5e3 style amp build with speaker-driven reverb and 6au6 tremolo * hum Issue*

[tony321owen]

I finally got around to a build Ive been dreaming up for awhile with some spare parts I had lying around from previous builds.(Mainly a chassis and Princeton-reverb style transformers.) This is also the first amp where Ive built a dove-tail jointed  pine cabinet with help from my buddy with a table saw.

The concept for the combo amp is a low powered 5e3 style circuit using octal-6sl7 tubes in the preamp with a speaker-driven reverb circuit i got a schematic for and the tremolo that was used in the silvertone 1482 which uses a 6au6 and is known to be deep and lush. The plan was to use the second input channel as a dedicated reverb return as opposed to bright and normal like on the 5e3. I also added a few mods that i think are cool for this kind of amp including switchable cathode bypass caps on the 1st and 2nd gain stage for a gain boost and a 6 position rotary switch to vary coupling cap values after the first stage which works well as a bass control  .

 I usually build my amps in small combo format so they are easier to move but this time I wanted to see what 10-15 watt amp sounds like in a little larger cab housing both a 12'' and an 8'' speaker! If i recall correctly majority of the classic amp companies at least fender rarely if ever offered lower powered amps (5-15 watt) with bigger cabs or dual speaker configurations probably for budget purposes. Im sure theres plenty of amps out there that do that I just cant think of..

So the last few weeks I finished wiring up the amp and by golly the thing sounded glorious. All the controls were smooth in terms of taper. The reverb sounds so great to me in a way different from fenders. The tremolo is also so deep and natural sounding in terms of waveform! Ive been searching for a good power tube bias-vary tremolo circuit for small cathode biased push-pull amps and i feel like i hit the jackpot with this one.  I was using the lower voltage taps  on my PT and with a 5y3 it sounded squishy and gave up the goods early. I may try to bring up the b+ a bit with a different rectifier tube..

Now heres the Issue!!!       It has a bad 60cycle hum and sounds like bacon sizzling. Ive been trying to trouble-shoot but at this point I think it may just be that I under-planned my layout and now perhaps theres some lead dress issues causing coupling...
I accidentally put my 6sl7 sockets a bit close so its difficult to lay things out in a way where wires don't run next to each other.. Perhaps my grid wire routing is bad on the first few stages.. The hum is there even if the first preamp tube is pulled though gets much louder with the first tube in and the volume/tone controls really make it louder and brings the sizzle out.

I have tried to troubleshooting: cleaning up wiring, rerouting, reworking the grounding scheme to closely match hoffmans design. I have grounded the tremolo circuit at the power section star ground instead of preamp bus...No luck... I have removed the reverb circuit and the cathode cap switch to see if it makes it better... No luck...
if I put my hand around the first preamp tube the hum gets louder. at higher settings this has happened too with the volume knob... I have switched in a few tubes to see if its just a bad 6sl7....Doesn't seem that way but will buy a few more to see since some of mine are a bit microphonic. The chassis is painted so no jacks or pots are touching metal and everything must be connected to one of the ground points.. could there be an issue there? maybe the backs of pots must be grounded or something i never thought of always using bare metal chassis's

At this point Im ready to just rebuild  the preamp with a layout more closely related to the traditional wiring diagrams I find online. Attached are a draft of my schematic and revised/proposed layout.
(revised layout is different from circuit in photo of inside the chassis attached)

Anyone see obvious issues in my layout that could be causing this hum?

Does my proposed grounding scheme/ filter cap placement make sense?

If anyone could give it a look and report any noticeable poor practices that would be awesome!
 
Thanks,

~TONY
 

[tony321owen]

pics

[tony321owen]

pics

[Esquirefreak]

I'm just guessing - But I know from experience that a reverb recovery circuit in most amps is highly sensitive/prone to noise. Therefore it needs a really quiet tube/driver and a careful lead dress.

Since some noise is present even without the tube, there might be some grounding issues as well. Maybe try moving grounds apart from/closer to eachother?

Cool build however!

/Max

[Williamblake]

Looking good! Congrats. What about heater elevation, that would only be two resistors and a cap ...

[Bieworm]

Humdinger pot with those 6SL7 helps a lot...

[Greg]

Try this easy fix first: unground the heater center tap and solder it to one of the 6v6's cathode. This will elevate your heater by 24-32volts DC and will tame most of the hum.

[tony321owen]

i tried elevating the heaters by running the the heater ct to the powertube cathode.. no significant hum reduction and it gave the tremolo a weird tick so i moved it back.. perhaps the humdinger pot could help?

[tony321owen]

I think I solved my problem after messing around tonight. It's a three part solution and I learned a a lot about different types of guitar amp hum in the process.

1. As I suspected, after doing all sorts of other things to try and troubleshoot the hum, my preamp layout was poor and I believe this was where most of the noise was entering the signal path. I should have seen it from the beginning before I started monkeying around with other fixes but perhaps sometimes I learn most about what not to do when I make mistakes. Im trying to see the silver lining on this one. haha.
I rerouted my input grid wire  to v1a instead of v1b which had previously forced it underneath the board where I think it was picking up both 120cycle and 60cycle heater hum. I think thats called capacitive  coupling. Components transmitting hum through electrical fields.. What clued me in i was that I had thought that I just had a case of 60cycle heater hum but when i tested it against a reference pitch It turned out to be a lot of 120 cycles as well. Everything I read said that pointed to the B+ supply as the likely source. 

2.Problem 2 was that I believe one of my 6sl7s is bad because when I put it in it really hums badly and has made suspicious noises.. I only had a 6sn7 to test it against which is a much lower gain tube so hum is expected to decrease as gain does in the first stage but before rerouting the v1a grid-wire even the 6sn7 had bad hum and now all you hear when the volume knob 10 is the gentle hiss that is to be expected in a quite amp. I hope its still quiet when I put a higher gain 6sl7 back in v1.



3. But there was still this egg frying sound similar to single coil guitar pickup buzz even with no cord plugged in and thats when I learned a good lesson about why they put a metal plate above the chassis in most amp cabinets.. I unintentionally hovered my hand above the preamp and for a moment the amp went dead silent. I did it again and kept it there and thats when I found the third source of hum besides the 60cycle/120 cycle hum source and the bad tube. RF interference I believe it's called.   


Through this process things got a little messy on my board unsoldering and desoldering components so I think I will still  fix the layout even more and clean things up a bit before I finish the tweed and mount everything in the cabinet. I like to know all connections are solid and neat in the amps I build so they have the potential to last the test of time and although its tedious go back and suck solder out of turrets and rewire components neatly, Its a good practice for minimizing headaches in the future.     

I will post pictures when I get it all rewired and mounted! 

[Lectroid]

I'll add this just for posterity.  My DR single channel build was a little scratchy/hissy/rough.  I got a huge reduction in noise just by lining the upper inside of my cabinet, above the preamp tubes, with that duct tape that's a thin metal strip. It's not cheap but it's easy to use and can be fit in anywhere.  I lined the pickup and switch/pot cavities on all my amps.  I can hear the difference immediately.