SUNN Model T 2nd Gen Power up ?

[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 ? Below is the link for the mods I did. I also replaced the bias caps ,and cans.
http://www.angelfire.com/blog/sunnmodelt/index.html

Thanks all !

[Gtr_tech]

Didn't put a cap in backwards did you?

I'd remove all the preamp tubes and see if the hum goes away. If it does, the output section/plate supply is doing its job. Next put in the phase inverter tube and see what happens. Keep "working backwards" inserting preamp tubes until you get the hum. Then you've localized it, now you can find out where its originating and whats causing it.

[gtrplayr1976]

Hmm...I don't think so, but ya never know. I'll perform the test you described also. Thanks Gtr_tech

[gtrplayr1976]

Ok..preamp tubes out...powered up ...still have a loud hum

[Gtr_tech]

Check the bias voltage at pin 5 of the output tube sockets. Set yer meter for AC volts. You should show *very* low readings....maybe just a few millivolts AC. Any more than 50mv is a problem and a potential source of hum. Either a cap is bad, installed backwards (bias is a *negative* voltage and the cap(s) should be connected so the positive terminal is at ground). If the caps are properly connected and you have more than normal AC ripple here, replace the diode.....it is either leaky or possibly shorted.

Don't rule out unknown condition output tubes. A failed/failing output tube can cause hum in the output. Got any to sub in?

[gtrplayr1976]

Ok..I'll do those checks. The bias caps were originally 50uf 150V, but I replaced them with 47uf 450V...it's what I could find. Will that make a difference ?

[Gtr_tech]

Nope.....as long as they're installed with correct polarity it won't make a lick-a difference.

[gtrplayr1976]

Looks like 1.1 VAC on pin 5 ,. The positive ends of bias caps are connected to the power tube socket.

[Jazz P Bass]

That is way too much ripple voltage.
Remove the output tubes & see what it reads.
It may be a leaky diode.

[Gtr_tech]

http://sunn.ampage.org/site/schematics/modelt.gif
You have them in reversed then. The cap is connected to negative voltage, So you need to reverse the polarity. You may have damaged them operating rev polarity, but just flip 'em around and see if it still hums. If it does, you ruined the caps and will need to replace them.

[gtrplayr1976]

http://sunn.ampage.org/site/schematics/modelt.gif
You have them in reversed then. The cap is connected to negative voltage, So you need to reverse the polarity. You may have damaged them operating rev polarity, but just flip 'em around and see if it still hums. If it does, you ruined the caps and will need to replace them.

DODO !!....so the negative end goes to the tube socket ?

[gtrplayr1976]

I've added an attachment of the original bias cap install. It looks like the positive end is connected at the tube socket. I've also attached a pic with new caps installed in the same direction. So...I think they're right ?

[gtrplayr1976]

That is way too much ripple voltage.
Remove the output tubes & see what it reads.
It may be a leaky diode.

Jazz, I pulled the tubes, and am reading 0.2 on two sockets, and 0.3 on the other two. I also noticed that with just the ground lead connected to the chassis it was reading 1.5 V ?

[Gtr_tech]

The socket is just used as a tie point, so that is the gnd connection. Both pos cap leads should show 0 ohms resistance to chassis gnd. Be sure both are grounded. So it appears they are in correctly. You threw me when you said "to the tube socket" which to me means tied to the bias feed resistors connected to pin 5.

[gtrplayr1976]

The socket is just used as a tie point, so that is the gnd connection. Both pos cap leads should show 0 ohms resistance to chassis gnd. Be sure both are grounded. So it appears they are in correctly. You threw me when you said "to the tube socket" which to me means tied to the bias feed resistors connected to pin 5.

Sorry. One cap reads 0, the other open....although I do not have the tubes in either.