Harmony 525 preamp with parallel triodes

[mustardbucket]

I recently acquired an original/unmodified 1966 Harmony 525 30w three input bass tube amp with a 15" Jensen that I plan on using with my Fender Strat and Gibson Les Paul guitars. This is my first tube amp modification project. If my terminology is incorrect, I apologize. So far I have replaced all filter and coupling caps, removed the death cap, added a grounded three-prong cord, and added hi-lo inputs with success.

I need your help trying to figure out why my amp went almost silent after I wired V1 12AX7 in parallel and then halved the plate resistor. I have a basic understanding as to why I need to experiment with cathode and plate resistor values, as well as halve them to get the same or similar bias as a single triode. To me, it appears my amp has two 100k plate resistors in series.

At first I experimented with numerous Rk values while leaving Ra alone. I listened to what the amp sounded like and recorded the voltages at the plate, cathode, and in between the two Ra after each change.

Next I changed the lower Ra to 51k while using a 2k Rk. The amp sounded really good. Lastly, I replaced the 51k Ra with just a wire from the plate to the top 100k resistor. The resistance from B+ to V1 pin 1 was 106 ohms. The plate and cathode voltages measured very close to the voltages of the single triode because the Rk and Ra were halved. Now the problem — I lost about 90% of the volume/output with this last change. Why?

1. Did my amp originally have 200k Ra because of the two 100k in series?
2. Do I need to replace both 100K with 51k instead?
3. Do I need to do anything with the 33uF bypass cap?
4. What does the .1uF coupling cap between the two 100k Ra do?

I could not find a schematic, so I had to draw one. While the schematic shows the triodes in parallel, the voltages, Rk, and Ra were from the single triode measurements for your reference.

Harmony-525-Schematic-Parallel-Triodes.gif

harmony525chassis.jpg

I want it to be known that my schematic was drawn using inspiration from numerous Harmony, Supro, Gretsch, Custom Kraft, and other Valco bass amp schematics. I could not find one for a 525. The closest one I found was a Gretsch Pro Bass 7160 schematic drawn by Chris Devine https://www.backfromthesixties.co.uk/ The resistors and cap specs, wiring color, connections, etc. were based off my amp.

[Stevem]

I can't find a schematic to 100% be sure what I will post will help you, but here it goes anyway.

If the amp has a treble and bass control then the second section of V1 is likely the signal recovery gain stage after the tone controls.

Your paralleling the A and B sections of V1 like you did knocked out tons of needed signal level.

[Masco]

Look at Supro Thunderbolt schematic, practically the same amp with a few cap value differences for bass.