuit. Bear in mind that most MVs only have a very limited sweep of usability with harp.
Anti Feedback - If you pull this off you are a VERY clever man indeed! No one else has built a circuit SOLELY to perform this function into an amp. Feedback is often as much (if not more) a symptom of the output amp (voltage, current and brand of power tubes). Ideally you should design the amp to be reasonably feedback resistant, but don't get too caught up in focussing on feedback killing schemes...9 times out of 10 you will just be robbing Peter to pay Paul.
Soul control - cool. A ratio of 10:1 upwards for the feedback loop would be good.
Lne out - cool, use a 25KL to 50KL pot, wired as a variable resistor for the dropper from the speaker, in series with a 2.2K fixed resistor. This will give you some flexibility when lining out to either mixing desks or bigger stage amps.
Multi-Z taps - ALWAYS run the speakers in parallel, rather than series or series-parallel for maximum signal transfer. Speaker load should also match secondary impedance UNLESS you specifically prefer the sound of a mismatch (not unheard of).
Compressor - leave it in the studio. Most guys can get a pair of 6L6s/5881 to compress without any outside help, especially with a tube recto, additional compression on stage will drop you back in the mix and do more harm than good.
Good luck.
Portable- This suggests to me say 2x10" or 3x10", light-ish alnico speakers, Eminence 102 (6oz magnets) are my preferred alnico 10"s. As previously mentioned, a 2x10" should have one speaker arranged vertically above the other, to reduce acoustic feedback and improve on-stage monitoring.
About 30W - How are you meauring this 30W? 30W AVERAGE is about as much as you will get from any "harp" amp as yet concieved. Power output of a harp amp is mostly dependent on B+ voltage. For a 2x10" I'd suggest 440-470v B+, if going 3x10" maybe aim for the higher figure...up to 500v (like a brown bandmaster). The higher B+ PTs will weigh more and you'll end up with a 5F6A style weight. 2x6L6 & 2x12AX7 preamp tubes will only need a BFDR style PT & chassis which makes for a lighter, more portable amp.
Lo gain preamp - of all the preamps I have tried, I prefer something along the lines of the 6G? (rather than 6G?A) Fenders. Relevant also to your tone stack point. Maybe make the middle control a tweakable value with a 25K pot rather than a fixed resistor. If you want to be able to play with differing mu tubes then try a cathode follower tone stack (5E7/5F6A). Using a singe 12A?7 for both preamp and tone stack recovery stages (like a tolex Fender) will limit your choice of Lo-mu tubes.
Gain & master - only really relevant if you go the CF tone stack route, I've tried many circuits and can only recommend the Marshall 2 input style, mount the master on a Push-Pull SPST with the switch disconnecting the ground reference, with the pot pushed in and the knob fully up the master is now COMPLETELY out of the circuit. Bear in mind that most MVs only have a very limited sweep of usability with harp.
Anti Feedback - If you pull this off you are a VERY clever man indeed! No one else has built a circuit SOLELY to perform this function into an amp. Feedback is often as much (if not more) a symptom of the output amp (voltage, current and brand of power tubes). Ideally you should design the amp to be reasonably feedback resistant, but don't get too caught up in focussing on feedback killing schemes...9 times out of 10 you will just be robbing Peter to pay Paul.
Soul control - cool. A ratio of 10:1 upwards for the feedback loop would be good.
Lne out - cool, use a 25KL to 50KL pot, wired as a variable resistor for the dropper from the speaker, in series with a 2.2K fixed resistor. This will give you some flexibility when lining out to either mixing desks or bigger stage amps.
Multi-Z taps - ALWAYS run the speakers in parallel, rather than series or series-parallel for maximum signal transfer. Speaker load should also match secondary impedance UNLESS you specifically prefer the sound of a mismatch (not unheard of).
Compressor - leave it in the studio. Most guys can get a pair of 6L6s/5881 to compress without any outside help, especially with a tube recto, additional compression on stage will drop you back in the mix and do more harm than good.
Good luck.
I was slipping into a cow pasture. 
I was actually thinking of slicing the top end and/or tightly controlling it through treble roll off caps on the plates and possible inputs. In addition to local feedback a la Dumble with the 22M/.01 cap loop and to top that off adding the lamp expandor/compressor at the output since he seems to think the power brake is the answer to taming the feedback. 
Comment