First build being strange

Arthur Dent · 12-29-2006, 06:55 PM

I just finished my first build, an Ampeg-ish circuit with two preamps with a 12ax7 each, another 12ax7 for the PI, and a quad of kt88's. For the debug process I only have two tubes in the power section running into an 8-ohm speaker.
The power supply is working, all the voltages are right, the B+ is 450 volts. The amp is star grounded.
No signal is getting through. It hums at a medium volume, and it stops making any noise at all when my bass is plugged into it. If I touch the tip of the cable it makes the hum slightly quieter. For some reason, ungrounding the ground side of one of the input jacks also stops the hum. This only happens with one of the two inputs on the first preamp. Whatever I do with the jacks, I cannot get a signal through.
Any suggestions would be greatly appreciated.

stokes · 12-29-2006, 07:20 PM

Are you using shorting jacks on the inputs?Sounds like the input jacks are wired wrong,your signal is likely getting grounded here.Why are you only running 2 power tubes to de-bug it,as you say?Did you adjust your bias for 2 tubes?

Arthur Dent · 12-29-2006, 07:50 PM

I am indeed using shorting jacks. I'll check my wiring on those again. Thanks for the reply.

I'm not actually sure if I'll end up using all four of those monstrous tubes. The bias hasn't realy been adjusted too extensively yet. I have it around -30 volts for now. I'm just trying to get some signal to the speaker before I start getting it to sound right.

Arthur Dent · 12-29-2006, 08:27 PM

Well, I checked the wiring on the jacks. The signal is not grounded. Still no sound out of the amp. I removed the jack with the hum-inducing ground side and disconnected the shorting thingy on the other jacks, so they are now non-shorting jacks. This made absolutely no difference in the function of the amp.

stokes · 12-29-2006, 09:05 PM

Then you need to trace your signal and see where it is getting lost.

Enzo · 12-30-2006, 04:56 AM

Pull the wires off the input jacks. With the ground wire and the hot wire exposed and hanging in mid air, use your finger and touch the end of the hot wire only. Does hum come out the amp speaker?

Arthur Dent · 12-30-2006, 08:59 PM

Nothing other than the faint, hardly-annoying hum that occurs otherwise.

If it's at all helpful, when I turn the B+ on, the amp hums loudly for a few seconds, then quiets down.

stokes · 12-31-2006, 02:30 AM

When you flip the standby on,the hum you hear is likely caused by the caps charging up,once they are fully charged the hum is gone.You are going to have to do some signal tracing to find out where you are losing the signal in the circuit.

Andrei_T · 01-23-2007, 08:15 PM

Maybe it's an oscillation problem. Are you using negative feedback? Have you tried moving wires around?

12-29-2006, 06:55 PM	#1
Arthur Dent Member Join Date: Oct 2006 Posts: 66	First build being strange I just finished my first build, an Ampeg-ish circuit with two preamps with a 12ax7 each, another 12ax7 for the PI, and a quad of kt88's. For the debug process I only have two tubes in the power section running into an 8-ohm speaker. The power supply is working, all the voltages are right, the B+ is 450 volts. The amp is star grounded. No signal is getting through. It hums at a medium volume, and it stops making any noise at all when my bass is plugged into it. If I touch the tip of the cable it makes the hum slightly quieter. For some reason, ungrounding the ground side of one of the input jacks also stops the hum. This only happens with one of the two inputs on the first preamp. Whatever I do with the jacks, I cannot get a signal through. Any suggestions would be greatly appreciated.

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12-29-2006, 07:20 PM	#2
stokes Old Timer Join Date: May 2006 Location: NYC Posts: 1,306	Are you using shorting jacks on the inputs?Sounds like the input jacks are wired wrong,your signal is likely getting grounded here.Why are you only running 2 power tubes to de-bug it,as you say?Did you adjust your bias for 2 tubes?

12-29-2006, 07:50 PM	#3
Arthur Dent Member Join Date: Oct 2006 Posts: 66	I am indeed using shorting jacks. I'll check my wiring on those again. Thanks for the reply. I'm not actually sure if I'll end up using all four of those monstrous tubes. The bias hasn't realy been adjusted too extensively yet. I have it around -30 volts for now. I'm just trying to get some signal to the speaker before I start getting it to sound right.

12-29-2006, 08:27 PM	#4
Arthur Dent Member Join Date: Oct 2006 Posts: 66	Well, I checked the wiring on the jacks. The signal is not grounded. Still no sound out of the amp. I removed the jack with the hum-inducing ground side and disconnected the shorting thingy on the other jacks, so they are now non-shorting jacks. This made absolutely no difference in the function of the amp.

12-29-2006, 09:05 PM	#5
stokes Old Timer Join Date: May 2006 Location: NYC Posts: 1,306	Then you need to trace your signal and see where it is getting lost.

12-30-2006, 04:56 AM	#6
Enzo Senior Member Join Date: May 2006 Location: Lansing, Michigan, USA Posts: 10,366	Pull the wires off the input jacks. With the ground wire and the hot wire exposed and hanging in mid air, use your finger and touch the end of the hot wire only. Does hum come out the amp speaker?

12-30-2006, 08:59 PM	#7
Arthur Dent Member Join Date: Oct 2006 Posts: 66	Nothing other than the faint, hardly-annoying hum that occurs otherwise. If it's at all helpful, when I turn the B+ on, the amp hums loudly for a few seconds, then quiets down.

12-31-2006, 02:30 AM	#8
stokes Old Timer Join Date: May 2006 Location: NYC Posts: 1,306	When you flip the standby on,the hum you hear is likely caused by the caps charging up,once they are fully charged the hum is gone.You are going to have to do some signal tracing to find out where you are losing the signal in the circuit.

01-23-2007, 08:15 PM	#9
Andrei_T Member Join Date: Jun 2006 Posts: 31	Maybe it's an oscillation problem. Are you using negative feedback? Have you tried moving wires around?

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