5F2A Hum

[rockybottom16]

I've got my deluxe quieted down - now for the 5F2A. It's not a real loud, obnoxious hum. But it is significantly louder than my 5E3. I pretty much went by the Weber schematic(on their website), using the brass plate. The only change I made was to ground the main filter cap at the PT bolt. It's got the hum as soon as I flip the switch and doesn't change much as it warms up. I tried elevating the heaters - no change. I tried moving the 6v6 cathode ground from the plate to the PT bolt - no change. This is the first time I've used the brass plate and I'm not impressed, to say the least. I'm thinking of removing the plate and using a buss on the back of the pots like I did on the deluxe, which is much quieter. I'm kind of at a loss.

[Bruce / Mission Amps]

I've got my deluxe quieted down - now for the 5F2A. It's not a real loud, obnoxious hum. But it is significantly louder than my 5E3. I pretty much went by the Weber schematic(on their website), using the brass plate. The only change I made was to ground the main filter cap at the PT bolt. It's got the hum as soon as I flip the switch and doesn't change much as it warms up. I tried elevating the heaters - no change. I tried moving the 6v6 cathode ground from the plate to the PT bolt - no change. This is the first time I've used the brass plate and I'm not impressed, to say the least. I'm thinking of removing the plate and using a buss on the back of the pots like I did on the deluxe, which is much quieter. I'm kind of at a loss.

The brass plate is a great idea... it is a giant star ground.
Try it with no power tubes and see if it still does that humming.
If you hear a hum as soon as you flip the on switch then your output tranny is picking up hum from your power tranny.
Unbolt it and move it around a little to see if it quiets down.
I sometimes rotate my OT about 30-45 degrees and can easily find a spot that is less hummy.

[tubeswell]

The only change I made was to ground the main filter cap at the PT bolt.

Try grounding the first/reservoir filter cap and the second filter cap grounds, and the 6V6 cathode resistor ground side, and the 6V6 grid leak resistor ground side, and the heater CT/100R ground side, and the High Tension CT all at that same PT chassis bolt, each via their own separate wires. Run the OT secondary ground to the output socket ground and leave that separate.

Then run all the other ground returns (including the third filter cap ground) each via their own separate wires to the input jack that is grounded.

[rockybottom16]

Bruce- Thanks for the tip! I rotated the OT about 40 degrees and now the startup hum(without power tube) is all but gone. Amazing.

Unfortunately, once warmed up with the power tube installed, the majority of the hum is still there. So: without power tube, no hum; with power tube and no preamp tube, hum; with both tubes, hum. And this is AFTER elevating the heater CT and making the changes recommended by Tubeswell. I also tried a different 6v6 with no change. Again, it's not a real loud hum; but sitting in a chair 5 feet away I can hear it. Whereas my 5e3 is virtually silent from that distance. The only difference now between the gounding scheme of the 2 amps is that in the 5e3 all the preamp stuff (including the input jacks and buss along the back of the pots) are grounded to a bolt through the chassis a couple inched from the jacks. In the Princeton the pots and jacks are grounded to the brass plate. Maybe I'm just being too picky. But I've hear that this amp can be great for the studio; and I've a feeling that the engineer would not be overjoyed to work with this one. (Not that I have any plans for studio work...)

I'd be happy to listen to any other suggestions. But I'm at the point where I'm thinking about yanking out the brass plate and using the buss method like I did in the 5e3. If that doesn't cure it, I'll just live with it.

[tubeswell]

In the Princeton the pots and jacks are grounded to the brass plate. Maybe I'm just being too picky. I'm at the point where I'm thinking about yanking out the brass plate and using the buss method like I did in the 5e3.

Try un-soldering the grounds from the brass plate and doing those grounds with their own separate wires going to the ground lug of the grounded input jack. (Make sure the other grounds are where I suggested putting them, by their own separate wires, at the PT bolt). Using a separate wire for each ground return path is quite critical. The aim here is to keep the ground return from each component separate from the rest of the ground returns as much as possible, thus avoiding potential micro-rises throughout the whole ground return system. It only takes a few micro volts of potential (VAC) difference in induce ground hum. (If all the 'low-current' ground returns are kept separate from all the 'high-current' ground returns, this further avoids micro-rises in ground potential - which is why the power amp grounds are kept separate)

Here's how I do the pre-amp grounds - I solder a series of brass 4mm eyelets mounted closely together at the input socket end of the main board so their flanges overlap, and I tin all those up with a nice and even coating of solder. Then I run each of the separate ground return wires to that 'bus', and one really short thick wire from there to the input jack ground lug. I use a big iron to melt enough (leaded) solder into those eyelets so that the whole joint is nice and clean. When I measure the resistance, the whole ground system at any point is no more than 1R. And there is hardly any hum (if any). Works for me.

Now so much for grounding.

Hum could also be coming from:

- unshielded signal wires; or

- undesirable sharing/coupling of EMF of the transformers - which location/orientation can assist with (but shielding is also important - are they shielded?); or

- from inadequate filtering - particularly in SE amps (in PP amps any residual power supply ripple is cancelled out through each half of the OT primary, which keeps them quiet). Whereas in SE amps you can improve that by adding a choke (by way of a CLC filter) between the rectifier and the OT supply. But if you put a CLC filter there, the choke needs to be big enough to handle the max current draw through the whole B+ (which, for a SE amp with 1 x 6V6 and 1 or 2 x 12AX7s, is not much and a small 50mA choke will do it).

[MWJB]

Make sure that all components have 1 path to ground, my 5F2A eyelet board had a screw hole that let a board screw touch a grouned eyelet = loop hum.

What cathode voltage do you have? You might need more than the typical 20-24vdc to bias the heaters?

I ran a 220K 2W resistor from the main B+ node to ONE side of the twisted pair heater wiring, then another resistor from the same side of the heaters to ground, I had a B+ of over 400vdc so a 22K worked for me, if you have 350-380vdc try 27K - IMPORTANT, the grounded 22K/27K resistor should be the ONLY ground reference for the heater winding & you want 35-40vdc, no more.