The Amp Garage

... if I run my pliers down the back of the volume pot then I get a crackling noise through the amp? Same if I touch the ground bus wire near the volume pot. I don't know if that means much... Metal on metal always seems to generate some noise... It could mean that the ground wire isn't making good...

I would try moving that red presence wire well away from the tonestack wiring (blue + yellow wires) and see it it helps with the 'going quiet'. Also, the soldering between the busbar and the pot casings doesn't seem to have taken. May not be a problem as the pots are grounded to the chassis, anyway....

The plate is actually glowing hot. OK, that's bad! See Jelle's suggestion above - you're looking for a capacitor connected between pins 3 and/or 8 and ground. (I would guess that 3 and 8 might be tied together, but I haven't looked for a schematic.) If you have a voltmeter you could check the volta...

lgehrig4 wrote:I can't imagine it being ok that the tube plate is red hot...

Is the plate structure *actually* glowing red hot?

Are your temperatures in °C or °F? "Red hot" is something around 600°C / 1100°F

The cathode structure will probably be ~ 900°C / 1600°F

I doubt that the zener has too much to do with it (but it might) - more likely the length of the connection from the gate to the resistor / zener and where that wiring is routed. Until the zener is conducting (either way) I don't think it does much more than add a bit of capacitance. To be 'text boo...

I don't really know - laziness? Ignorance? Or that nine times out of ten it doesn't cause any problems? I come at this from an electronics background, and the 'text book' designs all seem to advise a small resistor in the grid lead for FETs and MOSFETs. I suppose that the 100k resistor from the pot ...

Well, I hope it turns out to have solved the problem with your amp...

Mostro wrote:Should everyone start using gate stoppers to prevent this?

Yes

(I have been brought up to believe that it is good practice when designing with MOSFETS.)

thyforest wrote:marshall avt50 112 combo

.... it's just a general electric buzz sort of sound, but more than i've ever heard from any of my previous solid state amps.

That model has a cooling fan, I believe - is that what's making the noise?

Noise prior to the PI was my main issue. If I can get rid of that I'll be laughing! This is what it use to look like. Looks like you've grounded half the smoothing caps to the same ground as the preamp board , whereas I believe they should be grounded at the same place as the other smoothing capaci...

I could see that there could be oscillation due to the interaction between the choke, the VVR and supply sag - have you tried it without D3? Do you still get the problem with the MV wound to zero? (If not, then the problem could be the amount of filtering between the normal and regulated supplies). ...

Colossal wrote: Dead clean. Carbide bit is fine and it's done in about 10 seconds.

Router with a carbide cutter has worked fine for me on Al, but I always run it on the slowest speed and take a light cut.

And you'll be finding "glitter" all over your 'shop for years!

Something must be shorting in there...

Not wishing to insult your intelligence, but trying to cover all the bases: Are you sure that you have the correct bulb for the holder?

The 6.3V option has its attractions! (But check for shorts first! )

Is the lamp holder rated for 125V? (What does the writing say on the side?)

Maybe the "flash" you see from the lamp is really an arc inside the holder? - Are you sure that there's nothing inside the lamp holder that there shouldn't be? (Wire clippings, or crap from manufacturing?)

Bit of a puzzle!

martin manning wrote:I'm always interested to hear what you and others think, T-dog!

Why thank you! Likewise

This is one time KF screwed up... :^o :shock: :wink: You want to know what I think? (Well probably not, but this is the internet, so it's coming anyway....) The distortion on an Express kicks in when the power tubes are driven into grid conduction. Once this happens, because of the way the negative...