Other Amps

5F11 build - advice needed please.

1

Got a nice tweed Vibrolux, built for me by Mojotone. It's a great looking build, but there are a couple of minor issues. Firstly is the transformer/power supply hum. I assume that it's 50 cycle hum, (UK voltage)- can I safely increase the stock 16uF filter caps to say, 22uF to attempt to eliminate the hum.

Secondly, the low volume pulsing from the trem circuit. Is it an inherent characteristic, or can I build it out somehow? I've intentions to slow the trem with a couple of cap changes to make it more usable.

TIA.

2

the slight increase in cap value has little effect and won't fix the 50Hz hum much I guess. Has the heater wiring been grounded trough 2x100 ohm resistors of with a center tap on the transformer ?. It could be wire dressing issue. My 5E5-A build is dead quiet and looks quite alike this one.

3

The heater wires are grounded via a couple of resistors off of the pilot light holder.

4

You could check for ground loops. If you are grounding wiring based on a kit's recommendations, this would probably be more about checking for cold solder joints at any place where something is soldered. My personal preference is to ground all the filter caps to the chassis as directly as possible to a leg of the power transformer. And all the preamp grounding to a separate spot on the chassis. That way there aren't a lot of places for loops to occur or at least to check.

Some other sources of hum to check:

Filament supply: You mentioned that you created a virtual center tap using two resistors. Some people will actually go further with this and install a pot so you can vary the to resistor values to ground and balance the heater supply. This is called a 'hum dinger" and can sometimes get rid of hum due to unbalanced filament supply.

If your filament supply has a center tap off the power transformer, you can "elevate" it by connecting it to the cathode of your power tubes instead of directly to ground. This can sometimes help.

Also, I have heard mention that the filament supply needs to be "in phase" especially the supply to the preamp tubes. Basically the wire that connects to 4 and 5 should always be the same wire...I don't know if this is a myth or not but I always check for that instead of just twisting the wires together and using a random one when you get to the lugs.

One test to totally determine whether the filament supply is the possible source of hum is to connect a 12 volt lantern battery to all the filaments (instead of the power transformer's supply) and see if the hum goes away or not. This is pure DC with no AC ripple. If you still hear a hum after trying this...it's a hum source from somewhere else SUCH AS:

  1. Electromagnetic coupling between the power transformer and the output transformer. There's a reason that the power transformer and output transformer are bolted to the chassis at a 90 degree angle to each other. It is to eliminate this. But increasing the distance that they are apart and sometimes positioning the OT at a position slightly different from 90 degrees improves this. The tedious way you can test for this is to simply turn your amp on and physically move the OT around to see if the hum changes. Bolt it to the chassis where you hear the hum least.

2.Poor shielding. Using copper foil shielding inside the amp to create a faraday cage can help if the source of the hum is an outside streetlight, neon signs etc. Another possible source would be poor shielding from the input jack to the first gain stage of the first preamp tube. Also make sure that the shielded cables going to the reverb tank aren't corroded on an old amp and grounded well.

  1. Bad tubes. Swap out the tubes and see if the problem goes away. It happens.

  2. Bad filter caps. Check the polarity of all electrolytics to make sure they are all going in the right direction...negative towards ground. Smaller caps have a polarity to them that you can find with an oscilloscope. I've seen this done online but alas I don't have an oscilloscope so I have never tried this.

  3. Bad resistors along the power rail and to the plates of all tubes. Check voltages everywhere.

5

If it is 50cycle hum and not 100cycle upping the input filter is not the cure.Check where the filter caps are grounded,if its grounded like the schem,with the screen supply cap grounded with the preamp supply cap move the screen cap ground to the ground from the main supply cap.This is a common source of noise in these old Fenders.

6

Tubes can also be a source of 50/60 cycle hum, even if they test strong. I'd try swapping out each tube before firing up the soldering iron.

7

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.

Interestingly, the worst of the hum is diminished if I max the depth and speed pots with the trem turned off. It's not exactly the worst hum in the world, but I'm used to my Deluxe which is completely silent.

I'll try a tube swap and report back.

8

ALL the 16uF filter caps are supposed to be grounded on their negative sides. It shouldn't be that one is grounded and one isn't. Usually I tie all the negative poles of the filter caps together when joining them to the common ground on the transformer.

I'm looking at the schematic and something looks odd with this amp's design. The power tubes are cathode biased but the trem circuit is using the bias supply feed off of the power transformer's rectifier secondary to create the tremolo circuit. It seems to feed -31 volts up to the 250K "depth" pot and on to the power tube control grids. Anyway make sure that that 25uf/ 25v cap has the POSITIVE end facing ground or it might not filter well. Usually most caps in an amp have the negative side facing ground in an amp....except for fixed bias supply...and apparently this tremolo circuit which feeds an oscillating supply to the control grids I've never seen before.

Edit: Make sure the diode is also the right way. I don't know if they work both ways or not...just that it is polarized.

9

In a darkened room you can see the power tubes pulse with the trem engaged. I think that's the "bias Tremolo" effect. I've had a bit of a poke around and moved some of the flying wires from V1 a bit further away from the heater wires. The hum seems a bit better, or it could be that I'm getting used to it. It's no worse than say, a Blues Junior. I'm hoping that slowing the trem down will move the pulsing effect by giving me a more useful spread on the speed knob.

It's actually a great sounding little amp. Ten or twelve watts with the JJs, perfect for home use. A 10" driver the the Deluxe sized cab seems to give a more focused, tighter sound, and the whole thing sounds fresher than a Deluxe.

10

The voltages are critical on that amp to get the best out of the tremolo. I'm not an expert on that circuit, but you apparently really need to make sure that fixed bias going to the control grids is the right value.

11

The power tubes are fixed bias,not cathode bias.Dont slow that trem down too much,just change one of the caps,if you change more than one to try and get it slower it will thump worse.You can try putting a 100k resistor between the .02 and .1 cap feeding the depth control to alleviate some of the thumping.

12

The 5F11 circuit uses a 2 meg speed control pot. If you switch to the 3 meg used on later Fenders, the Tremolo will go slower. If that's not slow enough for you, PM me.
Take out one of the output transformer mounting screws and push the transformer on the diagonal and see if the hum changes.
Try a .047 cap from the wiper of the intensity pot to ground and see if that helps with the pulse.
Aside from that, show us better closeup photos of the inside of the amp. Maybe one of us will notice something.

13

I'm looking at the schematic and the cathodes are grounded. But the bias to the control grids oscillates. I guess that is fixed but variable. At any rate turning the tremolo knobs up with the switch off changes what is going to the grids. How that suppresses hum is above my pay scale.

14

A simpler way to slow it down is to change one of the .01 feedback caps to .02.When I did it to a 6g2 I built for my son I tried changing a second feedback cap and it got real slow but the thumping was unacceptable.I would try to minimize the thumping before you do any changes to slow it down.If you add some resistance as I suggested above it should decrease the thumping some.Look at the 6g2 schem and you'll see a 220k between the .1 cap feeding the depth pot and the plate.I suggested 100k in your case cause thats the value of the plate resistor.

15

I'm looking at the schematic and the cathodes are grounded. But the bias to the control grids oscillates. I guess that is fixed but variable. At any rate turning the tremolo knobs up with the switch off changes what is going to the grids. How that suppresses hum is above my pay scale.

– guitarcapo

Its called a bias wiggle.Yes the bias voltage varies,but when you switch the trem off the voltage stops wiggling.Its not intended to "suppress hum" it creates a tremolo by varying the bias.

16

The bias varies....its called Tremolo. It won't vary when the Tremolo is off.

17

Any idea why the hum reduces when he turns off the tremolo but turns up the intensity and speed pots? That's what I was getting at.

18

Any idea why the hum reduces when he turns off the tremolo but turns up the intensity and speed pots? That's what I was getting at.

– guitarcapo

Cant really comment on that,maybe Billy has an idea.I still think his excessive hum is caused by the preamp caps being grounded with the screen cap,like in the schem.Its quite common in these amps and the cure has often been by moving the ground from the screen cap to the ground with the main cap.Its difficult to diagnose via the internet,for all we know it could be 100 cycle and not 50 cycle,which is a whole different story.

19

Just re-read the posts,and I see you want to "I'm hoping that slowing the trem down will move the pulsing effect by giving me a more useful spread on the speed knob." Slowing the trem with a feedback cap change wont give you a wider spread on the speed knob as you say,it will just give you a slower rate over the same range on the knob.Billy's suggestion of using the 3m pot will give a wider range.If the low end is not slow enough then switch one of the .01's to a .02.

20

I tried the 0.047 cap from the wiper on the intensity, no difference apart from it seemed to make the trem wobble less intense. The background pulse is still there with nothing plugged in.

I noticed it's much worse, ie the pulse gets louder with single coils at the wrong angle in front of the amp, so I've ordered come copper foil to line the cab.

It's definitely 50Hz hum. It's not terribly loud, and I can learn to live with it. The trem pulse is another matter, and it's quite annoying. It seems to be linked to the depth control; with the trem on and the depth dialled down it's not there. It comes on at a quarter turn (about 4 on the dial), and gets louder, at about the point where the depth control seems to start working.

21

The power tubes are fixed bias,not cathode bias.Dont slow that trem down too much,just change one of the caps,if you change more than one to try and get it slower it will thump worse.You can try putting a 100k resistor between the .02 and .1 cap feeding the depth control to alleviate some of the thumping.

– stokes

Would putting a 100k resistor between the center and right lugs on the depth pot do the same?

22

No,its gotta go between the .1 cap and the .02 cap.If you look at the schem for the 6G2,there is a 220k cap there.On a 6G2 I built for my son I used 6BQ5's and had a serious thumping as you describe.I increased the 220k to 470k and the thumping subsided.I'm figuring since you have no resistor here adding a 100k may do the trick.

23

Any progress on this,Uncle D.?

24

Any progress on this,Uncle D.?

– stokes

I've upped the first two filter caps from 16 to 22uF, and that's made a bit of difference to the hum. It's there even with all the tubes out and the speaker unplugged, so I'm guessing it's a transformer issue. I can live with it now as it's virtually imperceptible from 5 feet away in a silent room. It's still much more than my 5e3, but that was built with a split grounding buss by someone who knew what they were doing!

However, in attempting to swap one of the caps that affect the trem speed, something seems to have failed and I've lost the trem altogether. As I'm now officially out of my depth it's off to the local tec's this afternoon.

25

If upping the filter caps improved the hum then its likely not 50 cycle,its 100 cycle.Its not likely a transformer issue,its those caps.I was going to comment on those caps you have in there,but you said it was 50 cycle.I see you have some sort of asian made caps.I would suggest using F&T's,much better caps,it makes a big difference.As for the loss of trem,its probably just a bad soldering job when you replaced one.


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