Here are the basic mods I did to both amps:
1) Upped the negative feedback resistor (connected to OT secondary) from 75k to 220k. Goal was no negative feedback for more gain. It is R222 on the schematic.
2) Lowered the V1B cathode bias resistor from 5.6k to 3.3k for more gain. R208.
3) Replaced the two 3w 350ohm ohmite resistors with 5w 360ohm wirewound cemented resistors. R240 and R228.
4) Rebuilt new plug in can type Filter caps (they plug into tube type sockets). 2 original cans were each three 20MFD 400v. I made new ones with 22uf 450v caps. C214 A/B/C and C216 A/B/C.
On the previous exact amp I did for my friend, I did the same mods as above, but not step 4, didn't change the filter caps. It sounded really good and he loves it.
I have tried to check and tighten every screw and socket to make sure nothing is rattling. I have looked at every new (and old) solder joint. I have measured the new filter caps out of the amp and they are all in spec with proper continuity for the plugs. I have looked for misplaced wires, solder joints, etc.
Both amps are having an identical problem, after identical mods, so I'm thinking it is related to the parts I put in.
I even changed the speaker in one of the amps in case that was it, but exact same problem persists.
I'm wondering if I have bad caps, or maybe I have created blocking distortion in the preamp by lowering the cathode bias resistor too much, or ??? I'm in the early stages of learning about amps so my knowledge is limited, thus my post asking for insight, encouraged by an 18watt.com friend to do so.
Would love any advice you can offer, questions you can ask, etc. - attached is a picture of the schematic, and the amps, and the caps I built.




Found the schematic online, a little easier to see, page 19:
http://www.acofs.org.au/part_5_files/Be ... 614EVM.pdf
I can attempt to make a sound sample tonight and upload it tomorrow.