5c3 Proluxe help

[Jason C]

Hey guys, I just finished putting together a 5c3 Proluxe (with a GA-75 preamp and tone circuit) together. The problem is a really loud hum, the signal gets through but with lots of hum and the tone controls don't seem to effect anything. Any ideas on where to look first? Also my voltages seem way high.....490v on pins 3 of the 6L6's. My first idea is a grounding issue, maybe? Thanks.
Jason

[tubeswell]

Got any pictures you can post?

[Jason C]

Here are some pics.
Jason

[Jason C]

And a couple more.
Jason

[tubeswell]

Where do you begin with any troubleshoot? Here's a couple of comments. (I might think of something else after a bit of a closer look). Got a copy of the proluxe layout and schematic there? - this is different from a Fender 5C1.

[Jason C]

Thanks Tubeswell. No those resistors aren't touching. This is the amp and layout I'm doing. https://taweber.powweb.com/store/5c3p_layout.jpg
And this is the preamp and tone circuit I'm using. http://www.schematicheaven.com/gibsonamps/ga75_old.pdf
I was going to run the input under the board but I thought it was bad to have it that close to the plate resistor.
Jason

[Jason C]

I pulled the phase inverter tube and the hum goes away. So that means my problem is in the preamp, right?
Jason

[tubeswell]

Hi Jason - Did you try another tube there?

[Jason C]

No I haven't but that tube came out of a working amp with no issues but I'll try another one. I did pull the preamp tube with no change in the hum. I also tried bypassing the tone controls with a jumper and still it hums. I dunno....just scratching my head here.
Jason

[tubeswell]

So everything is grounded to the brass grounding plate?

You could try a different grounding scheme and see if that reduces hum. Check out this one (courtesy of R.G. Keen - Thanks R.G.)

[loudthud]

In the picture prolux-010.jpg the top right jack appears to have a yellow wire that is not soldered. Also in that picture you have two wires from the volume control running right between the two pairs of input jacks. It would be better to route those wires from the volume control straight down towards the eyelet board, then along the chassis to the eyelets where they connect.

A couple of questions about the hum. When you flip the standby switch from Play to Standby, does the hum go away immediately or fade out over a period of a couple of seconds? Does the volume control have an effect on the hum? When you turn the volume all the way down does the hum go away, change slightly or get louder? Does the power transformer have a center tap for the heater winding (usually green with a yellow stripe) or is there a virtual center tap made up of two 100 ohm resistors?

[Jason C]

The input jacks are soldered. I took the picture just before I soldered them. I did reroute those wires going to the tone controls and also took the components they went to and mounted them on the pots. That shortened the grid wire to the PI pretty drastically (ugly now but what do you do?). The hum goes away imediately when the standby is flipped. The volume control has no effect on the hum. The PT has a filament CT that I grounded to one of the PT bolts along with the high voltage CT. Also, the hum did not stop when I pulled V1. I also swapped tubes and it still always stops humming when I pull the PI.
Jason

[Jason C]

I meant it does NOT go away immediately. It does take it little bit to fade away.
Jason

[loudthud]

I meant it does NOT go away immediately. It does take it little bit to fade away.
Jason

That usually points to a problem with the heater supply CT, possibly a bad tube, or a lead dress issue where a signal wire is too close to one of the heater wires. Your heater wiring looks pretty good. Make sure both of the green wires measure ~3.2VAC to ground. Another possibility is just a wiring error around the tone controls. Ohm all the points that are supposed to be grounded to the chassis.

A shielded wire from the tone controls to the phase inverter might help.

FYI: It looks like you used pin 1 on the 6L6 next to the rectifier as a B+ tie point. If you ever plug a metal 6L6 or 6V6 in there, the outside of the tube will have B+ on it!!! Another safety issue, you have the black wire from the line cord connected to the side terminal of the fuse holder. This allows you to touch the hot side of the line when inserting the fuse. Better to reverse those connections to the fuse holder.

[Jason C]

I tried clipping the filament CT to the positive terminal of a 9V battery with the negative clipped to the chassis....with no change. Thanks for the heads up on the fuse wiring. I'll never use metal power tubes in this but I may put that on some unused pin of the rectifier? I also fiddled with the bias and just made the hum a whole lot louder. Also, I'm not sure what you mean by ohm all the grounds. Do you mean measure the grounds resistance (sorry if that is a dumb question)? Here are my voltages V4 (6L6)pin 3=486 pin 4=463 pins 5 and 6=-61 pin 8=.011
V3 (6L6)pin 3=487 pin 4=465 pins 5 and 6=-61 pin 8=.008
V2 (6SL7GT)pin 2=277 pin 3 and 6=3.45 pin 5=279
V1 (6SL7GT)pin 2 and 5=441 pin3 and 6=7.91
all heaters are 3.3