jcm 800 clone for bass

[tubby]

Hi Everyone,

i just finished a jcm 800 (2203) clone and tweaked the circuit for bass:

- 0.1µF coupling caps except for the first coupling cap (0.047µF) and
tonestack (500pf, 0.047µF, 0.1µF).

- i omitted all of the treble peaking RC combinations (2x470k+470pf),
brightness cap on preamp pot.

The rest is stock 2203.

Upon first test, the amp sounded very thin, almost no bass, tone controls were rather uneffective, especially the bass control. distortion sounds awful, but it's not the farting-out type which would possibly imply blocking distortion. However, typical volume for a 100 watter is definitely there.

B+ and tube voltages are all in ballpark ranges, known good pre- and power tubes. Bias is set at 65% max dissipation. I went over the circuit again and again.

Could this issue be due to the (too) large coupling caps? What other component could it be? defunct OT?

thx for any ideas!

[loudthud]

If you are looking at the 2203 circuit, you must not be looking for a clean sounding bass. The caps in the tone stack should go back to stock values. The one that connects to the junction of the treble and bass controls will increase bass slightly if bumped up to 0.1uF. You should take another look at the treble peaking caps. Try them one at a time and experiment with the values. Amps like this need a little treble peaking somewhere or they sound pretty dull even for bass.