Hum in my Dave Hunter "Two-Stroke" build (First Po

[Axeplyr]

Hi everybody. Long time lurker, first time poster.

I am posting this here because you guys seem to have the market cornered on hum reduction. There are plenty of amp building furums on the net that I frequent, but you guys seem to be the "top dogs" so to speak!

First, a little background. I dove into my first amp build a couple of months ago. Knowing I wanted to tackle something easy to start with, I decided a single-ended Champ or Princeton style circuit would be the way to go. The Hunter Two-Stroke is a nice evolution of these basic circuits, so I decided to start with this basic circuit design.

I thought hard about a kit, but in the end decided to build from the ground up. I decided the build format (turret strips), the circuit layout, the chassis size, location of controls, faceplate design, head cab material, shape and design, tolex, aesthetics, and sourced all components. It is pricier this way - because I also had to buy the necessary tools and equipment. But I'm getting so much satisfaction with the finished product - which is almost totally unique. Only way it could be more so is if I designed the circuit! Maybe next time. Anyway, I digress. The negative aspect of the ground-up approach for a novice has unwittingly bit me in the a$$. That problem is HUM!!!

There is measureable 100hz hum (I'm in Australia - 240v 50hz) that is always present from power-up (no standby switch) and does not vary with the volume control. It is always there, always the same volume. It is also there even if I pull the single 12AX7 preamp tube. If I strum a cord and throw the power off, the hum stops instantly while the volume diminishes slowly. The hum is not unbearable, but it is loader than any of my other amps and I can hear it in the next room. So, to me it is too loud.

Anyway, those are the symptoms. I've done a fair amount of rewiring and rerouting. I believe the problem is poor grounding throughout - I used an aluminum Hammond chassis and did not follow star or buss grounding techniques. I just grounded to the chassis thru whatever turret lugs were nearby. My next step is to begin redoing my grounding system in accordance with many of the great references on the Net (i.e. Aiken and others floating around). But, I figured that a pic of my chassis and wiring here might help save alot of work that might send me to an early grave.

So, here are a few pics. Does anyone see anything obvious before I tear this thing apart???

[img778]http://www.tonecentral.net/gearpics/hom ... oke_18.JPG[/img]

[img778]http://www.tonecentral.net/gearpics/hom ... oke_19.JPG[/img]

[img778]http://www.tonecentral.net/gearpics/hom ... oke_20.JPG[/img]

[img778]http://www.tonecentral.net/gearpics/hom ... oke_21.JPG[/img]

[img778]http://www.tonecentral.net/gearpics/hom ... oke_22.JPG[/img]

[img778]http://www.tonecentral.net/gearpics/hom ... oke_23.JPG[/img]

[Andy Le Blanc]

if its twice the primary frequency then its probably getting in from the power
supply... the rectifier doubles it..... single ended amps can more sensitive to
supply ripple.... if you have a ground lug where youve attached the
neutral or ground from the house .... try and run individual ground wires
to that point from the neg. or ground from youre filter caps its a simple place to start.....
if its from the tube heaters ... install a hum pot. you can get a positive volt source from the
chathode of your output tube... to the wiper of the hum pot...
use a 100 to 500 ohm pot.

[nickt]

Grounding is a nasty mysterious beast. I've had an amp go from dead quiet to hummm and not been able to figure what I've screwed up - yes I'm also lazy - so take what I say with a grain of salt.

I'd suggest change one thing at a time and measure with a DVM as you go (not just listen). You definitely need to have a "goal". Doug Hoffman has a grounding paper on his site, Aiken has another on his. Either should work IMO.

http://www.el34world.com/charts/grounds.htm
http://www.aikenamps.com/StarGround.html

BTW Andy's 100R pot trick worked real well on my AC30 for many years.

Good Luck - your build looks REAL NICE! Hope you get it where you want it!

[triodecr]

It looks like your first two filter cap grounds and the power transformer centertaps are not grounded at the same point. Try moving your centertap wires over to the ground point of the 1st two filter caps and see if that helps.

triodecr

[jjman]

It looks like your first two filter cap grounds and the power transformer centertaps are not grounded at the same point. Try moving your centertap wires over to the ground point of the 1st two filter caps and see if that helps.

triodecr

I second that motion. When I built my 1st amp (Champ / Avatar) I used PTP with standoffs and I had the HV CT on the far side of the chassis from the filters. All the power supply current had to travel across the entire chassis and was modulating all the grounds. It took about a year for me to realize the mistake. I had thought that the HV CT was merely a "reference" like the heater CT. Now there is no hum and the tone is super fantasmo. http://www.geocities.com/jjsant/champo_sm58.mp3

Also try a different OP tube if the wire move doesn't work.[/url]

[FunkyE9th]

I've been drooling over the two-stroke and actually plan on basing my 1st build on that amp. I still don't know if I want to go the kit route or buy everything on my own. I'm in analysis paralysis mode. I hope you get your hum issue sorted out. Interesting though how Dave Hunter morphed the two-stroke into a one-stroke...

[rfgordon]

I'd first follow good grounding schemes, as recommended already.

Another easy trick to put some DC on the heaters is to disconnect the 6.3 V filament center tap (looks like green w/ yellow stripe?) from ground and put it on the top of the power tube's cathode resistor. This will give you an instant DC voltage (probably about 22 VDC in this amp) on the filaments, which will help them reject noise.

In amps that use a pair of 100 ohm resistors for an artificial center tap (or the 100 ohm pot trick) they can be tied to the cathode resistor for the same effect.

[Axeplyr]

Wow - I'm overwhelmed by the great responses!

I'll spend a little time absorbing these, and post replies over the weekend as I try them.

[Axeplyr]

OK... I started easy first. A couple of these were quick and easy, so I tried them first. I'll post inline to some of the original responses.

It looks like your first two filter cap grounds and the power transformer centertaps are not grounded at the same point. Try moving your centertap wires over to the ground point of the 1st two filter caps and see if that helps.

triodecr

I second that motion. When I built my 1st amp (Champ / Avatar) I used PTP with standoffs and I had the HV CT on the far side of the chassis from the filters. All the power supply current had to travel across the entire chassis and was modulating all the grounds. It took about a year for me to realize the mistake. I had thought that the HV CT was merely a "reference" like the heater CT. Now there is no hum and the tone is super fantasmo. http://www.geocities.com/jjsant/champo_sm58.mp3

Also try a different OP tube if the wire move doesn't work.[/url]

I tried these suggestions first, no help unfortunately. Thanks for the nice modulation explanation thing - that helps me visualize what may be going on here. And great sounding clip!

I'd first follow good grounding schemes, as recommended already.

Another easy trick to put some DC on the heaters is to disconnect the 6.3 V filament center tap (looks like green w/ yellow stripe?) from ground and put it on the top of the power tube's cathode resistor. This will give you an instant DC voltage (probably about 22 VDC in this amp) on the filaments, which will help them reject noise.

In amps that use a pair of 100 ohm resistors for an artificial center tap (or the 100 ohm pot trick) they can be tied to the cathode resistor for the same effect.

Tried this next. I measured 27v on the cathode of the power tube. Once again, no help!

Looks like I have to reground the whole amp, and follow good grounding practices as all of you have suggested. <sigh>

I've been drooling over the two-stroke and actually plan on basing my 1st build on that amp. I still don't know if I want to go the kit route or buy everything on my own. I'm in analysis paralysis mode. I hope you get your hum issue sorted out. Interesting though how Dave Hunter morphed the two-stroke into a one-stroke...

I'm happy with the voicing and the tone overall. However, the amp has a couple of things about it that require improvement - the hum of course, and there's a raspiness or brittleness to the overdriven tone that needs taming. I might have to yank these orange drops out of here and go with something better.

Back to the drawing board!

[rfgordon]

Since you have plenty of chassis space, you could add another filter and put a choke in. That should dramatically reduce power supply ripple.

[Phil_S]

For the uninitiated, implementing a ground scheme can be daunting. It looks to me as if you simply picked the most convenient local chassis bolt and that simply isn't a scheme. The probable result is that you have a ground loop. Others have already pointed this out.

IMO, what you should do is relatively simple. This amp doesn't have a lot of parts and you've got plenty of empty real estate inside the chassis. I suggest the following:

Pick one ungrounded lug on a terminal strip as your floating ground point. (If it turns out to be too much stuff, solder a bit of uninsulated 18awg copper wire between two adjacent ungrounded lugs and solder to the wire. This would be called a buss.) To this one floating ground, run all the pre-amp related grounds: input jack, preamp tube cathodes, filter cap that supplies the preamp, and any control pots. This is your low current ground point/buss. From this ground point, run only one wire to the high current chassis ground (described next).

Under a through-the-chassis bolt, use ring lugs to gather all the high current ground wires together, and the connection to the low current ground buss. The high current grounds are the output jacks, power tube cathode, HT center tap, and main filter cap for the power section. I would locate this bolt in the space between the rectifier socket and the PT. I see a bolt on that crossbar between the filter cap and the PT and that one is probably a good one to use, but I'm not clear what it is holding down. If that is just a support bar with no other function, it is an excellent spot for this ground. (Relocate the yellow wire from the rectifier so it runs under filament supply pair and lift the filament supply so it is in the air. IMO, you need to do this anyway, as it is a potential source of hum, so get it done.)

I hope I've covered all the things that should be grounded (see below). Make sure every ground goes to one point or the other. Post here if you are unsure what to do with it.

It appears you have an aluminum chassis. There is a possible problem with unlike metals in contact with each other creating oxidation over time which will eventually defeat the ground. To fix this problem, apply some NOALOX or similar compound to the ground bolt and between the bottom ring lug and the chassis.

Other grounds that are unrelated include the a/c power supply ground and the filament supply ground.

The a/c power supply ground (middle lug on the IEC inlet) must have it's own chassis bolt close to the IEC inlet. If this were my amp, I'd just put this on the rectifier socket retaining bolt near the fuse holder. Again, use NOALOX and a ring terminal. Use of a tranny bolt is common practice, but not recommended as the best practice.

It appears the Green/Yellow is the filament supply center tap? If so, put this on the high current ground bolt. Since I don't see an artificial CT for the filament supply, I'll assume I've got this right. If you do need to used two 100 ohm resistors for an artificial CT, these go to the chassis anywhere, but not on either of the two ground points I described above.

[Axeplyr]

Since you have plenty of chassis space, you could add another filter and put a choke in. That should dramatically reduce power supply ripple.

Rich,

I've considered this. However, I figured I probably have a real problem (i.e. ground loop or otherwise), and didn't want to take this approach until I actually exhausted everything else.

[Axeplyr]

For the uninitiated, implementing a ground scheme can be daunting. It looks to me as if you simply picked the most convenient local chassis bolt and that simply isn't a scheme. The probable result is that you have a ground loop. Others have already pointed this out.

IMO, what you should do is relatively simple. This amp doesn't have a lot of parts and you've got plenty of empty real estate inside the chassis. I suggest the following: ...

Thanks Phil. Looks like that is my next real step. I'll give this a shot - though a couple of parts I'm just a little confused about. Once I get started, I'll probably be back for greater clarification as required.

[Phil_S]

IMO, what you should do is relatively simple. This amp doesn't have a lot of parts and you've got plenty of empty real estate inside the chassis. I suggest the following: ...

Thanks Phil. Looks like that is my next real step. I'll give this a shot - though a couple of parts I'm just a little confused about. Once I get started, I'll probably be back for greater clarification as required.

I'll be happy to take email from you off line if that will help. I assume you know what to do with this email address: psymonds_removenospam@comcast.net

[notbob]

Hey, I'm new to the forum and fairly new to amp building. I've built 3 amps I'm on the third one now. All 3 of the amps I've built have the output transformer oriented perpindicular to the power transformer. Is it possible this could be causing the hum? My amp theory is a bit on the weak side but I'm trying to get educated.

Steve