Question Regarding Source of Hum

[AJD]

Hi All,
Longtime browser, first time poster. I recently built a single ended Princeton-like amp (my first build). The thing just smokes (in a good way) and I'm really happy with it overall, but I have a question regarding hum. When I have the amp plugged into a single 10" stock Fender speaker (from a Frontman 25R) the amp is virtually silent. There is a bit of shot noise when you crank it ALL the way up, but still way within reason. Now when I plug into my single 12" cabinet (loaded with an Eminence Swamp Thang) the hum is quite present. It doesn't matter if the the amp is at it's quietest setting or at it's loudest setting, the hum is present and unchanged in volume. The pitch of the hum is between B and B-flat as fretted on the third and fourth frets of the G string, so I believe that it is 120 cycle hum and not 60 cycle hum. I figured that I would beef up the filter caps or put in a choke but now I'm not so sure. Could this be due to the relative efficiencies/characteristics of the individual speakers? Any advice would be greatly appreciated. Currently the filter caps are 20uf&16uf, 16uf, and 8uf, with no choke. I don't have my camera with me so I can't take any gut shots right now but I do have some older photos taken during the build that I will post. Any advice or suggestions would be greatly appreciated. Thanks

[M Fowler]

Nice amp and did you build the head cab and speaker cab?

I noticed an impedence selector 4,8,16 and two speaker jacks. So when you use the 10" Fender speaker vs the Swamp Thang speaker are you using the same speaker wire, the same speaker jack on the amp and your speaker selector on the same impedence? Because I am wonder first about the cord and next if there could possibly be a bad connection in a jack or even the impedence switch with bad solder or broke wire etc.

Mark

[AJD]

Thanks very much M Fowler. Yeah, I built the head and amp cab and covered 'em.

Actually I am using different speaker cords now that you mention it. The Swamp Thang cab is out at my place in the desert where I can CRANK it and the little fender 10" is at my other place in town where I have to play at more reasonable/bedroom levels. I will definitely have to get both cabs side by side to A-B them this weekend. Definitely a good place to start. Thanks M Fowler. Here are a few shots of the cabinets while they were in progress.

[paulster]

Snap a picture when you can of the wiring from the output transformer to the speaker jacks, via the impedance selector if you've gone that route rather than individual jacks.

It sounds like it's the wiring there that has something potentially amiss.

Are the two cabs you've tested with both the same impedance?

[AJD]

Hi Paulster,
Yes, the speakers are both 8 ohms. I dug through my construction photos in the hopes of finding the shot that you requested, but no dice. I'll be sure I bring my camera back this week and take a few photos of the area that you suggested. Probably won't be until this coming Wed. or Thurs. though. Anything in particular that I should look for in this area?

[M Fowler]

You do good wood work and tolex work as well AJD, nice to see a real DIY with great results.

As I said prior I the believe the problem lies in the cords or the impedence switch and possible the jack on the speaker cab itself could be afoul.

Mark

[Structo]

This may or may not be the issue but even though nice right angle bends in the wiring look nice aesthetically, they sometimes are not the best electrically.

For example, the wires to the grids (pins 2 & 7) should be kept as short and as direct as possible and possibly could benefit from shielded cable there.

I see you used a star ground by the PT.
Do you have your preamp and input grounded there as well?

Or do you have your input jack grounded to the chassis or is it isolated?

[AJD]

Thanks for the response Structo. I had hemmed and hawed over the 90 degree angles, etc... for precisely the reason you mention, but as a novice it is not yet clear to me just when one wire might induce a voltage in another so I erred on the side of making it as pretty as I could. Probably not the soundest of methodologies. I was aware that the spaghetti looking interiors of 50's Fenders was not due to sloppiness but because the lead dress had been thoughfully planned out. I'm just not yet certain how to ascertain or apply the principles evident in such a layout. I do know that the shorter the run the better; if wires must cross do it at a 90 degree angle; separate wires with a great disparity in voltages; and use star grounding to avoid stray voltages traveling through the chassis.
The input grids of the 12ax7 do have shielded wire. I'll have to open up the amp to find out if my preamp and input are grounded at the star. I'm almost certain that they are. Is this a poor choice? The input jack might be grounded at the chassis as well. I'll check it out and get back to you all.
Thank you all very much for your input. It is greatly appreciated by this newbie. I have to go out of town for a few days starting tonight, but I will be sure to get some photos and check out the areas that you all mentioned and post my results as soon as I get back into town. Thanks for the help and support.

[Andy Le Blanc]

Take a look at your load too. 8 ohms on a speaker means from around 5 ohms
to around 8 ohms. Single ended amps can be touchy. It might be that the
hum was always there and was not revealed until it connected to a more
efficient speaker. I'd try shielded cable with the grid resistor connected
right to the socket pin on the first stage and then the same but with small
100 ohm resistors on the next couple stages. The plate leads and feed back
wire can raise hell if they are dressed with or close to input stages.
It is a wonderful looking amp.

[AJD]

Hi All,
Finally getting back to you with some photos...........

It looks like I have to post another set due to space limitations per post(?). Coming up.........

[AJD]

And here are the rest.............

FYI,
I checked the speakers side by side and I think that this problem is at least in part an issue of relative speaker efficiencies. The hum from the less efficient 10" Fender is quieter while the hum from the more effiecient Swamp Thang is louder.

I'm going to try your suggestions next Andy and then on to messing around with a choke or larger filter caps I suppose.

As it is, it still sounds great. It just bugs me that it isn't dead silent. Perhaps that is a bit much to ask for with a single ender???

Thanks to all for your generous help and sugggestions...............

[Kregg]

If one of the speakers is wired in reverse of what is indicated on the posts, could this cancel or cause a noticeable oscillation?
Shouldn't there be a ground wire (to chassis) for the speaker jack?

[Kregg]

Hmm ...

[LOUDthud]

The schematic shows one side of the heater supply grounded but in the pics it looks like there is a voltage divider off the main B+ and the center tap of the heater supply is hooked to that. If correct, that's good.

Are all the filter cap grounds connected together on the board? That could be the problem. There are several schools of thought on how to arrange grounds. If you insist on connecting the HV center tap (red w/yellow stripe) to a transformer bolt, try disconnecting the ground of the 8uF filter cap and grounding it at the input jack or where ever the first preamp tube gets it's ground. If that isn't quiet enough, move the red w/yellow stripe wire to the ground side of the 20uF cap. Run a wire from that some point to the chassis bolt of there isn't one already.

[AJD]

The schematic shows one side of the heater supply grounded but in the pics it looks like there is a voltage divider off the main B+ and the center tap of the heater supply is hooked to that. If correct, that's good.

Yes, that is correct. I placed a bit of positive voltage on the center tap via the B+.

Are all the filter cap grounds connected together on the board?

Yes, and then everything is connected to the star ground. Bad idea? Please see attached photos.

If you insist on connecting the HV center tap (red w/yellow stripe) to a transformer bolt, try disconnecting the ground of the 8uF filter cap and grounding it at the input jack or where ever the first preamp tube gets it's ground.

Again, they all terminate at the star ground. Should I alter this arrangement?

If that isn't quiet enough, move the red w/yellow stripe wire to the ground side of the 20uF cap. Run a wire from that some point to the chassis bolt of there isn't one already.

Are you suggesting that I connect the HV center tap directly to the ground side of the 20 uF cap and then on to the star ground? If so, how does this differ from what is already in place as currently the HV center tap and the 20uF cap both terminate at the same point? I'm not suggesting that it doesn't change anything electrically. It's just that if it does do so, I can't see how and I would like to know what difference it might make electrically.

Thanks for the help.