Blocking Distortion or mis-biased tube???

[ChrisM]

All voltages look fine. No leaky caps.

I'm pounding my head here!!

[LeftyStrat]

Do you have another preamp you can run into the effects return? This would at least determine if the problem was in the power amp or preamp.

Alternately run the preamp out to another power amp.

[ChrisM]

Do you have another preamp you can run into the effects return? This would at least determine if the problem was in the power amp or preamp.

Alternately run the preamp out to another power amp.

Tried this with an external preamp and the problem still exists. PI or output stage issue.

[LeftyStrat]

Tried this with an external preamp and the problem still exists. PI or output stage issue.

Okay, just throwing out some random things to try. It sounds like only one side of the PI is working properly. Maybe try disconnecting the negative feedback. Perhaps there is something wrong there.

Maybe measure AC volts after the PI coupling caps. Might check for DC volts in the same place to see if a cap is leaking.

[ChrisM]

Tried this with an external preamp and the problem still exists. PI or output stage issue.

Okay, just throwing out some random things to try. It sounds like only one side of the PI is working properly. Maybe try disconnecting the negative feedback. Perhaps there is something wrong there.

Maybe measure AC volts after the PI coupling caps. Might check for DC volts in the same place to see if a cap is leaking.

NFB is disconnected, did that a while ago.

I've tried 3 different sets of 100nF PI couplers. All 3 sets do the same thing. With the 220K/grid side disconnected the caps read 50V or so then drop to 0.5V then jump around from 0.5V to -0.3V. Very odd.

Does anyone have dumble PI voltages?

[Reeltarded]

Dood, it's a little like my last build. Stuff was all over the place and bias measurment was impossible though.

What is your output bias at again? Mine was in total cutoff until I swapped a 33k in place of the 56k range. Totally out to lunch and ugly.

[ChrisM]

Bias voltage is fine. I think the grid's are at -47V.

Tube's biased at 43mA at 430V on the plates.

[ChrisM]

Anyone have voltages for the PI in a Dumble amp?

[ChrisM]

1.8V dropped across the 820R base resistor in the PI. That's ~2mA flowing there. Isn't that a little high for the PI current draw?

Also increased grid stoppers to 470K. No change.

[ChrisM]

Anyone have voltages for the PI in a Dumble amp?

[Firestorm]

Anyone have voltages for the PI in a Dumble amp?

I know we obsess on voltages here, but a few volts either way don't matter. Big volt changes alter gain in a power amp, but not so much in a preamp. Does affect the PI, but if you're not off the charts there, no problem. Something else.

[ChrisM]

FML

[img1415]https://www.tube-town.net/ttstore/image ... 2204_1.jpg[/img]

White and Red primary wires reversed.

But to my credit, like wtf Hammond who makes a centre tap white instead of the usual red!

[cbass]

Glad you got it fixed.Yea I probaly would have assumed the red is the CT

[Reeltarded]

What's a CT?



YAY!

[cbass]

What's a CT?



YAY!

Crushed Testacle