I’ve built an amp with 2A3 and LL1660. I like the sound of it,
but was a little frustrated when I measured the frequency response
today. Around 9.5KHz I get a climbing response
that doesn’t go away until I’ve reached the end of the bandwidth of the
amp. With a few measurements, I located the problem to be the 1660. It’s
wired in the ALT-T hookup, to allow 20mA current through the 6H30.

Is this expected? If not (and also if so, I suppose :-), what can I do
about this deviation?

Hi Per,

Seems I already got this explained to me by an audioasylum inmate, it’s
an impedance problem. So I’ve got something to work with now, sorry for
wasting your time.In case you want to know, 24K resistance from the grid of the 2A3 to ground made the frequency response curve perfectly flat. I can’t wait to listen to it and see how this changed the sonics of the amp. BTW, thanks a lot for making quality products for tube amps. I already absolutely love the sound of my new amp.

Jens Axboe