fusedbrain wrote:
WTF happened there
It's too bad this thread has kinda "gone off the rails" so to speak, as I thought the modded amp sounded quite good, and I typically find the contributions to the forum from both V2A and NR to be of value and informative.
It would have been nice to have more of a discussion about the mods, the clipping circuit, the issues arising from adding NFB to an EL84 amp etc...
This is supposed to be the "Tech Corner" forum after all....
Oh well.... such is life on the interwebs in 2019

Hello fusebrain. I received a phone call from a prominent professional member of the world of music wanting to know if I could lift the dark sound (open it up Marshall-ish) and add more usable gain to this 18 watt amp. He named a few of the top hot rods he liked. I decided to take the job.
I reworked all preamp anodes, cathodes and couplers. In addition I created a couple assemblies to remain protected in heat shrink. Shunted voltages to ground in other places. The way I connected the different value couplers in combination with the anode values in series played a great part.
My past experiences aided in working outside of the box. I own 2 BE 100’s (2013 dual input & 2014 single input). Great amps as they were/are my live work horses when I don’t use my Marshall Pandora’s. Easily replaced if stolen. Dave uses a (voltage divider) 220k on the wiper of the BE 100’s I own. Reducing this value or even completely removing it not only increases volume as someone eluded to but allows the amp to react more like a Marshall tone stack.
On this PT20 the 220k voltage divider is located on the Master Volume. You can see the possibilities here using a on/off/on switch. I also used two different sets of clippers with my own choice of cap and resistor to offer distinctly different clipping flavors as requested by the customer.
I checked everything on the oscope to ensure stability and no crossover distortion when finished of course.
Just as importantly I used my ears to dial in what I found optimum for the Hopkins sound as we each seem to have a sonic place we like to end up. This ended up sonically much like my 50 & 100 watt Marshall modded Pandora’s.
You asked about the NFB and Presence.
This was my first experience on a 18 watt though I have thousands of hours repairing and modding larger amps.
I decided to try out a NFB and Presence Control. The PI configuration was a 100k/150k on anodes, 470 shared cathode and 27k tail.
I started with a 47k for a Presence resistor on a 5kL pot wired lol e a 1959 with a .1uf cap. I could not get a signal so I changed around the OT primaries on the EL84 pin sevens which resolved the issue.
I found that all combinations seemed to push the EL 84’s into clipping as suspected but confirmed for my self the distasteful results lol. I abandoned that idea realizing now why very few “mini” amps or 18 watt amps that produce over driven gain have this type of Presence. There is a Randal Smith method but I decided to pass on that option as I had other plans.
Once I abandoned the NFB Presence idea I fine tuned the preamp to my ear. Drilled my holes for the bells and whistles and put everything in place with proper lead dress. I did not use the clippers over my 2 hours of playing while starting the flow/burn in time settling all of the 25+ new components.
By the time I finished the HPT20 was sounding huge. My fiancée was laughing at how huge the tone was coming out of that little mini through my 2 UK V30 Marshall cabs as she has listened to the final production of every single one of the 60+ 50/100 Pandora’s, MorningStars and SoCaliDondo’s I created in 2018.
The owner is especially excited.
I talk in depth with every person I do work for. Many are members here. A couple are return customers requesting 2-3 amps. Back to the point. I had a blast with this little guy and have had a few inquiries to date with one inbound along with more Pandora requests.
I have been accused of “run on posts” lol. Non the less, if you have additional questions that I feel I can answer please ask fusebrain.
Thank you for your interest in this endeavor.
David