So I powered up the 2nd Gen Model T after mods, and Bias , Noise ?

[gtrplayr1976]

Ok...so I've powered the Model T after mods.  It has a lot of hum (at least to my ears)even with controls at zero, and no input.  I'm getting nearly 550 plate v on the bias probe, and have the bias at around 37ma on one tube ,and 28 on the other. This could be that the tubes are used. Too expensive to buy a new set just to test the amp.  I'm also seeing a negative voltage reading on the plate with the standby switch off. -022 . My understanding is you can adjust the hum with the controls at 10 , but that hum gets really loud. I have it plugged into a 1x12 with a 8 ohm V30 speaker.  I don't want to ruin the speaker.
Any thoughts ,or suggestions ?

Thanks

[Isaac]

Have you tried the hum balance control?

[gtrplayr1976]

Have you tried the hum balance control?

Yes sir I did.  I have all the preamp tubes out ,and still have the loud hum.  I don't know if it means anything , but I checked the resistance on the speaker outputs ,and they both read zero to ground, and from - to + ?   Could there be an issues with the impedance selector ?

[gtrplayr1976]

I think I've found the source of the hum.  Found one of the bias caps ungrounded.  I will check this further this evening when I get home.

[gtrplayr1976]

Ok..that was a small part of the problem...I now have both positive ends of the bias caps ground, but I still have some hum ,and when I turn the volume (s) up I get a really loud hum / buzz...the normal volume has sort of a phase shift sound...not prominent , but there.  I'm seeing a negative plate voltage (-22) with the standby off also.  Hum balance does nothing. 

[loudthud]

There are 47 or 50 ohm 5 watt resistors to each power tube plate and 1K 5 watt resistors to each power tube screen grid. Check every one of them for correct resistance.

[gtrplayr1976]

All resistors check ok.  thanks for the input.

[LPB]

 Most amp techs/repair shops should be able to test your power tubes very quickly at minimal expense. It seems doubtful that's the problem but it sounds like you want to be able to rule it out. Either way it's never a bad idea to make friends with the better(best)amp tech(s) in your area, if you gig with tube amps chances are you may need their services at some point down the line anyway in some capacity, whether it be actual repair, parts, or just advise. 

[gtrplayr1976]

We just don't have many amp techs around ,and what we do have aren't dependable.   I'm actually doing this for a friend , so I 'm trying to keep costs down to a minimum.  Thanks for the input.

[jaywalker]

http://www.geofex.com/ampdbug/ampdebug.htm see "hum"  you say hum gets louder when volume knob is turned up? check pre-tubes for starters (I re-read and see that it hummed with pre's out)

Also, in the picture from your earlier post I see the original two big brown coupling caps (.25uF ?) attached to two of the power tube sockets (I presume they're tied to an unused pin on the socket as a convenient connection point- see schematic). I replaced mine with orange drops 'cause they were 35 years old and od's are cheap.

[gtrplayr1976]

I found the issue.  dual cap was wired wrong.  Didn't know you could use ODs in place of electrolytics ....hmm......and OMG !!! if this doesn't crap out, it sounds GREAT ....I have some finishing touches, and cleanup, but this this is a monster.  I have to see if my bud will sell it to me 

[Isaac]

ODs as in Orange Drops? Aren't those pretty small (as in, low capacitance value) for the application?

[gtrplayr1976]

ODs as in Orange Drops? Aren't those pretty small (as in, low capacitance value) for the application?

I thought so too....the originals are only 50uf ,150V , but I've not seen an orange drop that uf.

[jaywalker]

ODs as in Orange Drops? Aren't those pretty small (as in, low capacitance value) for the application?

I thought so too....the originals are only 50uf ,150V , but I've not seen an orange drop that uf.

Hmmm??? .25 is called for on the schematic for the second gen. aka "model T super" and first gen. Coupling caps between PI and power tube grid. I used .22/ 600 OD's. What am I looking at?

OK now I see what you think I thought I said but meant something entirely different. Ah communication on the net   I sould have been more specific. Replacement probably isn't really necessary but I like to have that section solid.