tomlang wrote:I'm not sure I should ask this and probably normally wouldn't but since you posted a schematic of the KT88 amp could you possibly post a schematic of the headphone amp?
I am 100% NOT familiar with guitars or guitar amps. What do the Mode and Presence pots do?
ALL of your work is fabulous, just stellar!
sure sure no problem heres the schematic for one channel of that headphone amp, one change you should make is change volume pot to 100k, too much miller effect with a 500k pot there, zaps some presence on your music when not cranked
Heres a link, its a huge image
As far as the Mode and Presence are concerned. The Mode is an infinitely adjustable variation between Pentode and Triode operation on the EF86 driver tube. The EF86 is a small pentode tube often used in guitar amps dating back to some of the old Vox amps. Its also sometimes found in phono preamps or areas where extremely high gain is needed. Small signal pentodes can have gains of 200-300 compared to say a 12ax7 which has a theoretical max gain of 100 unloaded. So if you look at the mode when turned all the ways to one extreme, the Screen of the EF86 will be completely bypassed by the 470nF cap which is grounded while the 470nF cap on the plate is more or less floating with a 1M load to ground. This is pentode operation
When the Mode control is at the other extreme, the screen and plate caps are connected in series, and effectively from an AC standpoint, the screen and plate are shorted, which is Triode mode. However since they are not DC connected this produces an interesting triode characteristic where the tube curves are shifted by the screen voltage.
Now anywhere in between is some variation between those 2 modes, and even somewhere in the middle is a psuedo ultra linear operation. When operating in pentode, the stage has maximum gain, while in triode is has ~70% less gain, while in the psuedo ultralinear it is the minimum gain. This control allows you to adjust the gain into the phase inverter, effectivly controlling headroom. The clipping characteristics also change accordingly, and this makes the control very useful for tailoring the sound
Presence controls how much higher freq content in the feedback loop is cancelled in the long tail phase inverter. So what this does is boost the upper freq ranges in the power amp by not cancelling them in the feedback loop. When you shunt the highs to ground (max pres) all freqs below the cutoff are feedback while anything above is not. This is a very traditional guitar control dating back to early Fenders and Marshalls