Distortion - not the good kind - Rocket

[Blind Lemon]

I've built about a dozen of these Rockets and this is the first one with this wooden box drug across gravel noise. Mostly when the note is tailing off. Not all notes, F# on the G string you can really hear it. At first I thought it was speaker rub but I changed speaker cab and it's still there. I checked all the obvious thing, solder joints, tubes and the like. Any one had a similar experience?

BL

BTW I got another one setting next to it and its clear as a bell.

[Cliff Schecht]

Did you check all of the parts in the amp? You can check the plate/grid resistors with the tubes removed and check the caps coupling caps as well. What about the filter caps? Are they verified working? I'm guessing you've tried known good tube sets already. It would help to check the stuff you can easily fix before you start ripping the amp apart too far.

Post some pics and voltages, maybe some more detailed info (tube choice, parts used/subbed, etc) and we can probably help you more.

[Jana]

It sounds to me like a bad joint. Does it still do it if you completely isolate the amp from any vibrations? (amp in a different room, not sitting on a speaker cabinet, etc.)

Did you use orange drops? I have had them do exactly what you describe. It was caused by a lead on the cap not making good internal contact and with physical vibrations it would make that sound you described.

[JamesHealey]

I've had an orange drop do that also.

[Blind Lemon]

Nope not orange drops..........Mallory 150 look a likes. I thought about it today and the distortion is very much freq dependent. I'm going to change the caps out..........after I try another phase inverter, only tube I didn't swap out.

BL

[Blind Lemon]

Different PI didn't do the trick............

BL

[CapnCrunch]

You swapped the PI? I had an amp do the exact same thing to me. It was an AC15 clone. It sounded very much like a speaker rub. I went through the same dance you are doing right now. My problem was the PI tube. I swapped it out and problem gone. Funny thing is I've used that tube (an old GE long plate) in other amps with no problems, I just avoid V1 and the PI.

Like your distortion problem, mine, was extremely frequency dependent. I'd try another tube swap with known good tubes. Or maybe swap V2 for the PI tube. Good luck with it.

[Blind Lemon]

Tube swap no helpy, knwn good out off an 18w Plexi............ tone cap swap no help.

Guys it's behind the note, you really can't hear it until the note starts to fade.

BL

[CapnCrunch]

Guys it's behind the note, you really can't hear it until the note starts to fade.

BL

You are describing the exact problem I had. Certain notes would fade/decay into a raspy sort of distortion which I first thought was a speaker problem. Other notes were just fine. It would appear behind the note like I have experienced with a rubbing voice coil. It wasn't a speaker problem though.

I'm not saying that you have a tube problem for sure, but it was in my case. As I said above, the "bad" tube has worked fine in other amps and in other positions. However, that tube does the same thing in both my AC15 and my Rocket if I place it in the PI position. I found that it didn't make much difference if I took the head off the cabinet to isolate the head from the vibrating cab.

Have you re-touched all your solder joints? Do you have a scope?

[Blind Lemon]

Sounds like the exact same problem. As I said that was the first thing I did was put it through another speaker cab. Retouched all suspect solder joints, tried 2 different 12at7 and a12ax7 in the PI and change tone caps to Mallory 150s. I'm rechecking resistor values now.

BL

No scope..........

[Blind Lemon]

Resistors look correct, checked my voltages at start up and they all looked good.................

BL

[JamesHealey]

could be blocking distortion into the power tubes..

check out the 18watt.com paul ruby buzz fix.

http://www.18watt.com/modules.php?name= ... it&lid=125

[rooster]

James - Hey, do me a favor? Can you somehow post this PR piece without having to register on the 18watter site? I rarely visit there, although I think I did register a few years back. I would like to see what he says about the buzzes. Thanks!

BTW, this problem - if it is the tubes - is easily isolated with a long speaker cable and an independant speaker box. Somehow this smacks of a power tube rattle I ran into years ago. BTW, the customer never accepted it. Funny how that goes..... On the other hand, a chopstick should be able to duplicate a rattling part? Well, just throwing that out there.

[JamesHealey]

here u go.

[Richie]

could be many things, loose solder joint,funky tube,or vibrating componet,to even a loose speaker connection etc.. but one thing that makes the metalic speaker rub or sounds like a blown speaker,is the cathode resistor/s or bypass cap if you are using one on the preamp.
Try replacing those. Even though they measure or look good,may not be.