Other Amps

5F11 build - advice needed please.

26

It was F and T caps I used, a definite improvement. I was convinced it was 50Hz, maybe you're right about it being 100. I also lined out the upper part of the cab with copper tape, which seemed to help with noise from an external source.

27

I always use F+T 22uF caps in amps like these. It doesn't help you I know, but my 5F11 is dead quiet. I used a 1M tremolo pot because I couldn't find a PEC in 2M and I can get all of the range I need.

I would look at the preamp tube wiring. It seems to me that they have lined all of the wires up. Really you want the grid wires to cross the plate wires at 90 degrees, not be parallel. I put the plate and cathode wires flat against the chassis and the grid wires are as short as they can be and never parallel with the plate wires. Also make sure that the filament wiring is as far from the rest as can be, and if they cross again it should be at 90 degrees. AC wiring can induce hum in DC wiring.

Pulsing can be caused by a few things, but on amps with bias vary tremolo I have found it's important to ground the bias circuit away from the filters. The trouble with building amps is that each one is different. What works on one amp may not work on the next. It's frustrating!

28

Good points from Jimmy as well.When sheilding an amp like this,the sheilding,or copper tape has to go on the back cover so it covers the open side of the chassis.Also be sure the tape makes contact with a ground on the chassis or it will induce more external noise.I usually put a small self tapping screw in the edge of the chassis so it makes firm contact with the tape.Putting it on the upper part of the cab wont help in this type of cab and will likely act as an antenna and attract noise.

29

"I just checked the build, and the filter cap ground is coming of the first cap, nearest the transformer, rather than the second. The ground goes to a lug on the chassis, rather than to the common ground on the transformer."

I remember you posting this and I just want to make sure you are grounding all of the filter caps and not just the first one

30

I was a bit confused by that quote,too,guitarcapo.If those caps were not grounded at all,the noise would be unbearable.I'm not sure what he was saying there so I just advised to ground the main and screen together and the preamp cap seperately.

31

All the filter caps are grounded, but the builder connected them behind the board, and ran the ground off the first filter cap rather than the second, as per the layout.

32

If the ground for the preamp cap is grounded with the other 2 just lift the ground on the preamp cap and move it to the other end of the chassis near or at one of the grounds from the preamp circuitry.

33

I'm still confused. Are you saying the first filter cap is grounded separately from the other filter caps at a different location?

Usually I combine them all and ground them at the same location.

How do you "run the ground off the first rather than the second" when they are all combined and then grounded?

34

I take it to mean they are all grounded together at the main.But I could be wrong,his explanation is a bit confusing.The layout shows the main cap grounded at a point with other "higher current" components,and the screen cap grounded with the preamp cap at a different location.This is a commonly known mistake that causes noise in these amps.Anybody who has worked on these older Fenders knows this.Cure is to ground the screen cap with the main cap.

35

The first two filter caps should be grounded near the power transformer. The preamp filter cap should be grounded near the input.
The Tremolo is only a single stage and the oscillator pulses the output grids directly from the plate. Later and/or more expensive models use a second stage as a current amplifier which uses a low impedance cathode follower to pulse the output grids. It may be difficult to remove all of the pulse from the single stage Tremolo. It should be slight though. It sounds as though the Tremolo is pulsing the hum, and it might go away if you fix the hum. Of course, since on that circuit the oscillator is the other half of the preamp tube, having it at the same ground potential as the preamp half means it will be at a different ground potential than the output tube grids that the oscillator is pulsing. If splitting the cap grounds doesn't work, you could always try connecting the oscillator plate to the B+ at the screen grids and moving the oscillator cathode ground near the power transformer. There are a lot of things to try, but without having the amp here on my bench, it's difficult to be more helpful.


Register Sign in to join the conversation