Unbeatable hum problem in new build

[John Fisher]

Hello!
I am in the middle of building a high gain tube amp. The preamp is Soldano bassed. It is switchable to a clean channel Fender style channel.
In general the amp on both channels sounds great but on the high gain channel when the gain is turned up I get what sounds like a 120 cycle hum. I know it is comming from the first stage. I have star grounded the amp to death but still can't get rid of it. I even tried different tubes to no avail.
Any suggestions before tearing my hair out or suicide?
Thanks!

[Prattacaster]

Have you tried running shielded wire to your grids of the first couple gain stages? Ground only one end of the shield, center wire is your "hot". Usually 120Hz is attributed to the heaters.

[olddawg]

I don't know about the particular Soldano you are using, but a lot of high gain amps use DC filament voltage in the preamps just for that reason. Maybe you could float the heaters and sub a DC bench supply to see if it makes a difference? That and the shielding mentioned before. Anytime you make a lot of gain you amplify any hiss and hum present with it.

[Tonewood]

I was thinking heaters work straight off 60Hz (if not dc)& the rectified HT would be 120Hz. Can some explain if this is wrong?

Thanks

[Wilder Amplification]

I was thinking heaters work straight off 60Hz (if not dc)& the rectified HT would be 120Hz. Can some explain if this is wrong?

Thanks

I have no idea why Prattacaster even said that as AC heaters are in fact 60Hz. 120Hz is rectified DC ripple.

That being said, what type of rectifier circuit are you using? Also, did you ground the common/sleeve connection of the speaker jack?

[defaced]

Pics of the build would help. High res, straight down from the top, and tight shots of all chassis grounds.

A schematic wouldn't hurt. You may have a circuit error that you're trying to fight. Can't put out a fire on the house if you're spraying down the neighbor's smoking cigarette.

[tubeswell]

And on your schematic, if you can show how you have grounded what ground return points to where exactly, that would be good too.

[TransLucid]

Don't forget about magnetically-induced hum from the power supply circuit. Is the PT or choke near the input stage? While Jon's comment re 120Hz may also apply here, the current spikes caused by the rectifier (which are 120Hz) can cause magnetic radiation at 120Hz, it would seem to me...

Bob

[dai h.]

Soldano SLO100 heaters are AC. The amp is an offshoot of a Marshall 2203/4 but the layout is more rationalized (probably needs to be due to the high gain). For example, the preamp tubes are relocated, letting wire runs be shorter. Heaters are parallel and (more or less) at a strict right angle to the other wires (on the respective 12AX7s). It's possible the grounding is the cause of the problem hum (one possible cause). There should be a lot of DIY/cloning/reference info at SLOclone forums (or whatever it was called now. Note: you need to sign up to read the forums.).

[TransLucid]

Another thought - is the input jack isolated from the chassis and grounded to the star?

[Prattacaster]

I have no idea why Prattacaster even said that as AC heaters are in fact 60Hz. 120Hz is rectified DC ripple.

I appologize for that , i knew ripple was one, and heaters were the other, anyway shielded wires might help out a lot. Like transLucid said, make sure there is not a ground loop with the input jack, I had a sovtek that had massive hum because of the same problem. Easy fix.

[anson]

I had a hum in my build that was hard to diagnose. It also seemed to come and go as I moved the first stage preamp ground lead around. Lead dress is very important. Not just in the guitar signal path but also in the power supply. Star grounding doesn't really suit good lead dress to well, from my experience. The HT and ground wires should be twisted together inbetween filter caps so that induced power tranny hum isn't allowed to enter. HT and high current ground wire pairs shouldn't be allowed to make big loops. Same with signal and their respective return wires. Your problem might be of a different source though...
A

[loudthud]

When you switch the B+ off with the Standby switch, does the hum die instantly or does it fade out over a period of a couple of seconds? If it fades out, the problem is probably a lead dress issue with the heater wiring. If the hum dies instantly, it's usually a ground issue.

[John Fisher]

Well I really appreciate everyones feed back i have leared alot through it.
I have been out alot the last Couple of days so I havent had much time but I will tomorrow.
I did make sure that the input jack wasnt connected to the chassis and I even tried running DC to the heaters.
I have to confess that I made a mistake in the star grounding at the filter caps. I realized that I kind of lumped the grounds of the filter caps all together and that was my mistake. When I tried roughly seperating the grounds on each of the caps and then running them to the star ground that seemed to fix the problem. Hurray!
Tomorrow I will redo it all better and fix up the lead dresses a little better and I feel that I am on the road to sucess now.
I will report any further details.
Again thanks!

[Naoscaire]

The amp is an offshoot of a Marshall 2203/4...

Seriously?