Fender Hot Rod Deville Amp

[TwinBeech]

I have a Fender DeVille with 4x10's in it that has new tubes but is distorted. I can take the output at the pre Amp Out plug and it is clean. Going into the Power Amp in and the normal plug and the guitar is distorted. I have put a 500mv 1000Hz sine wave into the amp and it sounds clean and looks clean on the scope.

Does this make sense? I am missing something in trying to troubleshoot this amp.

Any tips are really appreciated.
I am attaching the schematic.

[rf7]

Try a different speaker cabinet, if you can. How does the power amp output look on the scope? Do you have a Dummy resistor?

[TwinBeech]

I plugged it into a different amp speakers with the same result. The power amp output looks the same as the Pre Amp out just higher voltages with the sine wave signal input. I don't have a dummy load.

[Randall]

New tubes doesn't mean there isn't a problem with one or both. Have you tried swapping in a different set? Also, the PI tube is after the power amp in jack, so try that one as well.

[bob p]

I'm not sure whether or not you did this, but have you tried injecting/tracing signal throughout every stage of the amp to determine which stage is introducing the signal distortion? Signal tracing should make it obvious exactly which stage the problem is coming from.

It might be a helpful exercise to create a voltage chart for all of the tubes, and plot the voltages for comparison to the schematic's reference voltages. That makes it pretty easy to recognize when a particular tube isn't biased correctly, when it's operating on the wrong load line, etc. Noticing an aberrant load line helps to identify exactly which component in which stage you should be looking at.

+1 on getting a dummy load resistor. Your ears will thank you.

[TwinBeech]

I looked again at the pin 5 grid inputs to the power tubes and they both look clean. Then on the plates pins 3 and the signal is distorted and weak by about a factor of 4. I guess I need to find something that can effect both plate circuits in common. I can see R61 is burned so I will go into it tomorrow to see what else I can find.

Thanks for the tips.

[mac dillard]

would be a good time to check the OT

[Chuck H]

I plugged it into a different amp speakers with the same result. The power amp output looks the same as the Pre Amp out just higher voltages with the sine wave signal input. I don't have a dummy load.

So can we assume that you left the speaker connected for testing? If so then did it actually sound distorted when the scope trace showed an undistorted sine wave? Because having tried the amp with another cabinet that wouldn't make any sense unless BOTH speaker loads were bad and I would sooner suspect a flaw in the testing procedure or reporting.

DO get a dummy load. It's not very expensive and testing amps at full volume through a speaker bugs everyone.

[Jazz P Bass]

I can see R61 is burned so I will go into it tomorrow to see what else I can find..

Is R61 (screen resistor) open?

If it is, there you go.
No screen resistor, no output on that tube.

[Chuck H]

Is R61 (screen resistor) open?

If it is, there you go.
No screen resistor, no output on that tube.

And shouldn't that show on a scope trace? Of course you're probably right, but why can't twinbeech see it? I was shooting for some proper testing to form a diagnosis on. Smarter of you to just interpret what information WAS available.

[TwinBeech]

So can we assume that you left the speaker connected for testing? If so then did it actually sound distorted when the scope trace showed an undistorted sine wave? Because having tried the amp with another cabinet that wouldn't make any sense unless BOTH speaker loads were bad and I would sooner suspect a flaw in the testing procedure or reporting.

DO get a dummy load. It's not very expensive and testing amps at full volume through a speaker bugs everyone.

The distortion is heard at low levels of volume. I did not look at the guitar audio on the scope; just the sine wave from the signal generator. I used the other cabinet to eliminate the speakers from the problem. I will replace the burned resistor and see what happens then.

[TwinBeech]

The problem showed up on the scope with the sine wave injected.

"I looked again at the pin 5 grid inputs to the power tubes and they both look clean. Then on the plates pins 3 and the signal is distorted and weak by about a factor of 4. I guess I need to find something that can effect both plate circuits in common. I can see R61 is burned so I will go into it tomorrow to see what else I can find."

[Chuck H]

It's doubtful the resistor is the problem so just replacing it probably won't fix the amp. As components go they don't often just fail because they are somehow inferior to the job. Much more likely some anomalous circuit behavior caused the resistor to blow. Almost certainly a bad power tube/s. Whenever power tubes fail it's a good idea to check the bias supply as well.

[TwinBeech]

I have changed the 6L6 power tubes to new ones. The old tubes had their plastic keyway bases broken off and had been installed with the pins incorrectly lined up. That may have caused the resistor to blow. As soon as I have it opened up I can look for more damage. I have never seen tubes broken like this.

[Justin Thomas]

I have never seen tubes broken like this.

I have, lots of times. Sometimes when a novice tube amp owner thinks the tubes screw in like light bulbs... other times, as the tubes age and cook, the plastic bases become brittle. Any stress on that base pin, they snap right off. Nearly every Svetlana (=C=) I've had that was used had this issue. But, they're still worth it. The bad part is, many tubes innards are symmetrical along one axis, so you can plug it in 180° wrong and it will still LOOK right. I also had a even t experience with an inexpensive new tube socket that was made of such soft plastic that the tube will easily go into the socket ANY way; the "right way" is just easiEST.

I'll occasionally draw a line onto the glass, on the base, and on the chassis to make sure the tube lines up. Course,that may not work with the next one... But I'm not paying $10/each for broken pin savers!

Yes, putting wacky-high voltages on certain tube elements can certainly make for all kinds of fun... when your 6L6 lights like a 100W bulb, you probably have it in wrong...

Justin