Rocket Build...noise issue.

[MGW]

SO...this is my first wreck build. I've done several though. This is something new. I'm seeing about 185 mV on V1 pin 7. The DC is making the volume pot on the guitar sound scratchy...kinda like a zvex box of rock. How the crap do I have voltage reading on the grid? Everything else with the amp is dead quiet. Dimed out it's quiet...till you play something Sounds great, but I have to get this quiet or it's gonna be an issue. The amp is to be used in church.

Thanks

[JazzGuitarGimp]

Have you tried a different tube at V1?

What are the idle voltages at V1, pins 6 and 7 ?

Do you have a VVR in this build?

[MGW]

Tried a different tube, same deal...DC on grid. No VVR. With the the input ungrounded, pin 6 is about 90 volts, pin 7 is 184 mV. If I let the input jack short to ground, the mV on pin 7 drops to about 11mV

[JazzGuitarGimp]

The cathode voltage is really low as compared to the stated voltage in the chart in the files section. The chart says 0.58V on the cathode. The plate voltage is pretty close at 90V vs 96V. I would check the resistor and capacitor in the cathode circuit. With the power off and the power supply caps discharged, check the resistance from the cathode to GND. Should be 1.5K. Also check the resistance of the 220K resistor in the plate circuit.

[MGW]

Plate 90.8 V plate resistor 217.6K
Grid -184.2 V (yes, negative) grid resistor 32.9K
Cathode .69 V cathode resistor 1.49K

B+5 is right at 183V before R4(220K). I'm stumped.

[MGW]

Here's the whole voltage chart.

[matt h]

(deleted)

[MGW]

No runaway blob of solder. Grid to ground is 1M. With tube in and out, there is 1.02M between the grid and cathode. Which to me seems about right with all 1M, 33K, and 1K5. The 33K isn't quite 33K. It's wired up just like the layout. V2 is only the CF.

[MGW]

Here's a gut shot of the area.

[MGW]

I added a cap in series between and the 33k and input jack. That keeps it off of the pot, but the voltage on the plate went up to about 125v, and the cathode is down to about .48v. So, the cathode seems a bit better, but the plate took quite the jump.

[Tillydog]

I don't see the 1M resistor on the input jack (R1). Is it hiding ?

I added a cap in series between and the 33k and input jack. That keeps it off of the pot, but the voltage on the plate went up to about 125v, and the cathode is down to about .48v. So, the cathode seems a bit better, but the plate took quite the jump.

If you added the cap, you should also add a grid leak resistor from the grid side of the cap to ground or the bias will go way off due to grid leak current biasing V1 (as would have appeared to have happened).

Do you have the connection between chassis and the B+ ground?

[JazzGuitarGimp]

I don't see the 1M resistor from the guitar input to GND. Is it there? I see that you are measuring about 1M, so obviously, it's there. Is the 1M resistor on the input jack side of the new cap, or the GND side?

[MGW]

Input jack is hiding it.

How do you determine the appropriate value for a grid leak resistor?

Connection between chassis and B+ ground? the filter caps are grounded at a lug on the recitifier socket. Is that what you mean? or am I completely missing something obvious?

[JazzGuitarGimp]

The grid leak resistor winds up in parallel with the guitar's output impedance, which is anywhere from zero (with the guitar's volume pot all the way down) to about 125K (with the guitar's volume pot centered) (assuming the guitar's volume pot has a value of 500k) to whatever the DC resistance of the pickup is (with the guitar volume all the way up) (likely in the area of 7K to 12K). So in practice, the value of the grid leak resistor is moot. This assumes that the input is not AC-coupled to the grid. If you are going to AC-couple the input, the 1M grid leak resistor really should be on the grid side of the capacitor.

One other shot in the dark (as I am stumped too): does your guitar have active pickups? If so, do you have the cable plugged all the way into the guitar's output jack? I have a strat with active EMG pups, and when the cable isn't inserted all the way, I get the scratchy volume symptom you have.

[MGW]

Not active pickups.

If I understand what you are saying about AC coupled input, I need to remove the 1 meg from the jack and add it to the grid side of the cap to ground. Is that correct?