HUUUUUMMY new build

[dehughes]

So, I swapped out preamp tubes and moved the PI electrolytic to the preamp buss bar ground, all to no avail. The hum hasn't changed a bit. I tried moving the wires around (tube leads, heater wires, etc..) and that didn't change anything either. Perhaps this PT is just hummy...but I doubt that, as I suspect something in my build is causing this excess hum.

I'm open to any and all suggestions...

[tubeswell]

Okay then it it probably not the tubes themselves

hum can come from bad grounding, bad wiring, bad transformer positioning, lack of appropriate shielding, or something bad in the signal path like a dodgy plate resistor (have you tried replacing the plate resistors in the PI? - CCs especially can get overcooked during soldering).

I see you started to change the ground wiring. The scheme I find works for me (and is totally silent), is having each grounding point going by its own separate return wire to the chassis ground point. In my builds to-date I have kept all of the grounds in the pre-amp (incl PI and pre-amp node filter/decoupling cap grounds) all going to one point on the chassis (at the input socket). And all the other grounds I have going (via their own separate wires) to another point on the chassis (at one of the PT bolts). Where I have had a tremolo, I have put the tremolo grounds going by their own separate wires to the tremolo socket footswitch ground. I have the mains power cord earth wire going to its own PT bolt. This system prevents to a large degree, the appearance of different voltages in the ground return path, which can cause hum

Other people use different systems. (BTW I have an article by R.G. which looks interesting and I might try that in my next build)

In terms of wiring, the others have already pointed out all the main points about keeping AC power wires away from signal leads etc.

Shielding input and grid signal cables (with cable shields grounded at one end only) helps keep stray EMR out of the signal path.

How are the transformers oriented w.r.t. one another? If the OT is too close to the PT, it can induce coupling of the EM fields, making hum.

Just some ideas to kick around

[loudthud]

At the input to the PI you have a .01 cap and two 220K resistors. As an experiment to see if there is ground noise, I would break the connection between the .01 and the 220Ks and see if running a wire from that cap to ground at the bottom of the tail resistor, and to the ground side of the cathode resistors of the previous two stages produces hum. Try connecting the wire to various other ground points to see if some or all of the grounds are noisey or quiet.

[dehughes]

Thanks tubes and loud. Much appreciated. I'll try your ideas out tomorrow or Wednesday when I have more time. I did, however, just re-wire the power amp electrolytics to have their own path to ground, so we'll see if that helps anything.

Here is a picture of how I have the trannies mounted:

[dehughes]

DUDES....I re-wired the electrolytics in the power section so that each has its own path to the ground, and that the PI electrolytic hangs off the preamp buss. Check this out....

0) No tubes in the amp = just the audible hum from the PT itself....nothing out of the speaker. Perfectly quiet.

1) Rectifier in: A very little swell of hum, but then perfectly quiet, just like 0.

2) Rectifier and Power tubes in: Hum is present now, albeit at no more than HALF the volume as with all the tubes in. This is way less hummy than before. If this was all the hum I ever heard, I could live with it, no problem.

3) Recto and Power Tubes and PI in: Hum is identical to #2.

4) Recto and Power Tubes and PI and other 12AX7 in: Hum is the a bit more than #2 and #3, albeit with some slight hiss (normal....not bad at all). OKAY, now THIS is better! This used to be waaaay hummier, having only the 6SN7 missing. It didn't use to matter which preamp tubes were pulled, for as long as the PI was in it still was a hummy little brat.

5) Recto, Power Tubes, PI, 12AX7, AND 6SN7 in (all tubes): Hum is ABOUT as bad as it was, though I think it is a bit less overall. INTERESTING....

I'm going to have to get my brain around all of this....I think I'm making progress (thanks nearly entirely to you all...) so I'm excited but still wanting to nail down the last hum problem. Maybe time for DC heaters? More ground tinkering? Very cool...I'll get to this more tomorrow or Thursday.

Please feel free to post more suggestions before then...they're greatly appreciated. It's getting close!

[dehughes]

Soooooo, I moved the ground point for the preamp buss bar to the same point that the power tube cathode and grid resistors are grounded, and guess what? It hums about the same with the volumes off, but when I turn them up, it is noticeably less hummy than when the preamp was grounded to the main lug. SO, progress! The 6SN7 channel is still injecting far more hum than the 12AX7 side, but less than before.

BTW, the amp sounds better now...which is amazing. The ground scheme really does help with tone. Super cool.

So, now what? Keep searching for the perfect ground scheme, eh? I'd rather not have to go with DC heaters...would rather solve this with a good ground scheme. I'm soooo freaking close...

[tubeswell]

FWIW I'd be inclined to try getting rid of the ground bus completely and have each ground return point going by its own separate wire to either the common pre-amp grounding point or the common hi-current grounding point.

[dehughes]

FWIW I'd be inclined to try getting rid of the ground bus completely and have each ground return point going by its own separate wire to either the common pre-amp grounding point or the common hi-current grounding point.

Sweet. I'll try that. I'll cut up the buss bar and sequester each stage off, and run leads to a common ground point (I'll try both, and see which is quieter).

THANKS!

[dehughes]

FIXED IT!!!!!


It WAS the 6SN7 heaters! The lantern battery solved alllllllllll the hum, save for a teeny bit which is normal for a cloned Brown Deluxe PT. AWESOME! Thanks SOOOOO freaking much to all of you. I owe you all a beer or something....thanks.

One thing of note: I noticed a drastic change in tone/feel of both channels depending on how I grounded them. That is, when the preamp buss bar was grounded to the same point as the power tubes cathode, the amp sounded and felt wonderful. But, when I ground the 12AX7 and 6SN7 stages to the other end (up by the jack), it felt kinda lame....harsh, midrangey, flat, flappy....just kinda "yech". Interesting. So, I guess I'll be re-doing my grounds for the sake of tone/feel now, huh.
_________________

[txstrat]

Glad you fixed it.
Mind to post some more pics of the inside?
Can't get enough of it Looks really great.

[dehughes]

Glad you fixed it.
Mind to post some more pics of the inside?
Can't get enough of it Looks really great.

Sure, I'll post more when I've wired in the DC heaters, etc.. Have to order parts for it...

[Enzo]

Try this, disconnect the 6v heater winding center tap from ground. Connect that center tap to the cathodes of the power tubes. Well, connect it to the top of the cathode resistor, that might be more conveninet than the tube socket.

Those cathodes are at a positive voltage - a positive voltage greater than the cathodes of the 6SN7. By elevating the heaters to this voltage, the electrons boiling off the heater in the 6SN7 will be FAR les likely to flow to the tube cathode.

Try it, one quick connection change. Might save you doing a ton of rewire and DC conversion. And if it doesn't help in your case, then move it back.

[dehughes]

Try this, disconnect the 6v heater winding center tap from ground. Connect that center tap to the cathodes of the power tubes. Well, connect it to the top of the cathode resistor, that might be more conveninet than the tube socket.

Those cathodes are at a positive voltage - a positive voltage greater than the cathodes of the 6SN7. By elevating the heaters to this voltage, the electrons boiling off the heater in the 6SN7 will be FAR les likely to flow to the tube cathode.

Try it, one quick connection change. Might save you doing a ton of rewire and DC conversion. And if it doesn't help in your case, then move it back.

Sweet. Will do. Could I just put a cap and resistor under the CT at the point where it's connected right now, should I find this to be the trick? I suppose my question is if there are disadvantages to putting the CT on top of the power tubes cathodes...

[tubeswell]

Sweet. Will do. Could I just put a cap and resistor under the CT at the point where it's connected right now, should I find this to be the trick? I suppose my question is if there are disadvantages to putting the CT on top of the power tubes cathodes...

There is no disadvantge - the output tube cathode is at a pretty constant voltage (especially if its bypassed), and you are only putting the heater winding's neutral reference point on the cathode.

Other methods of elevating the heater are described on the valve wizard site.

http://www.freewebs.com/valvewizard/heater.html

[Enzo]

A cap to ground from the center tap wouldn't do it. The whole point of the exercise is to make the heaters sit at some DC level higher than the preamp tube cathodes. The output tube cathode is a "free" source of such a voltage. You don;t need to make a DC supply to provide the voltage then.

The voltage itself doesn;t matter much, as long as it is more positive than the cathode voltage in the preamp tube, it will prevent electrons from flowing heater to cathode in that tube.