Of course, the GA-19 RVT version I was working on uses 6EU7s in the preamp with a 6C4 7-pin triode as the reverb driver.
(see schematic below which includes changes.)
A customer brought us the amp to be fitted with an Earthed 3-cond. AC cord, and general service. He complained that the amp lacked low end and asked if there was anything we could do to improve the overall gain and bass response, without drastically changing or modifying the amp (I think he was shy about footing the labor charge that would accompany anything to drastic. The general service on a 65 year old amp is time consuming enough).
After listening to the amp and looking at the schematic, I had a couple of ideas which were really simple to implement and wanted to try. Really nothing groundbreaking, but I love the result. Even into the original inefficient stock speaker.
The first was to remove the lossy tone network following the first stage coupling cap. Even with that removed, the -3dB roll off of the .001uF with a 1M load (2 x 2M pots) is 159Hz.
Changing the .001uF to .0047uF brings the HPF down to about 34Hz. Plenty of bass for guitar.
The last change was to solder a 56k resistor in parallel with the 100k grid stopper for input #1. It's a modest change but subjectively I think there's a bit more bite.
(edit: new schematic updated with RC network (R9,R10, C5, & C7) removed, and highlighting modifications with the additional change to C22, increasing the value from 500pF to .002µF as discussed below)
This is the original schematic and circuit.
https://el34world.com/charts/Schemat...on_ga19rvt.pdf
(see schematic below which includes changes.)
A customer brought us the amp to be fitted with an Earthed 3-cond. AC cord, and general service. He complained that the amp lacked low end and asked if there was anything we could do to improve the overall gain and bass response, without drastically changing or modifying the amp (I think he was shy about footing the labor charge that would accompany anything to drastic. The general service on a 65 year old amp is time consuming enough).
After listening to the amp and looking at the schematic, I had a couple of ideas which were really simple to implement and wanted to try. Really nothing groundbreaking, but I love the result. Even into the original inefficient stock speaker.
The first was to remove the lossy tone network following the first stage coupling cap. Even with that removed, the -3dB roll off of the .001uF with a 1M load (2 x 2M pots) is 159Hz.
Changing the .001uF to .0047uF brings the HPF down to about 34Hz. Plenty of bass for guitar.
The last change was to solder a 56k resistor in parallel with the 100k grid stopper for input #1. It's a modest change but subjectively I think there's a bit more bite.
(edit: new schematic updated with RC network (R9,R10, C5, & C7) removed, and highlighting modifications with the additional change to C22, increasing the value from 500pF to .002µF as discussed below)
This is the original schematic and circuit.
https://el34world.com/charts/Schemat...on_ga19rvt.pdf
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