Rocket revisited

[johnnyreece]

Well, my wife tells me it's time to thin out the herd, so I decided to go to this Rocket I built and nail down what's going on. I'm thinking I've probably got an oscillation issue. I have VVR installed, for full disclosure. For the most part, everything is fine. I seem to have trouble when the VVR is over half, volume is cranked, and the treble and hi-cut controls are turned up. It comes on suddenly. With the high cut at 10, volume at 10, and VVR about half, it hits at around 8 on the tone control. It sounds very much like 120hz hum. Bass control has no impact on the hum. Chopsticking isn't a very good option, because without a shield, I get tons of hum, which masks what's going on.

My first thoughts are to move the grid stopper off the board and down to the tube socket. My second thought is to replace the treble pot. Anyone else run into this?

Here's the link to the build thread with pics: https://tubeamparchive.com/viewtopic.ph ... et&start=0

[Littlewyan]

Have you tried pulling pre amp valves? Like pull V1 to see if it still oscillates with everything turned up (obviously volume wont make a difference at this point).

[johnnyreece]

Not yet, but I'll give it a go. I suppose I should have done some basic troubleshooting to start with.

[RJ Guitars]

I'll grab the low hanging fruit first... did you try temporarily try removing or bypassing the VVR and check to see if the problem is still here?

My hum problems are generally when something is losing a ground... not that that is revolutionary news but look at the knob settings and see if there is some odd combination of settings where the path to ground might be going to high resistance.

I've worked through this a time or two and found it with a reverse logic by using a small insulated screw driver to short to ground at various appropriate points (avoiding the heaters or B+)... like the tail of the phase inverter and such. Based on a recent personal experience when using insulated "cliff" jacks I'll ask, is your output tranny grounded? My triode Express hummed a tune for me until I figured that one out.

[johnnyreece]

I didn't try bypassing the VVR, either. I will try to pull the tubes one by one to see if I can determine a general area to look. I will also be sure I grounded the output jack, as well. Now that you mention it, I think I repurposed the original OT, and it was wired directly to the speakers. You might be on to something there! I was mostly confused about the sudden onset of the noise. Usually, hum increases gradually for me as I turn the dial, but this hits all at once. I'll poke around soon and keep everyone posted!

[johnnyreece]

Okay, after multiple tries of different ideas, I discovered it's the VVR. Well, likely not the VVR, but the circuitry I needed to add to V1, I'm guessing, was the issue. I may just leave it disconnected to get it buttoned up and try the VVR in another project. I dunno...it's friggin' loud!

I'm pretty sure it was some sort of oscillation, and I could get some really interesting stuff to happen if I moved the input wire, so I moved the grid stopper as close to the socket as I could...for no change. One of the caps was microphonic. Either way, it's not doing the craziness now. Thanks for the tips, guys!