Help With DIY Soldano X88R Preamp

[barryflan]

hi,
im looking for some help with an amp ive built which isnt quite performing.
The amp is a Soldano X88R Tube Preamp. ive built only the lead channel without the EQ stack. i will attach both the original pdf schematic i found online, and the modified version i built the amp from.
the problem with the amp is that it loses gain after warming up. it sounds amazing a second after switching on until about a minute later, at which time it begins to get slooshy sounding and steadily loses gain until after maybe 5 or ten minutes it eventually starts to sound like a resonator with no gain and very little volume but still a bit of compression. THEN, immediately after its switched off, it sounds ****ing awesome again, for as long as it takes the capacitors to discharge and the tube heaters to cool.

the capacitors are new and seem to be holding a big charge. theirs no odd smells or heat concentration inside the amp, and as i said, it sounds exactly as it should when warming up.

i would appreciate if someone who knows more about this than me could look over the schematic of the lead channel and psu to see if anything looks amiss.

if nessecary, i could upload sound samples of the decaying tone.

[Structo]

Kind of sounds like a leaky coupling cap.
Take another .022uf cap and put some test leads with clips on it and when the amp cuts out, clip the test cap across the suspect cap to see if it is the culprit.

[markr14850]

Kind of sounds like a leaky coupling cap.
Take another .022uf cap and put some test leads with clips on it and when the amp cuts out, clip the test cap across the suspect cap to see if it is the culprit.

If it is a leaky cap, wouldn't the DC still be leaking through it?

[Structo]

Good point.

So I guess you would have to lift a leg on each one first.

[markr14850]

My guess is that you need a coupling cap added on the exit from the U5 cathode follower.

[da Geezer]

My guess is that you need a coupling cap added on the exit from the U5 cathode follower.

Yep....when you removed the TS, you lost your coupler. Try different values (.047/.022/.0047/.002) until you find the response you're looking for.

[barryflan]

thanks for the help

unfortunotely, its getting more complicated.
while waiting for a reply, i disconnected the three lower voltage snubber caps in the psu and the amp seemed to be working fine, except for the expected added noise, with no time related issues anymore.

next i tried reconnecting them aswell as adding a .002 coupling cap as suggested. now it sounded relatively ok except for a lot of squeeling, relative to the gain selected.

next i bypassed the coupling cap to see if the new squeeling was its fault. now the amp was theoretically back to its original state, but when tested, some of the squeeling is still there but relatively quiet and it seems to be initiated by feedback from the guitar. also it seems now the original problem of dropping gain is gone or diminished.

now im really fuckin confused.
any ideas ??

[markr14850]

Maybe the tonestack that you removed was dropping the signal level enough that, without it, you have too much gain, and are getting internal feedback as a result?

[drz400]

You are probably getting oscillations by removing that tone stack and probably slamming the grid. Also need a coupling cap as previously mentioned.
Calculate the amount of db loss usually obtained by the tone stack and match it with a T voltage divider before the Level, R43 is kind of redundant, C25 is kind of low and you could use a build out resistor for protection.

If you have oscillations whatever you change in the filtering and coupling will make it change it's behavior, how clean and thought out is the layout?