Adjusting High Frequency Response in ST120

I love my ST120 and current tube/ coupling cap combo.  Lots of sparkle but compared to other amps may have a little too much mustard on the hot dog and seems the highs are tilted up slightly.  Cymbal splashes are prominent but vocal sibilance can be slightly exaggerated is the main complaint.  Can I increase the NFB coupling cap or lower its associative series resistor to adjust the highs down?   Any other way?  Suggested component value range and obvious pitfalls to avoid?

The sparkle is one area I truly enjoy and makes my ST120 stand out.  So looking for very slight changes and nothing drastic.  I may hate the change and put it back immediately LOL.

Two things:

1) "tilted up highs" describes some effect you are hearing--and may be related to high frequency response... or something else (as you I suspect you suspect--NFB over correction, instability, microphonics, etc.). My guess is that a healthy VTA-120 is linear between 20-20kHz.

2) Bandwidth limiting of Stereo 70 amps was a fairly common practice back in the 1980s. This was intended to reduce the stress on an inadequate power supply at frequency extremes. I believe that the VTA-120 features a direct coupled input. You could insert simple first order low pass filters to the L and R inputs of your amp and see if you like the result. Easy to add, easy to remove.

...j

Three words: Center Driver Tube.
Easiest and cheapest tone-modifier. Try a batch of old 12BH7s. TungSols did it for me. RCAs and Sylvanias and Raytheons - oh my!!!

Funny, I just swapped out my vintage Mullard 12au7's for GE 12bh7's in driver/inverter positions this morning.  (chasing rainbows with my new speakers....)

IMO BH7 types generally more laid back than AU7's which happens to be desirable with my new speaks.  I agree with Kently that these might be an easy fix.

Nothing wrong with mullards BTW, very sweet midrange but they do fetch serious coin these days.

Difficult to imagine an otherwise well-functioning amp within specs being the source of unpleasant or accentuated high-end. Musicians are constantly tweaking caps resistors and other mods to get the sound they want in their Fenders and Marshalls but this is in the preamp section and I would think a hi fi power amp is reproducing pretty much what it's being fed? Might be other, more simple solutions per Bob and may even consider adding a sophisticated high-end equalizer or parametric to get the sound you are seeking rather than doing mods in the power amp? It's an interesting theory but actually modding the power amp for me would be a last resort.

It's all about the tubes, eh.

Similar situation here, and I went with a 12BH7 center with 5963's in the sides.

Which ended up a bit too dark.

Solution there was to put a brighter tube in my DAC/Tube buffer. Pure bliss!

... and another roller is born ...

PS ... if a 12BH7 is too dark, a pair of RCA clear tops in the sides should accomplish the same thing.

Thanks everyone. I have some Mullard's I will try first. They were too dark for me 4 years ago but I always like to experiment. Thanks for the suggestion.

It's very unlikely that there is a frequency deficiency in the st-120.
You might have distortion coming from the source, or speakers that is unfit or damaged.

Start with turning down treble control on your preamp, if problem goes away, enjoy the music.