Express build Hum.. help troubleshooting.

[Kassie]

Hey people,

I just finished building an expres.. Well I built it before and it worked. But I decided to rewire everything.

I now get a loud hum.

Things I have done to troubleshoot.
Measured all the voltages, with tubes installed. Preamp stage is a little off on most voltages strangely.. Please check the attached file.

I have pulled v1, no result, v2 no result, pulled PI.. ahhh almost all the hum is gone. Though I still hear the same frequency hum barely hearable through the speaker, it's almost not there.

Ohh and I also rotated the OT 180 degrees versus the last time, due to better wiring inside the chassis.

I'ts really a kind of hum you hear from the power network here at home. Because on the same network upstairs we have a hot air heating and it makes a similar type of noise.

Also when i changed ohmage on the OT secondary selector switch, it started oscilating on all the 3 positions.. then after switching off and on it was fine again... hmmmm.

Furthermore my end tubes are not consistend. I supposed to have bought a matched pair. But one is biased on 37.9 and the other on 42.7. I biased through 1 ohm metal film resistors on pin 8.

Could it be faulty filter caps? Ground loop? I will post some pics in a minute. I build it a little different then standard Express since I used a jtm45 chassis. First time it worked fine so it is possible.

Pic's coming....

[Kassie]

In the mean time disconnected the negative FB wire from the presence pot and that seems to be the problem.. that wire is running under and over a couple of AC cables.. under the board.

Edit: It's not the problem. Back with all the preamp tubes installed I get massive amounts of hum when turning the drive pot open. Changing tubes did not work. And I am barely hearing signal when I try to play guitar.

[Allynmey]

Voltages are off in your PI. Check the plate and cathode resistor values. What trannys are you using? Your voltages are low on the heaters but a little high across the board. Take a few high res. pics of the preamp and power board and post them.

Allyn

[Kassie]

Ok here are some pics..

I forgot to wire the ground wire from the speakerjack to the chassis. So did it and it helped to bring the hum level down. Now I have no hum with all the potentiometers at 0. But I introduce hum by opening them up. I rewired the 1.5/22uf to the ground bus on the pots (like standard) instead of on the chassis... also no result. Will post soon about other findings..

Those resistors that feed the filter caps are 18k in parallel. And the last one that feeds B5 is 2x 4k7. Actually the 18k in parallel all measure 8.8K. So they are a little off.. why I didn't use 9k1? Not to be found here in Netherlands.

The PT is a Hammond 372JX, OT MercuryMagnetics KF..
Quite weird indeed... It's suposed to be 600Volts on the secondaries but it says 612 volts or so... while the heaters are 5.9AC

[Kassie]

Ok so I measured continuity... and right off the bat the grid wire from v1.1 was broken off... that solid core wire sure breaks fast when you are busy with the inputjack and such... soldered in a normal stranded wire and it works.. the thing is loud... also added a 15k grid stopper on v1.1. and planning on a 820ohm on the v1.2 grid, try to avoid any more shielding, but perhaps the negative feedback wire needs to be shielded. I am thinking about running it from the speaker jack instead off the ohm selector switch. That way it has to travel less distance and it will also be further from AC wires..

Since I didn't add the ppimv that I used to have my ears are kinda hurting allready yikes...

No more hum... well exept if I stand right on the chassis with my guitar... but thats normal. Hiss levels are pretty high, but I have lousy preamp tubes in it... still have a high grade TAD for v1 that worked well.

All in all pretty satisfied so far... need to recheck the bias..

It's hard checking voltages because the wall voltage just now is 220ac.. yesterday it was 235. The PT wiring is set for 240 actually... so maybe that's an explanation for a lower filament voltage. Or maybe Hammond has just given me a mondaymorning model.

[hans-jörg]

Hi Kassie,

take care with this solid core. At first it is way to oversized (at least for the signal) and second, they brake very fast if you start to move them around. If you dont take care you could breake an other one. I would change the wire around the tubes, stranded or at least thinner solid ones.
Don´t shield the neg. feedback wire. There are a lot threads which result is not to do it.

BTW: does this Hammond don´t have tabs for 230V or 220 VAC? If jes try the 230. I have a similar one from Hammond and i am using the 230 with good results.

Best

Hans-Jörg

[Kassie]

Hi Kassie,

take care with this solid core. At first it is way to oversized (at least for the signal) and second, they brake very fast if you start to move them around. If you dont take care you could breake an other one. I would change the wire around the tubes, stranded or at least thinner solid ones.
Don´t shield the neg. feedback wire. There are a lot threads which result is not to do it.

BTW: does this Hammond don´t have tabs for 230V or 220 VAC? If jes try the 230. I have a similar one from Hammond and i am using the 230 with good results.

Best

Hans-Jörg

Hey Hans-Jorg. Indeed the wire is oversized. I ordered it but had no Idea it would be so large. Actually it is solid copper, then a white insulation, and on top of that cloth. So that makes it quite big. I actually like it for the tubes, since I can bend it firmly to the chassis. The wire has les movement and thus more stable and less prone to osclitation. Tight to the chassis also has it's shielding properties. My first build I could not do without shielding the grid wire to the volume pot. Now that is not a problem.

I played it yesterday and today on full volume for a couple of hours.. it's very stable. Only a little oscilation with all the controls maxed out (yes also the volume, presence and treble) and the 500pf switch on, it was an out of control sound, and the amp thought so too. maybe I'll add an extra grid resistor between volume/grid. I allready have a 15k grid resistor on the input.

The negative feedback wire is in place and unshielded. I found a good place for it but attached it to the speaker jack. (I play on 8ohms anyways). That negative feedback loop tightens the amp up considerably, and also brings the noise floor in the power amp down. I could hear the difference right away.

My first build sounded like many of ceriatone clones on youtube. More trebly and high fi... this build actually comes into the territory of Glen Kuykendals amp. So I am quite pleased. I wonder if it's the solid core wire, but I think it's a combination of multiple factors, like better soldering haha. All in all it's a better build compared to the last one...

I am still thinking about that power transformer. 230 should be ok, but many amptechs in NL advise to use the 240. Since we are going steadily to 240 anyways. Often I measure 235 in the evenings. But during the day it fluctuates a lot between 220 and 240.

I am not too worried about the heater filaments since they are quite happy between 5 and 6.3 volts. Actually there are benefits of lower heater voltage and the difference is not suposed to be audible. The PT spits out a higher secondary voltage then what it says on the tin. Wiring it to 230 will increase all voltages maybe too much.

I re-measured all the voltages of the tubes and they are up to spec now.
Also I am almost done constructing my cab, nice dovetail joints. Then looking for a nice finish. I am thinking about giving it a black stain treatment, then sanding, then a warm yellow. Kinda like Joe Perry's boneyard Les Paul.

[hans-jörg]

Hi
good to hear that everything is ok.
On behalf of the "thik" core: the problems accure when you have to move them around because of tweaking as ex.
No doubt they are fine and robust.
Because of the mains: I will install in the future a swich in my amps to have the option of 230 or 240. Till now I was to lazy for such ...
But you are right, the futur will bring higher mains.

Best

Hans-Jörg

[Kassie]

Hey guys,

thought you might like to see my finished amp.

I went for the vintage brown look with copper.

[IMG:800:600]http://i932.photobucket.com/albums/ad16 ... C02147.jpg[/img]

[IMG:800:600]http://i932.photobucket.com/albums/ad16 ... C02143.jpg[/img]

[IMG:800:600]http://i932.photobucket.com/albums/ad16 ... C02149.jpg[/img]

[IMG:800:600]http://i932.photobucket.com/albums/ad16 ... C02151.jpg[/img]

[Bob S]

Man - that is beautiful !
Absolutely stunning.
The copper looks great with the dark wood cab.
Me likes very, very mucho much.
Well Done!

[Reeltarded]

Good job!! Looks GREAT!! Better work than all of my other Dutch friends could muster! lmao!!

Love the look of the finished amp... I can't even say how much. Very nice!

[Kassie]

Thanks guys!

@ reeltarded you know quite a few Dutch people?

Yeah I first tained it with black ink actually... then sanded that off till the grains so you get like a bit more definition of the grains.. then put some redish lacquer on it (as a bit per original).. but it just turned brown actually...

I scaved and sanded two beautifull wooden faceplates, only to see them split with drilling the holes... so in order for me to not eat my shoe I thought, hey If I go for that old whiskey barrel look... some copper might suit it... and voila... copper is really soft though so for the faceplate I pressed and glued the copper on an aluminium plate first... letters and the locomotive are decals... I cutted as much away as possible otherwise you see this 'rollertape like thingy' through the lacquer.

Thought about the logo, but I just added it.. I think the amp looks not original at all so I think it's not bad. Plus people actually reckognise Trainwreck with the beautifull sound... so I give credits to Ken and his legacy (and the company). I really do like that logo actually

I used a jtm45 chassis which is a bit less deep. Which I like cuz it saves space... my mum said: That amp doesn't look big at all... I said "yeah wait how it sounds" haha.