102 build hum at startup
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102 build hum at startup
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: And a video of the hum (tubes are in): Thanks in advance for any feedback on where to look next.
Cheers,
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: And a video of the hum (tubes are in): Thanks in advance for any feedback on where to look next.
Cheers,
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Phil
Re: 102 build hum at startup
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.
Phil
- martin manning
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Re: 102 build hum at startup
I would suspect the output transformer or its connections. Measure the resistance on each side of the primary and see how they compare.
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Re: 102 build hum at startup
Seems like something in the output section... It almost sounds like a really loud transformer hum. Also, the preamp voltages seem high.
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Re: 102 build hum at startup
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.
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Re: 102 build hum at startup
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.martin manning wrote: ↑Sat Oct 15, 2022 8:05 pm I would suspect the output transformer or its connections. Measure the resistance on each side of the primary and see how they compare.
OT primaries the same at 15 ohms (hooked up).
Phil
Re: 102 build hum at startup
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.Chris Brown wrote: ↑Sat Oct 15, 2022 8:15 pm Seems like something in the output section... It almost sounds like a really loud transformer hum. Also, the preamp voltages seem high.
Thanks for the ideas...
Phil
- martin manning
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Re: 102 build hum at startup
OT secondary grounded?
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Re: 102 build hum at startup
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.
When they are not connected correctly (in series) the choke could be the cause of the hum you hear without the tubes in.
Re: 102 build hum at startup
Hi Richard,Richard1001 wrote: ↑Sun Oct 16, 2022 11:38 am 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.
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.
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Phil
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Re: 102 build hum at startup
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,
Cheers,
Phil
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Re: 102 build hum at startup
Bad cap out the gate from F&T?! no way ! must have been a repackaged Illinois cap !philbard wrote: ↑Sun Oct 16, 2022 2:48 pm 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,
Charlie
Re: 102 build hum at startup
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.WhopperPlate wrote: ↑Sun Oct 16, 2022 3:20 pmBad cap out the gate from F&T?! no way ! must have been a repackaged Illinois cap !philbard wrote: ↑Sun Oct 16, 2022 2:48 pm 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,
Phil