express hum issue, run out of ideas!

[tomugli]

hi, first time poster here, though i've been reading for a while.

I've just finished an express clone with kt66's and a few little mods and sods, but nothing drastic!

it sounds awesome as far as tone goes, but there is an awful hum that gets louder both with volume increase and presence increase (possibly just more toppy with presence, but perceived louder). At 0 volume, it's gone. Swapped the tubes around, including the power tubes to el34's; no difference. removed v1; gone. i poked around with a stick, and the only thing that seemed to make any difference was when i touched the C1/R2 to ground bus wire. changed this though, and it's still the same.

so it's pre volume, and possibly tone stack related i think. Also swapped the jack from an isolated one to a chassis grounded version and again no difference.

in my ts i'm running a 250p .047u and .047u. jj gold v1, eh v2, tungsol v3 (12AX7) sovtek kt66's tubes.

any help would be most gratefully appreciated, as i'm a bit stuck now.

thanks in advance, tom

[stubbyfex]

Hello!
Welcome to the forum. Sometimes when I have a problem with a new build, especially one with mods, I remove the mods! I guess it is like starting with a clean slate. I first try to get the "stock" circuit up and running before I start tweeking or changing things. Once I have it running, then I do my mods so I have a baseline reference to compare to.
That said, you might try wiring around the tones. You could use one of the .047uf caps that are in the tone control circuit. First unhook the 100K slope resistor, and the 500pf treble cap. Then wire the .047uf cap from the first plate of V1 to the input of the volume control. It might hum worse if your lead dress is too long!
Another thing to check is the last filter cap, the one that supplies power to V1. If it did not have a proper ground, it might cause hum too.
Good luck in your quest.

[fishy]

Any pics you can share. Sometimes the more knowledgable guys can point you in the right direction with some decent photos and get you where you need to be faster.

How is the output tranny referenced to ground. At the speaker jack or is it isolated from the chassis and grounded elsewhere?

[tomugli]

thanks for the quick replies!

the ot is grounded to the output jacks, but the hum is pre ot i think. would it perhaps be wise to try altering this anyway as better practice; no stone left un turned and all that!

i will try changing the v1 supplying filter cap ground, as this is on a seperate wire to the 2 next to it. i will also try the tone rewire.

the mods are really only in terms of component value choices, nothing really wild's been done, and i like the tone so much, i think it'll stay this way (when i've found th noise!)

thanks again for any help and knowledge you can offer, i will report back soon.......... (and try to get some pics up)

[dartanion]

If everything is wired correctly and the layout is good, I would keep trying different tubes in V1. Wrecks are very choosy about V1 tubes, so keep trying until you find one that will work. I usually end up going through a dozen or so tubes before I find one that works in a particular amp.

As well, are you testing the amp with the chassis open and under fluorescent lights? This causes hum for sure. Simple test is to turn the lights off.

Pictures of the guts would definitely help us help you.

[Bruciep07]

Tom,
With my first express build, I swapped the OT primaries without realizing it. Playing with the volume or presence control would induce (or defeat) the hum, with V1 pulled it stopped... I know you said you think it's pre-OT, but just make sure you're OT is wired up correctly...
Pacific or Toneslut's Heyboer Vintage spec OT: Green to V4, Purple to V5
Heyboer non vintage spec: Brown to V4, Blue to V5
Bruce

[fishy]

If everything is wired correctly and the layout is good, I would keep trying different tubes in V1. Wrecks are very choosy about V1 tubes, so keep trying until you find one that will work. I usually end up going through a dozen or so tubes before I find one that works in a particular amp.

As well, are you testing the amp with the chassis open and under fluorescent lights? This causes hum for sure. Simple test is to turn the lights off.

Pictures of the guts would definitely help us help you.

1+
Nothing like checking against what works. Tubes are a big part of these amps.

My comments regarding the output ground came about because you mentioned initially having an isolated jack on the input so I was wondering what the story was with the OT which is usually chassis grounded on one of the speaker jacks. If was isolated on the jack and grounded elsewhere, you might / might not induce some issues.

[tomugli]

[IMG768]http://i567.photobucket.com/albums/ss11 ... CN4593.jpg[/img]

[IMG768]http://i567.photobucket.com/albums/ss11 ... CN4596.jpg[/img]

(jack changed to chassis grounded version now, no difference)

[IMG768]http://i567.photobucket.com/albums/ss11 ... CN4597.jpg[/img]

[IMG768]http://i567.photobucket.com/albums/ss11 ... CN4598.jpg[/img]

[IMG768]http://i567.photobucket.com/albums/ss11 ... CN4600.jpg[/img]

the parallel caps in ts/high pass and the series resistors are a stop gap fix at the mo, just trying to find values to best suit my desired tone.

tried the cap re ground tonight, but sadly to no avail. when i get home tomorrow, i'll be fiddling with the ts and ot grounds.

hope these help you help me. thanks again for the input.

[fishy]

Are you sure your ground connections truly iare at ground. The chassis looks as though it is powder coated (at least it does to me). If so, did you prep the holes and make sure you have good contact to the chassis?

The filament wiring from the preamp side of the output. It seems as though the wiring on V4/5 is very close to the signal wiring. It also looks as though the filament wiring at I assume is V1 is close the board and not against the chassis.

Filament wiring close to signal wiring will usually induce hum.

Also looks to me as though V1 gets filament before V3 - any reason for that or am I mistaken. Not sure if that is significant but it is against the norm

Lead dress around V1 to V3 is hidden so it might be tough if you have to go there.

[M Fowler]

Try ground the input jack to the grounding bus on the puts for a test. Also you do not have C7 .22uf cap on the board goes from v1 pin 3 to cap then to ground on treble pot.

If your layout works ok but man I had a hard time with the layout flipped end to end and also the board components are inverted as well?

Mark

[Structo]

Most likely lead dress.
That amp is in need of some serious finesse.

[tomugli]

ok cheers for the suggestions, i will experiment over the weekend. lead dress and grounding seem to be the main suggestions, i shall tidy up and re dress my ground points.

do you think the proximity of b1 supply lead to feedback loop to presence lead might also be inducing some troubles?

cheers again
tom

[M Fowler]

What about that missing cap off V1 pin 3?

Mark

[tomugli]

that came off for tonal prefference, but the hum was unchanged, and present both before and after. tried a few different values here and on the other triode of v1, but it made no difference to the noise.