102 build hum at startup

[philbard]

Hello All,

Just finished a 102 build, same configuration as I've been using with success for some time now. Went through the standard startup process, bringing the voltage up slowly and letting the caps form, etc. Plugged a speaker in and I'm getting pretty loud hum when I start up. It happens immediately after flipping on the power, both with tubes in and with tubes out (?). With tubes in it gets louder when they warm up. Faint sound from the guitar when jacked in, and some popping. I've been over the grounds, checked all the power supply wiring, OT transformer wiring, all seems correct. I'm using a Hammond 290FX PT, which is 348v, my PS voltages are as follows. Without tubes, they match the tubes-in voltages I normally get, but when I install tubes all are below normal.

Tubes out:
B+1 438
B+2 438
B+3 432
B+4 380
B+5 373

Tubes in:
B+1 392
B+2 391
B+3 375
B+4 283
B+5 276

Here is a gunshot:

Gut Shot.jpg

And a video of the hum (tubes are in):

Hum Video 480p.mov

Thanks in advance for any feedback on where to look next.

Cheers,

[philbard]

Update: Just determined that the preamp works fine, no hum, by plugging into the Return on another 102. Would seem to indicate the source is the Power Section.

[martin manning]

I would suspect the output transformer or its connections. Measure the resistance on each side of the primary and see how they compare.

[Chris Brown]

Seems like something in the output section... It almost sounds like a really loud transformer hum. Also, the preamp voltages seem high.

[Chris Brown]

Random thought.. anything weird happening with the switchable fet? Even with the input grounded at the jack, that could be feeding something into the signal path if it's engaged.

[philbard]

I would suspect the output transformer or its connections. Measure the resistance on each side of the primary and see how they compare.

I thought that too. Initially I hooked up the OT's common wire to the output terminal on the impedance switch. Moving that to ground made no difference. I've even swapped the OT for another in case I had damaged it, no change. I did find a couple of wires that were omitted in the preamp (mid boost and FET) because the layout software I'm using is a little flakey when it comes to printing out the diagrams, sometimes drops stuff. That solved the problem with the Mid Boost not working and also the FET. But there was no change to the hum.

OT primaries the same at 15 ohms (hooked up).

[philbard]

Seems like something in the output section... It almost sounds like a really loud transformer hum. Also, the preamp voltages seem high.

I agree on the character of the hum. However, the preamp voltages are lower if anything than my usual builds. Also, I don't think it could be FET bleed as the preamp section is hum free when output to a different amp's PS.

Thanks for the ideas...

[martin manning]

OT secondary grounded?

[philbard]

OT secondary grounded?

Yes, and just reflowed it to make sure.

[Richard1001]

Looking at te voltages, with and without powertubes, there might be a problem with the first two filtercaps. How did you wire them?

When they are not connected correctly (in series) the choke could be the cause of the hum you hear without the tubes in.

[philbard]

Looking at te voltages, with and without powertubes, there might be a problem with the first two filtercaps. How did you wire them?

When they are not connected correctly (in series) the choke could be the cause of the hum you hear without the tubes in.

Hi Richard,

Here is the wiring diagram for the PS. I just checked with an ohmmeter and also lifted the board to confirm the wiring. Its correct. I am concerned that the B+ voltages seem low, however, and don't know what could be the cause of that yet.

Power Supply.jpg

[philbard]

Got it. I got to thinking that if somehow AC was showing up in the OT and all the wiring was correct and grounds solid, perhaps it was some DC ripple from the power supply. Sure enough, the first large filter cap was bad. Swapped it out and the amp works perfectly. I always measure my smaller caps and resistors before installing, but neglect to do so with the large filter caps. Lesson learned... Thanks to all who helped out with feedback. Its fantastic to have such a knowledgeable available on TAG.

Cheers,

[WhopperPlate]

Got it. I got to thinking that if somehow AC was showing up in the OT and all the wiring was correct and grounds solid, perhaps it was some DC ripple from the power supply. Sure enough, the first large filter cap was bad. Swapped it out and the amp works perfectly. I always measure my smaller caps and resistors before installing, but neglect to do so with the large filter caps. Lesson learned... Thanks to all who helped out with feedback. Its fantastic to have such a knowledgeable available on TAG.

Cheers,

Bad cap out the gate from F&T?! no way ! must have been a repackaged Illinois cap !

[philbard]

Got it. I got to thinking that if somehow AC was showing up in the OT and all the wiring was correct and grounds solid, perhaps it was some DC ripple from the power supply. Sure enough, the first large filter cap was bad. Swapped it out and the amp works perfectly. I always measure my smaller caps and resistors before installing, but neglect to do so with the large filter caps. Lesson learned... Thanks to all who helped out with feedback. Its fantastic to have such a knowledgeable available on TAG.

Cheers,

Bad cap out the gate from F&T?! no way ! must have been a repackaged Illinois cap !

Yeah, I was pretty surprised too. It was part of the reason I didn't bother to test it beforehand. But then ya never know.