Best and quietest layout for a Fender 5E3 amp.

[pdf64]

My $0.02....

It may not be an option but I've built my 5E3 derivatives in BF Vibrochamp aluminum chassis AND every design that utilizes a cathode follower (cathodyne PI) should have heater elevation, IMHO. Noise and hum levels are noticeably less than in my 5E3 in the original type chassis - I think it may be due the proximity of the PT to other components whereas in the original every thing is much closer - even thought the dimensions of the VC chassis are not much different - just that tubes and PT are on the same plane. ...

By the time of the black panel redesign, Fender finally turned the mains transformer 90°, so that its core wasn't firing its strongest magnetic flux field along the amp chassis, into the OT and amp circuitry.
So that chassis design has an inherently lower magnetically induced hum level than tweeds.
Plus it's not a 'ship in a bottle' build process, so facilitating better lead dress etc.

[pdf64]

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The problem with Rob’s MV circuit is leakage. At zero volume the 1M pot forms a voltage divider with the 56K resistor.
This isn’t a big problem when the amp is “clean” but when the amp is driven hard the volume is louder than desired.

I think the issue is bootstrapping messing the pot taper, such that even audio taper ends up more like reverse audio.

[martin manning]

Aren't you creating a situation that will not occur in normal use?

Some guitars, even some freestanding valve preamps etc, use a 1M volume control, so resistance at the amp input could be up to 250k when that's set electrically halfway.
Plus it's common to swap in a 12AX7 into V1 of a 5E3, which could exacerbate the low margin of stability issue.

True, but 250k is not infinite.

Mark, does the problem occur when a guitar is plugged in and its volume is turned down?

[Mark]

...but the location of the 0.022uF is problematic as far as I can tell. It forms a feedback loop with the input jacks in the bright channel. The issue isn’t noticeable unless the contacts of the jack are apart and the resistance of the input is high. I suppose that I could ignore it, but I’d rather not.

Aren't you creating a situation that will not occur in normal use?

Agreed Martin it is a situation that normally doesn’t occur, though I don’t know if it affects the sound of the guitar in an adverse way. I have the attitude that I don’t want an oscillation in the amp regardless of the likelihood of it happening. The guy who owns the amp reckoning he heard the oscillation from time to time.

[Stevem]

Any oscillation riding on a amps output wave form will modulate the audio signwave coming out of the amp.
Look up the word frequently modulation.

Some guitar effects stomp boxs are designed to do this .

[psb962]

Very pleased with the elevated heater center tap on my 5e3 build. It's a quiet amp. An easy way to implement is to put both 100 ohm resistors on one of the power tubes, with one end of each on their respective heater connection, and have both connected to pin 8.