Trainwreck Express hum problem

[Movie85]

Hey out there,

I've finally built my Trainwreck Express clone, and I am very happy with my built. But I have a big hum problem: I have the PPIMV-Mod installed. When I turn it all the way up, and with the normal "Volume" at 0, then it doesnt hum. When I turn the Volume up it starts to hum very loudly. It's a very raspy hum sound, I've attached a sound file so you get an idea of the noise.
Some things I did, maybe it helps: At first, the OT wires were used the wrong way round, so I fixed that. I pulled the PI tube, and the hum was gone. I also moved some wires in the inside around, but the hum didnt change. I use the proper star ground leading.

I've also attached some pictures of the layout, maybe someone sees the problem right away, or has some tipps for me how to make it better. The Voltage chart is also included, maybe somebody can have a brief look over it, for me it looks fine except of the PI cathode and grid Voltage, its a bit low isnt it? Also the Heater Voltage is about 6.5VAc, is that too high? How could I fix that?

Thanks in advance,

Tom

[Tillydog]

Since nobody has chipped in yet:

I'm not an expert, but the hum just sounds like mains pick-up to me.

I can't really tell much from the photos, to be honest (they're too fuzzy), but the heater wiring looks funky around the PI and power valves.

What happens to the hum if you flip the power off? - Does the hum fade out or stop dead? If it stops dead, it's likely caused by pick-up from the PT or heater wiring.

Does it hum with / without a lead plugged in?

What's the grounding arrangement like at your input jack?

(I haven't looked at the voltages)

Andy

[Bob S]

6.5 vac is ok on the heaters.
Please be very careful - safety first.
Try chop sticking around V1 & the Vol / Treble pot wiring.
The Bright switch wiring gave me fits one time - you might want to chop stick here as well.
Remove V1 & see if the hum is gone - V1 is a problem until you find the right low noise tube.
Good Luck.

[kwijabo]

i'm going to throw the caution flag first - be exceptionally careful when probing a live amp. i take no responsibility for your safety!

something i learned from this forum and helped me once - the first thing to figure about hum is if its 60hz or 120hz. 60Hz suggests heaters, 120hz suggests the power supply dropping voltage(s). if its the 60, do you have a center tap on the heaters or the 100ohm's to ground?

if its 120, examine the power line and see what kind of ripple is on it. in general (not sure what express values are) should be at least <15%, if not <10%. if its high, i would suspect a power cap.

i was able to find out mine was 60hz with a multi-meter set to detect frequency. i removed the center tap and replaced it with the two 100ohm configuration.

if its neither 60 or 120 or the above things are OK, explore what tillydog asked.
input cord vs. none? input grounding arrangement.

[Tillydog]

The noise in the file posted is 50Hz, so places to look are heater wiring, heater elevation, or poor grounding practices (the hum seems really loud).

If the hum only comes with increasing the volume pot, it's getting in at the tone stack, or earlier (V1a & input).

Double check that you have followed the correct grounding scheme exactly (amp off!)

0.02p

Andy

Edit to add - I can't see anything untoward in the voltage readings.

[Movie85]

Hey, thanks for the replies! Sorry for the bad pictures, I'll make some new tomorrow(Cellphone cam..).
I tested what Tillydog said, and the hum stops right away. If no cord is plugged in, then the 50Hz(thanks Tillydog) is still there, with a guitar plugged in(like in the soundfile) then some additional noise appears, but in "normal" volume. When I turn the Volume(PPIMV) all the way up, its too loud for my ears...
The hum went away when I remove V1, but I also used some different tubes, so no bad tube. I also moved the heater wiring a bit around, no difference...

Is there a difference when I use the 100Ohm config? I use the taps at the moment... I basicly followed the ceriatone layout.

Tomorrow I'll double check everything again, anyhow the hum seems not like wires which lay incorrectly but something other...

Good night

[Movie85]

Hey guys! I found the source for the hum: I double checked my layout, and I discovered that I've forgott(!) the center tap! Because anyhow I tought it was the centertap of the high voltage wires they've always mentioned. Now I installed two 100Ohm resistors and now this loud hum is gone. Still some noises, but "normal" ones

Thanks for bringing me to this!

Bye,

Tom