rattling from amp?

[bal704]

I'm in the middle of another new build. This one is basically a 5E3, with a 5879 tube in the preamp rather than a 12AX7.

A few differences:
I'm using a choke instead of a resistor in the PS
Diode rectifier
My PS caps are 32/16/16 rather than the 16/16/16 on the Fender version.
I added a 470 ohm resistor to pin 4 of each power tube.

It sounds good at what I call 'loud bedroom' volume. However, when I push it more, I get a sound that sounds like something loose in the speaker cabinet that rides on top of the note.

I initially thought it was some kind of sympathetic vibration from the amp cab sitting on top of the speaker cab, so I moved the amp onto a carpeted surface a few feet away from the speaker. Didn't help. Tried a different cab. No help. Swapped all tubes. No help. Reflowed all connections. No help.

To make sure it wasn't something in the cabs or room vibrating, I tried both cabs with different amps, and they didn't have the 'rattling' issue at all, at even louder volumes.

So...I've heard of blocking distortion and other types of issues, but I don't know what they sound like. I'm pretty sure the sound I'm hearing is coming from my amp, and not a mechanical vibration from speakers or other items in the room. Are there any ELECTRICAL causes for the sounds I'm hearing?

[Phil_S]

Amateur opinion here. Your 5879 has gobs more gain than a 12AY7. You are stuffing gobs more gain into the circuit and probably overwhelming it. To boot, a concertina isn't a great choice for a hot preamp. Figure out how to reduce gain on the 5879 and give it more clean head room at the same time. Or give it a different tube. Have you got an as-built schematic?

[Stevem]

What does the amp use for a output tube?

[bal704]

I've tried both 6V6 and 6L6 tubes in it, with the same results.

I'll have to make a schematic...

[M Fowler]

A cold or poor solder joint can make an amp sound like it has speaker rattle or cabinet rattle too.

Mark

[bal704]

I looked up Cathodyne PI's, and here is mentioned something that sounds similar to what I think I here (towards the bottom of the page):

http://www.valvewizard.co.uk/cathodyne.html

If I understand this correctly, it would be worth adding a 470k-1M resistor in series with PI pin 7. Am I understanding this correctly, or would I add that between the PI input and pin 2?

[M Fowler]

470k to 1m grid blocker between the anode (plate) pin 2 to pin 6 grid.

[Stevem]

6v6 and 6l6 type tubes are of the beam pentode type, this means they drive to there peak output with not much audio drive signal and like any tube sound like shit once over drivin passed a certain point!

On average a 6v6 needs only some 13 volts of drive audio and a 6l6 type needs around 18 volts.
Your new pentode driver is likely putting out a ton more than the Triode set up was!
Hook up a volts meter set for AC volts to the pin 5 on the output tubes socket and crank the amp open when you play while hitting full chords hard, do you see the drive signal go above 18 volts? I bet you do, right?

[dinkotom]

470k to 1m grid blocker between the anode (plate) pin 2 to pin 6 grid.

This doesn't seem correct.

[Structo]

Pins 2 and 7 are for the heater.

I think Mark meant pin 6 to pin 4.

Pin 6 is not connected to anything and is frequently used for a tie point for the screen resistor.
Use a 1w 470-500 resistor from pin 4 to the B+ screen supply.

I think you need to try a grid stopper on the power tubes.

Using at least a 1 watt resistor, try 1K5 to 3k3 from pin 5 to the 220K resistors on the board.

[bal704]

As I stated in my first post, I added a 470 ohm resistor to pin 4 of each power tube. I also have a 1k5 on pin 5 of each power tube.

From what I've read of Merlin's site, sound like I need a grid stopper on the PI tube. I'm going to try that when I get a chance, as well as measure some voltages.

[M Fowler]

Assuming we are still talking about the PI using a 12AX7 type tube the heaters are 4&5.

Also, depending on which side of the preamp tube is being used first the pin numbers can change so I'll edit my post. Anode of first 1/2 triode to grid of second 1/2 of triode used in the PI.

[Structo]

When we troubleshoot we have to eliminate the obvious mistakes most often made.

Did you wire the first tube for the 5879?

If you overdrive the PI too much it will cut off.

[bal704]

Preamp is a 5879, and wired for such. I used the schematic for channel 1 of the Gibson GA-40 for the wiring. Also, I used an audio probe on it, and it sounds fine at the preamp level.

PI is a 12ax7.

The power tubes have been both 6l6 and 6V6.

I'm really interested to try the grid stopper on PI trick to see if it helps, as the symptoms seem to mimic the ones discussed on Merlin's site.

I was originally going to use a floating paraphrase PI on this one, as I'd used it on a bass amp I built and liked the way it sounded. Not sure why I ended up using this PI....

[Structo]

So are you using the 5E3 phase inverter or a long tailed pair?

Not sure what you mean about a grid stopper on the PI.

You can also try a larger grid stopper to the power tubes.

Try a 3K3 to see if it affects the noise.