New Build JTM45 Hum - Need Suggestions! See pics

[kburks1230]

Am I missing a preamp filter capacitor right in the middle of the board?
It would be the largest one like on this guys board:
http://vintageamps.com/plexiboard/viewt ... 4&p=822955

[xtian]

Is there a cap can under the middle of the board, mounted on the outside of the chassis?

[kburks1230]

Is there a cap can under the middle of the board, mounted on the outside of the chassis?

No, i checked. Could this explain the extra noise I'm getting in the preamp circuit?

[ToneMerc]

Is there a cap can under the middle of the board, mounted on the outside of the chassis?

No, i checked. Could this explain the extra noise I'm getting in the preamp circuit?

No, a JTM45/100 is a totally different animal than what you have. Don't drift off track chasing things that are non-applicable to your JTM45 build. It's obvious of what needs to occur with your build. I think you are wasting your time looking for a single simple magic bullet to resolve your issues here. If are not comfortable with doing the work yourself, there's nothing wrong with that. However, from a suggestion and help standpoint from the outside looking in, it's not much more we can do until that amp is cleaned up.

Maybe there's someone local that can help.

TM



TM

[Phil_S]

I've been away for a few days. I'm an amateur builder. IMO, you are getting good advice here. The collective wisdom is that you should concentrate on fixing the ground layout. In fairness, if you are new to this, it can be hard to wrap your mind around this aspect of amp building. You may think there is 0 potential on a ground, but that's not right. There is relatively low potential at the input and first preamp tube cathode, and it increases as you move towards the power section (cathodes, filter caps, OT). To oversimplify, the goal is to avoid a ground loop. A ground loop happens when you get the ground potentials out of order or mix low potential and high potential grounds. There are different views of how to layout the grounds. Soma says to ground to chassis at the input jack and this is a commonly accepted method; proven and we know it works. I go the other way and ground near the power section/first filter cap, which also works. So, you've got to choose. Because you appear to be quite new at this, FWIW, I'm going to attempt to describe what I'd do to fix it.

Before going further, I suggest going to tedweber.com to find the 6M45 layout, which is essentially this amp, probably not exactly, but close enough. Print the layout. On that layout, find every ground symbol and mark it with a highlighter. Then find each of those grounds in your amp. Look for any other grounds in your amp. Essentially, make a visual inventory of grounds. You may want to make a single overview picture of your amp, print it and use the highlighter on that picture.

Places to look for grounds are input jacks, cathodes, pots (presence, mid, volume), speaker jacks or output transformer (not both; one or the other), power tube cathodes, power supply filter caps, bias supply cap(s). Preamp cathodes usually have a resistor between the tube socket and ground or a resistor and a cap in parallel.

I like to use a ground buss. They make "buss wire" but I often use a piece of solid copper taken from ordinary 14ga house supply wire "Romex" or the like. Either remove the naked ground wire or use a piece of the black or white and remove all the insulation. The challenge you've got is where to put the buss wire. I suggest you can attach one end at the - side of the cathode bypass cap located at the edge of the board near the input jacks. The other end can probably be attached to the ground bolt that is between the power transformer and the board, near where the OT wires enter the chassis. This ground bolt is the ONLY chassis ground for the amplifier CIRCUIT. Bend it and place it so that it is clear of all contact with turrets and other components. Wire this heavy should stay where you place it.

There are two other chassis grounds that are not CIRCUIT grounds and these have a separate ground point on the chassis. One is the mains power supply -- the "green wire" which requires a dedicated ground bolt through the chassis at a point near the IEC power inlet. The wire should have enough slack (not very much slack) in the even of a catastrophic drop or pull that the mains ground will be the last thing to experience a tug. I can't see it very well, but the center lug on the IEC inlet looks to me like there is more than one wire and that just can't be a) correct and/or b) safe. Fix that if it's wrong.

The other non-circuit ground is the artificial center tap for the heater supply. This is a pair of 100 ohm resistors in a "V" shape, were the bottom of the V is a single point ground to the chassis and each resistor connected to one leg of the heater supply. I'm not seeing it in your build, but maybe I missed it. This particular ground can be done at any convenient point on the heater circuit. In your build, I'd put a solderable ring lug under the screw that is retaining the V1 socket and terminate it there.

For the circuit ground, I would start by lifting from the chassis all the inappropriate ground connections.

I would connect directly to the chassis circuit ground point the following:
The two JJ can caps above the PT. You can chain these together if you like.
Output jacks or Output transformer (one or the other depending on how the circuit was built).
Power tube cathodes.
Bias supply.
Ground buss.
PT center tap or if the CT is fused, ground the fuse.
PT shield if there is one (generally there is not).
Use one or more ring lugs (not spade lugs which can slip off the post) under the bolt to make the ground connections.

On the ground buss, there are basically three groups of grounds to be make, each one corresponding to a "stage." Each stage ends with a preamp tube cathode, but for this amp, it is probably OK to treat both cathodes from a given tube as a logical cluster -- three tubes, three groups. When I say "group" I mean put the wires on the buss as close together as is practical. and then move to the next cluster. Don't over think the grouping...if you get the group to take up about 1" of buss wire, that's fine. If you need to spread out a little more, don't be concerned. Remember, you'll need to solder to the buss wire, so do this in a way that won't require a blow torch to make a proper solder joint.

V1 group: input jacks, V1 cathode resistors.
V2 group: V2 cathode resistors, volume pots.
V3 group: V3 cathodes (through the presence pot), .1uf cap on the presence pot, mid pot.

Do this. It should cure your ground loops and kill the hum.

To give you some idea of how this works, I'm attaching a picture of my JTM45-like amp. The biggest difference is that I used individual filter caps that are located with the related section, so the grounding of those is different. You can see the copper buss wire and how I (loosely) grouped the grounds. Your amp is different, so you can't copy it, but it shows the basic concept. I'm not the most gifted or tidiest builder. If mine doesn't hum, yours can be made hum free, too.

If I missed a component ground and you are unsure what to do, just ask.

I feel certain not everyone will agree with my post. That's OK. As I said, there is more than one way to do it. Views and experience on this vary. I'm an amateur, so I suppose you should take all of this with a grain of salt.