New clone....hummm

[Normster]

This might be a long shot, but it's easy enough to try. Try separating your shielded wires. Notice how Brandon pairs up the inputs to the gain and level pots, but keeps the sends routed completely separate. I have had amps that I could induce hum in by moving the shielded wires together. (I'm sure I read that coax only provides RF shielding, not electromagnetic.)

[soje]

Thanks for all your input.

So far the hum is still there. I tried disconnecting the shielded wire from input with interresting results. Of course it makes a racket with the resistor flying loose I grounded the resistoer to various places, but the hum increased to more than with the isolated and grounded input.

The jazz setting does diminish the hum a bit, but it's still there. Diming the bass pot, or grounding the .1 cap in the TS does kill the hum thou.

I've even tried disconnecting the ground wire to the realy board, but no change.

I'm thinking about trying dc heaters for V1 and V2.


Any other ideas?
Regards
soje

[Structo]

Changing the heaters to DC is not going to resolve this problem.

You have a wiring error somewhere in the tonestack or relay wiring.

[talbany]

Yeah I doubt changing the heaters will resolve anything too..since it's a low frequency hum it will only be passed by the .1 cap...It's more than likely everywhere ..Just being filtered out by the mid and treble cap so that doesn't really tell us anything... I also forgot the grid on the input needs to see a 1 meg load or it will squeel on you sorry...I took a closer look at your pictures again and didn't see anything out of the ordinary... The reason why the the noise is not so bad in the jazz position is the switch pads that frequency but it's still there.. You got me.... Not being able to actually hear it can't tell you where it's being generated..Layout noise or power supply.. If you want to tack in a 20uf filter cap across the existing cap for V1 to see if it's a bad filter cap that may be worth a try..Check make sure you don't have a leaky cap---Scratchy pots or snapping switches...Also the shielded wire doesn't have anything to do with the clean channel.. accept going to the input jack.. Most of that is OD related.. This kind of sh#% will drive you NUTS!! wont it.. Sorry I couldn't help more..


Tony VVT

[BobW]

The coax shield doesn't need to be connected to the volume pot, connect it only to the ground buss.
Check (measure) your V1 cathode grounds to the Volume coax shield and insure they are connected to the ground buss. I would also lift the preamp ground lug off the chassis and remeasure. The pics are difficult to see, but you could be grounding the coax via the volume pot only.

Also since you have a few grounds at the same lug, re-flow the solder again to insure the joint is solid.

Temporarily remove the input jack from the chassis, to determine if the hum goes away.

As Heisthl suggested you may have a ground loop, but simply measuring ohmage won't tell you if you truly do have a loop. You'll need to temporarily lift grounds / input jack ,etc. and remeasure. If you get the same reading with lifted grounds et. then you have a ground loop.

Using an inspection mirror, check under the board to insure nothing is touching the chassis. clipped leads, etc.

You can place a metal shim in between the relays, then ground it to the chassis to determine if relay / noise coupling is the issue. (this seems the least likely). hth

[groovtubin]

Thanks for all your input.

So far the hum is still there. I tried disconnecting the shielded wire from input with interresting results. Of course it makes a racket with the resistor flying loose I grounded the resistoer to various places, but the hum increased to more than with the isolated and grounded input.

The jazz setting does diminish the hum a bit, but it's still there. Diming the bass pot, or grounding the .1 cap in the TS does kill the hum thou.

I've even tried disconnecting the ground wire to the realy board, but no change.

I'm thinking about trying dc heaters for V1 and V2.


Any other ideas?
Regards
soje

you`ve got the preamp ground going to the HEATER FLOAT RESISTOR GROUND, at least it LOOKS like it, that would NOT be right!

jim@Omegaamps

[BobW]

Thanks for all your input.

So far the hum is still there. I tried disconnecting the shielded wire from input with interresting results. Of course it makes a racket with the resistor flying loose I grounded the resistoer to various places, but the hum increased to more than with the isolated and grounded input.

The jazz setting does diminish the hum a bit, but it's still there. Diming the bass pot, or grounding the .1 cap in the TS does kill the hum thou.

I've even tried disconnecting the ground wire to the realy board, but no change.

I'm thinking about trying dc heaters for V1 and V2.


Any other ideas?
Regards
soje

you`ve got the preamp ground going to the HEATER FLOAT RESISTOR GROUND, at least it LOOKS like it, that would NOT be right!

jim@Omegaamps

Jim is right. Since you already have multiple grounds located at that lug as well, try lifting both resistors from the lug, attach a ground wire to both resistors and run the wire over to the AC ground lug. Then try disconnecting the black preamp ground wire from the heater float resistors ground lug. Reconnect the black filter cap gound wire to the screw closest to the 22uF ground buss.

Ideally you want, AC ground and heater float ground paths to be closest to the Pwr transformer. It provides the lowest impedance path for current induced noise, and placing preamp and other audio signal path grounds farther away insures the power and heater current paths will not interfere. At the same time, you also want to keep the audio ground paths as small as possible, which is why some designs place the preamp filter caps close to the preamp components.

[soje]

I've been moving grounds around like a madman, and ended up pulling the relay board out and hardwiring the clean channel.

By now my grounding is thus:

1) PT ct, caps before choke and relay rect are grounded to PT bolt at the edge of the chassis.
2) Power tube cathodes, bias supply, 1. cap after choke and heater elevation are grounded on the next PT bolt.
3) The next 3 caps and the presence pot are grounded at the OT bolt between these caps and the preamp board.
4) The output jack is connected to the chassis.
5) The input jack, ground bus for the pots, cathodes for V1-2, od trim and OD relay ground are grounded at a bolt close to the input jack.

As indicated I elevated the heaters to about 30V. Also, I ended up removing the grid resistor on V1. I tried different resistors, but nothing beat simply removing it.

These three things brought the hum down to reasonable levels. I then proceede to reinstall the relays, and the clean channel is good. However, there's a lot of "rough" hiss on the OD channel.

It's still there if I turn down the volume knob, but turning down the first OD knob, the hiss disapears. I've tried hardwiring the OD channel (remove relay wires, and jumper the .047 cap to the 220k resistor), but nothing changed. The noise resebles the harsh uneven noise I hear when throwing the standby switch. This last mentioned noise fades.

Any ideas?

Regards
soje