How much hum is acceptable in a Fender Red Knob Twin

[jalexquijano]

I want to be able to turn the gain up to 10, pull the treble boost, and raise the volume over 4 until 10 with no squeal at all ? I have already tightened the tube sockets and have controlled the squeal at some point. I have also replaced the brand new 9 pin stereo input jacks with the former original input jacks made in japan that have the transparent plastic on the bottom and resoldered the points. All the filter caps are also brand new.

Where should i look then? Should i replace all the tube sockets specially V1, V7 and V2? What about the orange drop caps C21 and C16 which are 0.68 250V just beside the gain pot on channel 2? I have no oscilloscope and can only rely on your guidance. I really want to keep the amp stock with no mods and just eliminate the squealing in channel 2. Channel 1 is running perfectly.

Please see schematic:


http://www.blueguitar.org/new/schem/fender/the_twin.gif

[acorkos]

it's easy enough to try soundguruman's advice of clipping in a 6pF cap across pins 1 and 2 of V1...

[km6xz]

Instead of random shotgunning, try isolating the problem to the stage that is oscillating? Should an amp squeal? No but there are lots of opportunities for one to squeal with the right combination of phase shift and gain. You keep mentioning the control positions but not the gain that represents. The first step is to measure stage gain. Just what IS the gain of the amp though that signal path. It is easy to measure and would tell you if there is a problem with excess positive feedback in a normal gain amp or you are dealing with a normal level of positive feedback but excess gain.
If you just start adding parts until it goes away, who knows how it is going to impact performance or reliability. There is no free lunch with any mod, seeking to gain something usually requires loosing something else, usually reliability.
So stop swapping parts. Isolate the feedback path. You mention running presence control up which indicates the output is in involved which also means load is involved. The amp has a design gain, is it meeting or exceeding that gain?

[ogeecheeman]

I know I'm late to the party but thought this might be of value-had one of these red knob twin beast on the bench for a month with the same problem.finally soldered in a 390pf silver mica cap from + pin of input jack 1 to ground. nailed it. crap circuit boards,crap components,crap wire,crap jacks = one big headache