weird effect on my tube preamp

[dschwartz]

Hi all!!
i finished my new tube preamp (double shot 2), at least the circuit and tone-tweaking, it sounds really, really good!!!, clean is percussive and sparkly, and high gain tones are superb, tight, chest- kicking bottom end, and extreme low noise!! soon i´ll post some pics and sound samples..

the clean channel is made bypassing stages 2 and 3 of the high gain part, and changing the 1M pot for a 10K pot..(important info for my question..)

now, the thing is that when i turn the gain down on the clean channel, the sound comes out clean, but gatey, and anemic...not a really nice tone.. but gets better when i turn up the master volume, it gets tight and percussive..

also, i noticed a slight hum when y turn down the master vol that fades away when y turn it up..it feels like the preamp doest´t like to be played at low volumes..it becomes a bit noisier, with some fur and gatey...

i thought that the gatey sound on the clean channel is there cause the triode on stage 4 is configured for high gain, and to get clean tone i have to lower the input voltage too much and make the triode work too much on a too small signal..but since it gets better when i turn the vol up i´m kind of confused and suspect impedance problems...

oh..and another weird thing.. the preamp works like a charm on the return input of my peavey bandit..but my tube triumph 120 just make a loud hum!!!! no sound at all, loud hum, like touching the tip of the plug all the time..plugging the guitar sounds great, but it rejects the preamp!!!!

any help would me appreciated...

[chillhuman]

Check for DC on all grids (pin 2 and 7) and also on the output jack. If you have a signal generator, put a sine wave into the grids going backwards from the output until you find the stage which is giving you problems. You also should post your voltages.

[dschwartz]

good idea!
i will check
i know than anithing different to 0 means bad coupling cap...

[dschwartz]

OK i checked for DC and nopes...there´s no DC on the grids or the output.. (well, 0.3mV, maybe DMM error)..

someone told me that maybe i´m "stressing" the mosfet buffer cause i have an scaled down tonestack and a low output pot (50K pot) but i don´t want to scale up the tonestack (wiring and changing a LOT of )parts

my source resistor for the mosfet is 100K (too big?) and the problem appears when i lower the output volume (lowering the output impedance)..

my guess is that i´m driving with the mosfet a circuit with lower imnpedance than the output buffer impedance, so the buffer can´t "feed" enough this electron-hungry circuit (sorry for my technical language) and start sounding like crap...

Am I sort of near the problem?  should i lower the buffer´s source cap to maybe..1K? or add a resistor somewhere to make the impedance of the tonestack bigger w/out changing everything?

plz help