Monkeymatic Rocket 64

Express, Liverpool, Rocket, Dirty Little Monster, etc.

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joeboo88
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Re: Monkeymatic Rocket 64

Post by joeboo88 »

I don’t know if it makes a difference, I tightened all the screws for the top plate of the chassis.
Just something to check....
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xtian
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Re: Monkeymatic Rocket 64

Post by xtian »

Thanks, but the hum is not vibration.
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xtian
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Re: Monkeymatic Rocket 64

Post by xtian »

Hum is busting my chops.

Hum does not change with PI pulled, so that limits it to the power section.

No hum in standby, so it’s not the proximity of the PT vs. OT.

Hum does not change with extra 40uF clipped into reservoir.

No improvement with different recto tube and power tubes.

Things I can try:

Isolate output jack from chassis (there’s no NFB to worry about).

Anything else?
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xtian
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Re: Monkeymatic Rocket 64

Post by xtian »

I have 5vAC ripple on my reservoir cap. Not too surprising, I guess, because the reservoir is connected to one leg of the 5vAC rectifier heater supply. Is that much ripple expected?

The ripple on the next node, the screen supply, is about 100mVAC. I tried adding an additional 470R/47uF node right after the rectifier, and it did not reduce the hum. (Though I was dropping 50v through the 470R/5W resistor, which is...5.3 watts. Hot.


Note that I'm using 5U4GB rectos, not GZ34. The 5U4 drops 50v, which is needed here. Wiring is same for both tubes, right?
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JazzGuitarGimp
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Re: Monkeymatic Rocket 64

Post by JazzGuitarGimp »

xtian, have you determined whether the hum is 60Hz or 120Hz? For reference, B at the 2nd fret of the A string is close to 120Hz.
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xtian
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Re: Monkeymatic Rocket 64

Post by xtian »

I scoped it. The answer is both. 60Hz with 120Hz at about half the amplitude.
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didit
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Re: Monkeymatic Rocket 64

Post by didit »

Heaters appear ungrounded? Might contribute to 60Hz.

Best .. Ian
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xtian
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Re: Monkeymatic Rocket 64

Post by xtian »

Heaters have artificial CT grounded to power tube cathode ground. I tried moving that ground point around with no change. Thanks, Ian.
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didit
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Re: Monkeymatic Rocket 64

Post by didit »

OK--hiding in plain sight, tucked-in off PI. Unclear what goes on under that PCB, specifically the corner with apparently the can filter cap ground attached. Having yet to build a Rocket type, cannot offer first-hand experience but underwhelmed by Fischer's original grounding scheme as illustrated in TAG layout. PCBs are great, but those post+screws to ground inspire concern. Tried grounding PI with the power tubes? Tried using elevated cathode bias ground for heaters (easier reach to relocated terminal strip in 2nd bolt hole)? Tried moving first ground over closer to the center? Reread Blencowe's chapter on grounding?

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JazzGuitarGimp
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Re: Monkeymatic Rocket 64

Post by JazzGuitarGimp »

You might try lifting one end of the coupling caps between the PI and the PA. I know you have the PI valve removed, but the hum may be coming in from the B+ net. Alternatively, you could ground the ends of these two caps electrically nearest the PI.
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xtian
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Re: Monkeymatic Rocket 64

Post by xtian »

I will work my way thru your suggestions, thanks guys.

One clue is that the hum vanishes instantly when I cut the mains. So, definitely heaters. Heaters measure 6.9vAC, so I used Variac to drop to 6.3v, with no change in hum.

I moved the artificial CT (aCT) to the end and to the beginning of the heater chain, and used alligator clip to move the ground point around--no change at all. Only change is when no aCT is connected, then much bigger hum.
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pompeiisneaks
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Re: Monkeymatic Rocket 64

Post by pompeiisneaks »

Have you tried a hum balance pot? Maybe that would help minimize it.

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xtian
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Re: Monkeymatic Rocket 64

Post by xtian »

Props for a great suggestion, Phil, but balance pot sounds best right in the middle. No hum reduction.
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xtian
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Re: Monkeymatic Rocket 64

Post by xtian »

Man, I tried rearranging the entire preamp grounding scheme, and then moving the can cap ground around, and noting I do makes any difference in the hum, for the better or worse!

I thought MAYBE the pilot light and MOV were worth bypassing, so I wired the PT primary directly to my mains. No change.

Quickly running out of things to try!
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didit
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Re: Monkeymatic Rocket 64

Post by didit »

Unbolt power transformer, and rotate to the limit wires allow on all three axis, pitch yaw and roll. If hum changes notably you know what's required.
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