Some hum problems...

[smurf]

Hi,
After replacing my PT (see other topic), my new 36watter marwatt is working !
Only problem is that there's a loud hum in the amp, which doesnt change with the volume knob.
I noticed when i was checking the tube voltages, that when i touched pin 2 of the PI, the amp was dead quiet, all the hum was completely gone.
So, where should i be looking to make it quiet without having to to touch pin2 ?

And another thing i found when measuring the voltages, is that on one side of the OT primary on pin 7, i couldnt measure any voltage, my DMM gave a read that was out of range(on the 500VDC setting), while on the other side of the primary i measure 344VDC.
Could it be fried ? I turned the amp on for a second without a load, resulting in a spark at the speaker output jacks

Here are the voltages i measured at the tubes:
V1 - 5751
1: 155
3: 1.84
6: 156
8: 1.74

V2 - 12au7
1: 171
2: 58
3: 121
6: 170
7: 57
8: 123

V3 + V4 - EL84
3: 11.4
7: -
9: 320

V5 + V6 - EL84
3: 11.4
7: 344
9: 321

[Defdaddy]

Well, having no voltages on the plates of V3 and V4 is definitely a problem, one which I would tackle before further investigating the hum from the PI.

Are you sure you made a good solder joint there when you wired in the replacement OT? Have you tried reflowing it?

An easy method to check the OT for shorts is, with the amp off and drained, measure the resistance from the primary center tap (usually red, from the filter cap) to the one plate connection (usually blue) and then from the center tap to the other plate connection (usually brown). The impedances, typically a couple hundred ohms, should be within 20% of each other. On a shorted OT, one measurement will usually be way different from the other.

This is not a definitive way to prove that an OT is good, but it will identify one that is bad.

Def

[smurf]

I'm measuring 38 Ohm on the OT primary on V3&V4, and 42 Ohm at V5&6, which is the same as i remember when i measured it a couple of days ago.
From time to time i do get a read on my DMM, but it seems pretty random:
normally i just get the "1", indicating out of range, and then it jumps to "1250", "1550", ... for an instant and then back to "1".

Also, when i measure on V3&4 ( the problematic ones), the volume of the buzz is lowered very noticably when i make contact with my DMM probe, which doesnt happen at the other side of the primary, on V5&6.
Don't know if thats of any help.

[Defdaddy]

I'm measuring 38 Ohm on the OT primary on V3&V4, and 42 Ohm at V5&6

I assume you mean that each reading is with respect to the center tap. If so, that test looks good.

For any further testing, I would remove the tubes. The voltages will be higher with it unloaded, but you will keep the tubes out of harm's way.

You didn't say if you reflowed the solder joints - I'll assume you did.

Have you visually inspected the V3&4 plate leads, from the tube socket all the way to the transformer? Any nicks or possible areas where it is contacting ground or another component? There aren't but so many opportunities for failure there because it should be a straight run from the OT to the plate connections. One thing you could do is to swap the blue lead for the brown and see if the voltage problem stays with the socket or follows the tranny lead.

Beyond that, I'm out of ideas. Someone else chime in please.

D2

[smurf]

I assume you mean that each reading is with respect to the center tap. If so, that test looks good.

Indeed with respect to CT

You didn't say if you reflowed the solder joints - I'll assume you did.

I did

Have you visually inspected the V3&4 plate leads, from the tube socket all the way to the transformer? Any nicks or possible areas where it is contacting ground or another component?

no visual nor measureable shorts there

There aren't but so many opportunities for failure there because it should be a straight run from the OT to the plate connections. One thing you could do is to swap the blue lead for the brown and see if the voltage problem stays with the socket or follows the tranny lead.

I had just swapped the leads of the primary, because i was getting positive feedback, as described in this topic: http://www.18watt.com/modules.php?name= ... sc&start=0
swapping the leads solved that problem.

I gotta say i played the amp for a few minutes yesterday, and apart from the hum, it sounded great. I already made an 18watter marwatt, and this sounds exactly like it.

[zaphod_phil]

So you still have the same hum? Is it in both channels? Can you post a schematic for your amp, or is it the same Marwatt design as in the Downloads section, with the addition of two more power tubes?

[smurf]

It's a one channel marwatt, the 'watt' side ( i left out the 100k, 1K5x2 and the 5u and 1u caps), from the one in the downloads section with the poweramp of the 36watt tmb layout v2.0 from the download section.
And the hum is still there yeah.
cheers

[57chevy]

I noticed when i was checking the tube voltages, that when i touched pin 2 of the PI, the amp was dead quiet, all the hum was completely gone.

I noticed you built a single channel amp. Is pin 2 of the PI not being used? If so, connect a .1uF cap between pin 2 and ground.

[smurf]

I noticed when i was checking the tube voltages, that when i touched pin 2 of the PI, the amp was dead quiet, all the hum was completely gone.

I noticed you built a single channel amp. Is pin 2 of the PI not being used? If so, connect a .1uF cap between pin 2 and ground.

Hi, pin 2 was being used allright, but i forgot to connect the cap there to ground. result: no more hum
I switched the power tubes at the same time ( moved V3&4 to V5&6 and vice versa) , and i got readings on pin7 of all the tubes.
Amp is done and sounding great!
Thanks for the help people!