Finished Amp/hum issue!

[basschops1528]

How would it make a difference being too long?/its the B+, primaries , and speakers wires. The hum is as loud as it was soldered in and mounted before. Anything getting in those long would be too small for the power tubes to make a hum as significant as it was before, correct?

[Willabe]

Anything getting in those long would be too small for the power tubes to make a hum as significant as it was before, correct?

The street goes both ways, what about anything coming OUT of those wires and getting into other wires/circuits?   

[basschops1528]

Well I tried to drape them as far awar as possible. The primaries were kinda near the inputs but I knew that and kept em as far away as possible, plus with no input they're grounded

[basschops1528]

I want to keep moving forward with eliminating the hum from my amp. I've decided to make another video and perform a few "hum tests" for you all to listen to. I've discovered during the test that when turning up each channel' volume, at a certain point the hum gets louder; this is quite different than I had stated and assumed before. Listen to the video, do any sound analyzing you guys want like HotBluePlates did before and hopefully someone can help me narrow down where the affected area might be.

To recap:
The hum gets louder past a certain volume.
Tone Pots affect the sound of the hum.
Each channels' hum sounds different.
With V1 removed only, there is hum; V2 removed stops the hum (can't verify if no sound or typical hiss; listen to last video)
Hum is mainly 120Hz at approx. -28 dB, and 60Hz -54 dB

P.S. What is acceptable in terms of dB for any kinda of hum? I know it's more of a subjective opinion but is there an established number by consensus?

http://www.youtube.com/embed/f6EJuSs8U2k

[basschops1528]

Bump

[shooter]

What is acceptable

not a *standard* but for me, if the guitar vol is 1 and my 1st gain/vol pot is 1 and I can't hear the hum, I'm happy - most of my amps are <20W

[basschops1528]

Hey guys,

I had an epiphany today but I'm not sure it will solve this hum issue. I recently realized that one of the output tubes is not part of the matched quad because one of the originals failed short between plate and filament.    I want to try pulling that tube, and one on the "other side" to see if two matched tubes will function without the hum. I just learned that going down to "50watts" should be accompanied with a halving in the impedance so I will observe that as well. Maybe a four ohm load into the eight ohm tap.

[sluckey]

I just learned that going down to "50watts" should be accompanied with a halving in the impedance so I will observe that as well. Maybe a four ohm load into the eight ohm tap.

That's backwards. If you pull two tubes in a 4 tube parallel/push/pull amp, the primary impedance will DOUBLE. Therefore, you would use an 8Ω speaker on the 4Ω tap.

[basschops1528]

Thanks for the correction, I knew what I meant but got it backwards!

[basschops1528]

I had another thought as well right after my last post. What causes the DC voltage (thus scratchiness) on my presence pot? Doesn't the transformer block DC, even though the B+ potentially sets up an "electromagnet" in the primary?

[sluckey]

Cathode current for the PI tube must flow through the presence pot. This current flow creates a voltage drop across the pot.

[basschops1528]

Is there any way to block it?

[sluckey]

Yes. Modify the presence circuit so the PI current does not flow thru the pot. Like this...

     http://el34world.com/charts/Schematics/files/marshall/Marshall_jmp_mastervol_50w_2204u.pdf

[basschops1528]

Sluckey, I just finished watching a video on bypass/coupling cap proper installation. I never paid attention to the "out foil" lead on any cap since I never knew about it, other than electrolytic cap alignment of course. Could wiring multiple caps "backwards" increase the chances of 120Hz hum in a tube amp at AF? I could see RF being a problem.

On the same note, I understand how you would install a bypass cap to ground; with the outer foil lead going to...well.. ground of course. BUT how do you install a coupling cap between stages or even in other situations I cannot recall, where it is unknown to me where to align the outer foil?

[Paul1453]

I think they recommend the foil side goes on the side with the least resistance to ground.

I have not paid much attention to accurately aligning all the foil sides of my caps yet.
Probably because I haven't had much problem with hum.
There is usually another reason if you have bad hum, IMO. 

[pompeiisneaks]

I think the foil issue reduces overall noise, but it is a subtle tweak, it doesn't somehow magnify it.  If there is some source of noise and the foil side goes to the higher impedance area, it can send it on, instead of shunting it away.  The source is what should be fixed, but if it can't, setting the outside foil right reduces or can remove the noise source.

~Phil