Non-HRM Build Squeals when on...

[v00d00blues79]

I've finished wiring up my latest non-hrm build and this little bugger has no sound, but it does have a low squeal to it when the volume gets cranked a bit. Just curious on where i should start on the debug for this...


Thanks,

Andy

[keithrick]

I would double check all your wiring...obviously, but if you used any shielded cable make sure you are not grounding out your signal!

I am sure others will have more specific advice. What layout did you use? Good luck!

[v00d00blues79]

Yeah... i used some shielded cable. Ill have to check to see if the signal is getting grounded anywhere.


Thanks,

Andy

[Bob-I]

I'd disconnect the NFB. If it's wired backwards, meaning the leads to the OT are reversed you'll get this. If the squeel goes away reverse the OT leads to the output tubes. Then you can start troubleshooting the rest.

[v00d00blues79]

I gave the phase reversal on the PI a try and it was not it. I did notice though some very strange voltages when i checked it out.

Here they are:

V1:
Pin 1 - 196
Pin 3 - 1.79
Pin 6 - 140 <-- does not look right to me at all...
Pin 8 - .97

V2:
Pin 1 - 257
Pin 3 - 2.18
Pin 6 - 351 <-- another out of wack voltage...
Pin 8 - 0 <-- thinking this is the cause of my V2 problems.

V3:
Pin 1 - 291
Pin 2 - 38
Pin 3 - 59
Pin 6 - 288
Pin 7 - 38
Pin 8 - 59

V4-7:
Pin 3 - 444
Pin 4 - 438
Pin 5 - -37

I re-flowed the connections for V2 hoping that would correct it, but no luck. I also tried a couple of different tubes to see if that would help, but again no luck. Could it be possible that the 5uF cap i have in there is bad?

I also have a 150k hanging of of the B+5 to ground for the FET simulation.

Thanks,

Andy

[Bob-I]

Here they are:

V1:
Pin 1 - 196
Pin 3 - 1.79
Pin 6 - 140 <-- does not look right to me at all...
Pin 8 - .97

Not way outta wack.

V2:
Pin 1 - 257
Pin 3 - 2.18
Pin 6 - 351 <-- another out of wack voltage...
Pin 8 - 0 <-- thinking this is the cause of my V2 problems.

This one you gotta figure out. I'd start by removing the bypass cap and just run with a cathode resistor. If the voltage is still off you might have a bad tube socket since you already tried the tube.

V3:
Pin 1 - 291
Pin 2 - 38
Pin 3 - 59
Pin 6 - 288
Pin 7 - 38
Pin 8 - 59

V4-7:
Pin 3 - 444
Pin 4 - 438
Pin 5 - -37

I re-flowed the connections for V2 hoping that would correct it, but no luck. I also tried a couple of different tubes to see if that would help, but again no luck. Could it be possible that the 5uF cap i have in there is bad?

I also have a 150k hanging of of the B+5 to ground for the FET simulation.

Thanks,

Andy

The rest look ok.

One troubleshooting method I like to use it to check the signal stage by stage.

I built a set of test leads with a 1/4" jack, a .047uF 600V cap and a pair of clip leads. Connect one clip lead to the cap and then to the HOT lead on the plug, connect the other clip lead to the ground.

Now connect the plug to another amp, even a stereo is fine. Connect the ground clip to the chassis.

Fire up the amp to be tested and plug in a sound source. You can use a signal generator if you have one, a CD player or a guitar. The HOT lead will allow you to test if the signal is getting through.

1) Connect the HOT to the input jack and make sure you can hear sound from the other amp.

2) Connect to the first plate of V1a, pin 1. You should get a louder sound.

3) Connect to the GRID of V1b, pin 7. This will allow you to see if signal is passing through the tone stack. You can also test tone stack function this way.

Continue through the signal chain, even listening to the sides of the PI.

I hope that helps.

[v00d00blues79]

I figured out what i had wrong on V2... the 100k input trimmer wasn't hooked up on the ground side. I'll test out your suggestions on the signal testing.


Thanks,

Andy

[v00d00blues79]

Its interesting though that when i switch the amp to the OD side it gets a sound like a shortwave radio being tuned to a frequency... its most interesting...


Thanks,

Andy

[dogears]

That is way out.... You should have no more than several volts different than V1a. A mismatched tube may give 7 or 8 volts different. Not 50-60v Something is very wrong.

Pin 6 - 140 <-- does not look right to me at all...
Pin 8 - .97

[keithrick]

If you can post some pic's of your build, fresh eyes sometimes help!

[v00d00blues79]

I'll try and post some pictures as soon as i can.

Dogears, I kinda figured something was up. I'm not sure though were to start on that one...

I'll tell ya, these Dumbles are a bugger to get going, or i'm just a bit out of sync with them...


Thanks,

Andy

[Normster]

V1:
Pin 1 - 196
Pin 3 - 1.79
Pin 6 - 140 <-- does not look right to me at all...
Pin 8 - .97

Looks like you may have your plate resistors reversed.

V2:
Pin 1 - 257
Pin 3 - 2.18
Pin 6 - 351 <-- another out of wack voltage...
Pin 8 - 0 <-- thinking this is the cause of my V2 problems.

I'm thinking a wiring or tube socket problem. If it was a shorted cap, I think your plate voltage would be lower. Make sure the sleeve in pin 6 is actually making contact with the tube pin. That would also explain the high voltage on pin 1 since you're only drawing current through half the circuit.

Edit: Never mind. I just saw your post about fixing V2.

[v00d00blues79]

I've swapped the plate resistors and the voltages are still a little off, but its just in tweaking the values to get them in range. I'm still trying to get some pictures of the inside of the chassis to see if anyone can spot something...

Thanks,

Andy