Starting a 2204 build

  • Thread starter Thread starter ledvedder
  • Start date Start date
glpg80 said:
The resistance to ground isn’t the cause of the buzz, it’s the loops of currents within the grounds that cause a problem. That’s why myself on the headfirst page and everyone else here has been talking about grounding - it matters a lot not only where grounds are attached but also how you can reduce or at least minimize ground loops of current.

Another source of buzz is wire location. Have you chopsticked each of the preamp tube wires to see what changes the buzz? Is the buzz 60Hz or 120Hz?
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I'm pretty sure this is 60Hz? I have chopsticked and moved around each of the wires and nothing seems to change it. Also, the gain knob scratches a bit when adjusting it.

 
Make sure to check for hum/buzz without anything plugged into amp's input.
 
Definitely louder with guitar plugged in. Is there anything in your room that's causing RF/EMI? Florescent lights? Neon signs? A transformer? Fan? TV? Something's definitely up with that gain control though. Might help track the source of noise to V1/V2.
 
Scratchy gain knob means you have DC on it. What’s the DC voltage on the gain knob coming from the previous stage coupling capacitor? Measure both sides of it - you should have B+ - Ibias*100k on one side and 0V DC on the other.

Put a large sheet of metal over the chassis and see if the hum goes away.
 
FourT6and2 said:
Definitely louder with guitar plugged in. Is there anything in your room that's causing RF/EMI? Florescent lights? Neon signs? A transformer? Fan? TV? Something's definitely up with that gain control though. Might help track the source of noise to V1/V2.
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There are florescent lights in my workshop, but the hum stayed the same when I shut them off.
 
Go back to that connection of the board mounted filter cap. It looks wrong. Look at the diagram @FourT6and2 posted in post #37 on page 2 of this thread. That's the way it should be. That red wire from PI plate resistors needs to go to a 10k resistor (no cap) first, then the other side of the 10k gets the cap and another 10k to the last cap lead.

EDIT: Also, the lead that goes to V2 pin 6 needs to be connected to the end of that 10k resistor that is connected to the cap.
 
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SpiderWars said:
Go back to that connection of the board mounted filter cap. It looks wrong. Look at the diagram @FourT6and2 posted in post #37 on page 2 of this thread. That's the way it should be. That red wire from PI plate resistors needs to go to a 10k resistor (no cap) first, then the other side of the 10k gets the cap and another 10k to the last cap lead.

EDIT: Also, the lead that goes to V2 pin 6 needs to be connected to the end of that 10k resistor that is connected to the cap.
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It looks wonky in the photo but I think he just has the connections mirrored/reversed to the other side of the cap?
 
Also please verify how you have the other two filter caps connected. Which layout diagram did you wind up using? Because I see some potential issues if you combined different layouts.
 
FourT6and2 said:
Also please verify how you have the other two filter caps connected. Which layout diagram did you wind up using? Because I see some potential issues if you combined different layouts.
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I used this layout for the power section. But I only have one dual preamp cap, so that's wired a bit different.
 
SpiderWars said:
Go back to that connection of the board mounted filter cap. It looks wrong. Look at the diagram @FourT6and2 posted in post #37 on page 2 of this thread. That's the way it should be. That red wire from PI plate resistors needs to go to a 10k resistor (no cap) first, then the other side of the 10k gets the cap and another 10k to the last cap lead.

EDIT: Also, the lead that goes to V2 pin 6 needs to be connected to the end of that 10k resistor that is connected to the cap.
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This is the preamp filter cap.
20220823_140718_copy_4000x3000.jpg
Screenshot_20220828_002225_Gallery_copy_1080x2340.jpg
 
glpg80 said:
Scratchy gain knob means you have DC on it. What’s the DC voltage on the gain knob coming from the previous stage coupling capacitor? Measure both sides of it - you should have B+ - Ibias*100k on one side and 0V DC on the other.

Put a large sheet of metal over the chassis and see if the hum goes away.
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There's 206V on one side and 6mV on the other side.
 
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glpg80 said:
6mV is on the opposite outer leg of the pregain pot and not the wiper?

Check the ground for that pregain pot.
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Oh, I need to measure on the gain pot? I measured both sides of the 2nd 0.022uF cap.
 
ledvedder said:
Oh, I need to measure on the gain pot? I measured both sides of the 2nd 0.022uF cap.
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Nah you’re good. Still verify the pregain pot isn’t floating and is grounded properly.

If you’re getting 6mV on the other side of that cap then I’ll let the others chime in, but I think that cap is bad and needs replaced. I think you have a leaky coupling cap putting DC on the pregain pot. 6mV is small but depending on your meter quality it could point to a component problem or percent error on the meter.

Even then, if it takes your meter a second it should still be 0V DC on one side and roughly 200V on the other.
 
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How do I test the filter caps? Everything I've researched about 120Hz hum points to bad filtering.
 
ledvedder said:
How do I test the filter caps? Everything I've researched about 120Hz hum points to bad filtering.
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In circuit?

Have you reflowed everything yet?

You need look at your ripple voltage on top of your DC voltage and see if it’s within reason. To do this you’ll need an oscope, a very large value DC blocking cap (exact electrical value doesn’t matter, it needs to be rated at 1kv or more) where you connect your probe to one side and the other side of the cap becomes your circuit probe.

To test it out of circuit turn it off unplug the amp and drain the caps As normal. Lift the positive side and measure the ESR. They should all be almost dead shorts. A bad cap will have a larger ESR than the others and also possibly show signs of overheating such as buldging or deformation.

Have you put a metal cover over the chassis yet? I’m dead serious about this. You may be chasing your tail.
 
Please provide voltages for every tube/pin. V1-1, 2, 3, 4, 5, 6, 7, 8, 9; V2-1, 2, 3, etc.
And voltages for every B+ node. Each filter cap lug, PI node, preamp cap nodes, and V1 node before plate resistors.
 
Ok, here's some voltages that I got.

V1 pins 1-9 -
238v
1.2mv
2.6v
-1.8mv
-1.8mv
203v
-0.4mv
1.7
0.4mv

V2 pins 1-9 -
155v
4.7mv
1v
-1.8mv
0.5mv
276v
157v
-1.8mv

V3 pins 1-9 -
216v
1.5mv
34.6v
-1.6mv
-0.5mv
201v
0.7mv
34.6v
-1.6mv

Power filter caps -
442v
442v
312v
440v

Preamp filter cap -
267v
275v

V1 node -
266v

B+ onto turret board -
441 v

B+ after 1st dropping resister (22k) and at PI node -
312v
 
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