Rocket Build...noise issue.

[matt h]

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[MGW]

Pulled all the cap/resistor crap that I added. Reworked the input jack. with nothing plugged in to the jack I get 34.24K (grid stop + cathode). With the cable I get 1.011M (input + grid stop + cathode).

The plate resistor measures 217.2K, cathode measures 1.491K, the grid stopper measures 32.72K, input jack resistor measures .976M.

B+ 1=303V, 2=295v, 3=223v, 4=196v, 5=192v.

V1 with cable in input - no guitar.
1=121v, 2=0v, 3=.97v, 6=93v, 7=-116mv, 8=.68v.

So...apparently I need to check the board and sockets for voltage at this point.

[matt h]

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[JazzGuitarGimp]

that would be correct. any of those mica caps leaky? what happens if the input goes direct to the tube pin, and no connection to the board? maybe take a dental mirror and look under the board for a miswire or a piece of wire or solder.

+1 on connecting a wire from tip of jack to 33K right at the tube pin, bypassing the board entirely. It's possible you've got one of those slightly conductive boards.

[MGW]

Not sure I get your meaning..."through" the cathode resistor, but the 1Meg grounds to the chassis at the input jack. They just sum because the circuit is made complete by the chassis...as far as the continuity is concerned. Grid to ground through input jack...if cable plugged in it has to go through a 1M to get to ground. Without a cable the tip is shorted to ground leaving out the 1M. As to the cathode...it goes to ground through the 1K5 and grounds to the buss bar on the back of the pots.

Bypassed the board entirely as you suggested and there was no change.

[matt h]

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[Richie]

The bypass cap is correct. I reverified last night. The black band furthest from the tubes is ground. Not sure how a leaky mica cap on the pin 6 would impact pin 7 of the same triode. I see how it would leak voltage into the next stage, but it is clean.

Can you just try this? Lift the mica cap or break the connection from the board to the pot,on both the mica caps. Then take your readings,and , see if the voltage changes. Most likely the mica cap closest to the input will be the one causing the problem. These have been know for a while to be leaky and cause problems. At least if you try it, you can rule that out,and move on. Do the control pots vol bass treble have any scratchiness to them? Any voltage on the pots?

You see where your ground is for the cathode of that section of tube? Do you see the connection of the ground of that pot? Same spot. If that mica cap is leaky, it can cause weird things to happen.

[MGW]

Pulled the mica and put in a 1000pf 150M. No change. Swapped out the cathode bypass cap for a different model altogether...no change. Added another Rp...now we're cooking. The negative grid voltage dropped WAY down...or up as it were. There is still a little bit there. Swapped the socket...it isn't that.
OH...YES...several guitars. None active, all exhibit the same behavior. Safe to say that isn't it.

Thoughts? and MANY thanks!!!!

[Tillydog]

Just looking for clues:

Where *exactly* are you connecting the negative lead of your meter when you are checking grid voltage?

[MGW]

I am connecting it to the lug that grounds the output tube R/C. Right in the middle of the chassis.

[Firestorm]

This is too weird. Gotta be one of those phantom coupling things Weber talks about. Speaking of Weber, he says running shielded wire with the shield connected to the plate cures it. Just pointing it out. Weber, after all.

[Tillydog]

I am connecting it to the lug that grounds the output tube R/C. Right in the middle of the chassis.

Do you measure dead-on zero volts on the turret at the grounded end of V1a cathode resistor/bypass cap from that same point? How about the voltage on the ground lug of the input jack?

If you see any voltage, it points to a bad ground connection somewhere. If not, I'd be back to thinking it might be two dud tubes...

[matt h]

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[Blackburn]

Interesting that I happened to come across this post today over on TGP, by Hogy. I couldn't help but remember this thread.

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"You can't forget that Ken absolutely loved AC30s.

I have had the good fortune of owning a couple of particularly great sounding AC30s, and I can tell you that if you find a good one, it's as good as a Rocket in terms of tone. In fact, for certain things, it might be better.

I think what Ken wanted in the Rocket was an amp that sounded consistently as good as the greatest vintage AC30s. Very few of those actually do.

So take a killer AC30, optimize the layout, correct some issues like excessive ghost noting and flubby bass response, "trainwreck" the circuit by careful parts selection, and there you are.

Note that Ken didn't just up the cap values in the power supply, but completely changed the design from a radial supply to a totem pole style. This is one reason the voltages in the preamp are what they are, and that is deliberate.

This circuit has its quirks, too. A properly running Rocket leaks a small amount of DC onto the guitar's volume pot, making it scratchy. Doesn't happen with all preamp tubes, but interestingly more often than not with the ones Ken liked best in V1, Bugle Boy ECC83s."
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You don't happen to have that tube in V1, do you?

[matt h]

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