Wednesday, 4 January 2012

Jet City JC20H output harshness

The Jet City JC20H is an EL84 based guitar amplifier which has had some criticism for sounding a bit harsh or brittle. Mine was no exception. The stock EL84s proved to be substantially mismatched and were ditched for some matched JJ Tesla. However the brittleness proved unmoveable despite many changes to the pre-amp section.
The solution was with the power amp section. A 330pf capacitor across the 47K negative feedback (NFB) resistor marked R12 in the service schematic. This removes the unnecessarily high bandwidth of the output stage, and will also increase high frequency stability. Whatever the cause, the brittleness is gone and the amp sounds a whole lot better. Roll off is 1/ 2Pi * R *C  , so 330pF and 47K will equal about 10kHz which is ample for a guitar amplifier. 

The simulation below shows the frequency response with and without the 330pF capacitor.  The peaking at 14kHz is reduced by about 9dB. Scale is 3dB per division vertically and 2Hz to 20kHz.


  1. Tried this mod and it worked great! Totally removed any trace of the harsh jagged edge to the high end... That weird glassy jangle on top of the note that I couldn't stand. Sounds amazing now! Btw, I have the 20hv. All stock except new tubes and rebiased to 70%. Thanks for this info!

    1. Hi, Glad this worked for you. The issue I have since found is that _any_ increased coupling losses (fractions of a percent) in the transformer coupled with the feedback loop can produce peaking. You can fix it either with a big lump of cash and buy a new transformer or fix it in the feedback with a capacitor for a few pence and a lot less hastle. For some reason most vocal folk go for the transformer and then broadcast it as a result all over the net. Further testing rested on a 220pF capacitor in my amp.

  2. If I remember correctly, I think I wound up with 220pf as well (closest I had on hand). Fairly transparent mod.

    Quick question for you... I have another single ended EL84 amp that does not have negative feedback (AX84 P1-ish build... Started out as a high octane, but I reduced some of the octane). Is there a way to do something similar or would I need to add negative feedback? Currently when I run the master wide open it gets harsh sounding.


  3. Hi, It may be best to seek help on the AX84 forum. There were a ton of revisions of this amp. There was some work done with a Zobel Network (tuned C/R in series) connected to the primary side, but I can't find it now. Not totally related but Google Beam Blocker Guitar. It may help in other ways. Good Luck.

  4. Hey, I have JCA22... does this work on my amp too? R12 is the NF resistor?