New Build JTM45 Hum - Need Suggestions! See pics

[soma_hero]

I build around an amp every other day for a living, and i wouldn't blink twice about the lead dress in that amp as a cause of hum. Oscillation or other instability maybe but that you can figure out by pushing wires around.

GROUNDING is your problem here. Grounding is all about knowing the rules so you know when/how to break them. Read up here:

http://geofex.com/Article_Folders/stargnd/stargnd.htm
http://www.valvewizard.co.uk/Grounding.html

There are plenty of "grounding schemes" you can find online to follow but that is more like giving someone a fish instead of teaching them to fish. You need to understand the "Flow" of the grounding river. The Valvewizard article is excellent at illustrating this.

In short some bullets:

-rectifier ground (in this case your PT center tap) goes straight to the first filter cap ground

- try to keep your grounds from that point "in-line." For instance, first filter cap ground to second cap ground, to PI cap ground, to preamp cap ground, etc. It's hard to keep them inline like this if you ground to the chassis. The chassis lets current flow all over the place and not necessarily where you want it to go.

A trick! Take a 0.1uF 600V cap, and ground one end with an alligator clip/piece of wire. Now you can use the other end of the cap to AC ground the signal at any place you touch the cap lead. So with the amp on, humming, work your way from the input to the PI to see where you can get rid of the hum.

If you ground the grid of the second stage and the hum goes away, your hum is originating at the first stage. If you ground the input to the PI andthe hum goes away, your hum is coming before the PI. This way you can narrow down where the hum starts, and move your grounds around until you quiet things down.

[xtian]

Nice post, Soma. Where do you like to ground the input jacks and the heater CT?

And what amps do you build? Do you have a web site?

[soma_hero]

I build for a company in seattle, but I don't know whether or not I should advertise. I haven't been told not too, but i've been known to shoot my mouth off from time to time so I wouldn't want that to come back to my employers.

Anyway, I like to ground to the chassis at the input jack. Usually I ground the heater CT at the rectifier ground, and then work my way from filter cap to filter cap. The individual cathode grounds/volume/tonestack grounds go to the respective filter cap with a separate wire than the wire going from filter cap to filter cap.

I don't usually isolate the output jacks from the chassis which sometimes makes me have to split the grounds in half (around the pi) and ground the input section at the input jack and use a ground lug near the other end of the chassis for the rest of the grounds. This way the flow from the OT doesn't have to go all the way through the preamp grounds in order to make it to the reservoir cap.

[LeftyStrat]

I build for a company in seattle, but I don't know whether or not I should advertise.

THD? Soldano? Bet it starts with a 'V'.

[kburks1230]

I build for a company in seattle, but I don't know whether or not I should advertise. I haven't been told not too, but i've been known to shoot my mouth off from time to time so I wouldn't want that to come back to my employers.

Anyway, I like to ground to the chassis at the input jack. Usually I ground the heater CT at the rectifier ground, and then work my way from filter cap to filter cap. The individual cathode grounds/volume/tonestack grounds go to the respective filter cap with a separate wire than the wire going from filter cap to filter cap.

I don't usually isolate the output jacks from the chassis which sometimes makes me have to split the grounds in half (around the pi) and ground the input section at the input jack and use a ground lug near the other end of the chassis for the rest of the grounds. This way the flow from the OT doesn't have to go all the way through the preamp grounds in order to make it to the reservoir cap.

Soma,

Can you draw it out on my pic so I know what I should do?
I'm a newbie. I got zapped for the first time yesterday by one of the filter caps. I now know which ones are the filter caps. Yikes!

[Firestorm]

It usually doesn't affect JTM 45s (more a problem with JTM 50s with split cathodes), but sometimes elevating the heaters will keep 60 cycle hum off the cathodes. Since you also have the cathode follower in the circuit, it's not such a bad idea to do it anyway to improve heater/cathode voltages. Modern tubes are pretty intolerant of this.

[Tillydog]

Can you draw it out on my pic so I know what I should do?

I concur with the advice that grounding is your problem - Three people above have posted pictures or links to the same, proven grounding scheme for this amp (generally in line with the principles outlined by soma_hero and the Valve Wizard document). It would be an instructive exercise for you to figure out where and how your grounding is different.

0.02p

Andy

[Structo]

Another thing I have found that can induce hum is the wire that goes to the Standby switch, if it is near wires carrying AC, it can pick up hum from those.

Be CAREFUL and learn how to drain your filter caps.

Make a tool like this to bleed the current from the caps.

Clip the black end to a bare metal part on the chassis and clip the other end to the + side of the cap.

Hook up your volt meter, black to chassis ground and red to the + side of the cap and watch the voltage bleed down.

Leave the bleeder wire connected while working on the amp to prevent recharging of the caps.

Click on the picture below.

[soma_hero]

Figure 13.14 in the Valvewizard article.

Also the image for RG's grounding of the vox amp are very illustrative in the first link i posted.

[soma_hero]

this is a good picture for a high gain amp like the SLO-100. You obviously won't have this many stages but it shows you how you would connect from one cap to the next in order.

[Firestorm]

Another thing I have found that can induce hum is the wire that goes to the Standby switch, if it is near wires carrying AC, it can pick up hum from those.

Be CAREFUL and learn how to drain your filter caps.

Make a tool like this to bleed the current from the caps.

Clip the black end to a bare metal part on the chassis and clip the other end to the + side of the cap.

Hook up your volt meter, black to chassis ground and red to the + side of the cap and watch the voltage bleed down.

Leave the bleeder wire connected while working on the amp to prevent recharging of the caps.

Click on the picture below.

That's not a bad tool, but can just clip ground to plate and drain through the OT. Then ground the preamp loads. Either way, dump the charge before you stick your fingers in there.

[epis]

Hi kburks1230,

there is a critical error in your grounding scheme. Mid and presence pots shouldn't be grounded at same point with OT secondaries and heathers ground.
Rearrange the grounding scheme as per second image posted by Structo.
That will help. Cheers

[Structo]

That's not a bad tool, but can just clip ground to plate and drain through the OT. Then ground the preamp loads. Either way, dump the charge before you stick your fingers in there.

Yeah, I wanted to keep it simple for him and this way he will learn about caps and how long they take to drain.

[Firestorm]

And as you said, you've got to leave the bleeder connected or the caps will rebuild some of their charge. One important caveat in amps modded by Dan Torres (or according to his advice) is there may be a diode in the dropping string which will prevent the preamp caps from discharging normally. I always check for for DC in the B+ rail before proceeding.

[Reeltarded]

Electricity tastes horrible here. I got ganked by 48v once last year. ONCE.