Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: phsyconoodler on October 02, 2014, 06:08:10 pm
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I have a vibroverb build that sound superb eccept it has hum coming from the reverb recovery tube.
Its the bias-vary tremolo circuit and uses all 12ax7 tubes.
Pulling the recovery tube stops the hum dead in its tracks.
Its confusing cause I've tried filter caps, lead dress, tubes, reducing gain (that does help) even trying extra filtering and nothing seems to work. Removed the wires entirely from the pots at the board, even coupling caps.
Nothing works. its definitely isolated to the reverb return circuit.
The only thing i havent done is try new plate resistors.
Any ideas?
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[ I am going to assume you have a male RCA-RCA cable from the output of your reverb can to a chassis-mounted RCA jack, and that jack is grounded by virtue of being screwed to the sheet metal of the chassis. ]
Possible ground loop formed by superfluous ground on the output of the reverb tank?
I seem to recall a few posts on that topic around here somewhere.
What happens if you pull the RCA "rev out" connection out of the rev tank? Still get that hum?
Can you feed that "rev out" into another independent amp via RCA cable & adapter? (To help determine whether your hum is absolutely, without question in the rev recovery tube)
Maybe try a different RCA-RCA cable where the ground connection from end to end is broken.
Jus' thinking out loud.
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maybe you tried this: ground the grid input to the recovery triode. that'll rule out the tank, the pedal, and the wire leads that go to them.
if you zero out the reverb control knob, all hum goes away? and the amp sounds good otherwise?
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Try terminalgs' recommendation first; if that stop the hum, but unplugging the tank does not stop the hum, try the below:
The 220kΩ grid reference resistor is located on the RCA jack. Some have found that having this ground separate from the cathode ground for that triode causes hum (and it doesn't always cause hum, which often makes it a bear to find).
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It hums sitting on the bench with no reverb cables plugged in. As soon as that tube gets removed the hum is gone completely.
So i know it isn't the tank at all.
If anything bypasses the circuit its fine. As soon as that tube is in it hums.
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there was a thread on cathode caps n hum. I *THINK* a 250uf cap was used to eliminate the hum. Pretty sure it was a V1A issue, but the recovery tube is sorta a V1a because signal is pretty small. so pop one on the recovery tubes cathode to ground n see.
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Did that. No change. Putting in a 12au7 made it less noisy but still nasty.
Going to try new plate resistors next i guess.
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there was a thread on cathode caps n hum. I *THINK* a 250uf cap was used to eliminate the hum.
That doesn't apply here.
Did that. No change. Putting in a 12au7 made it less noisy but still nasty.
Going to try new plate resistors next i guess.
Did what? Just swapping tube type and/or messing with bypass cap? Did you try moving the 220kΩ resistor to be directly at the tube or moving its ground to be at the same eyelet as the recovery stage cathode resistor?
You already know a plate load resistor won't cause or change hum.
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If you want to see if the noise is coming via the input (from tank) or if the noise has to do with the gain stage itself (plate/cathode/etc), unplug the tank and plug in a known-quiet signal source (guitar, signal generator, etc).
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Moved the 220k resistor to the tube and grounded at the same spot as the cathode resistor. A very small almost unnoticeable reduction in hum.
im at a loss at this point.
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Does the reverb recovery tube share a cathode 820Ω/25µF with the post wet/dry amp stage like the original? If so, try using a separate 1.5K/25uf for each cathode. Maybe that will help. No guarantees.
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Yes it does indeed. I shall give that a shot tomorrow.
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Have you tried shielded wire to and from the Reverb pot? You might see it as a Band-Aid but it helped me reduce the noise level on two Blackface builds.
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Not sure if my build is similar to yours but I had a Nasty hum in My Revibe. Sluckey mentioned With the removal of the gorund loop and swapping a few different 12ax7's for the reverb I got rid of the hum. That was V4 on the revibe build, good luck
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Any solution?
Brad :dontknow:
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Not yet. I can remove the wires to the reverb pot and it does nothing. I havent separated the cathodes yet as sluckey mentioned. No time for this til next week.
The noise comes only when the tube is in. It does not have the tank cables hooked up on the bench.
my rerouting of wires has made the trem thump so i have to put it back the way it was.
Update coming as soon as i can find the time.
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Eagerly awaiting your solution to what seems like a very irritating put-it-on-the-shelf-for-5-years or tear-it-all-apart problem!
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Well i finally got around to separating the cathodes but no change at all. The noise is still there.
This bizarre issue is odd because i have built so many reverb amps over the years and none have ever had thid issue.
Puzzling......
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I can remove the wires to the reverb pot and it does nothing.
Do you mean removing the 100K reverb potentiometer from the circuit doesn't stop the hum?
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I can remove the wires to the reverb pot and it does nothing.
Doing that pretty much totally isolates the reverb recovery circuit. That means the other half of that tube circuit, the post wet/dry mixer amp, is probably causing the hum.
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That's why I wanted to clarify his statement.
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Yeah i kinda figured it must be the drive tube that puts the noise in and the recovery circuit amplifies it.
Im going to replace some components in that circuit and see if i can isolate it.
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Yeah i kinda figured it must be the drive tube that puts the noise in and the recovery circuit amplifies it.
Im going to replace some components in that circuit and see if i can isolate it.
I'm not talking about the driver tube. That's all been eliminated when you unplugged the tank. I'm talking about the other half of the recovery tube. The wiper of the reverb pot connects thru a 470K to the grid of the wet/dry mixer, which is the other half of the recovery tube. You say (I think) that disconnecting the reverb pot does not eliminate the hum, but pulling the tube does. So, look at that part of the circuit.
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Hi guys, I had a hum in an old amp and it turned out to be the input of the tank was close to the PT. This was kinda unavoidable as to tank was jammed in the front of the cab.
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Ok third tube from the right is the driver tube, correct? T
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Ok third tube from the right is the driver tube, correct? T
It is on Fender's original Vibroverb. Why do you keep asking about the driver? We're trying to steer you to look at V4 pins 6, 7, and 8 and associated components.
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And thats what i have been doing. The driver tube is the one that has the triodes for the plates and cathodes connected. I tried separating the cathodes.
The recovery tube does not have shared cathodes.
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And thats what i have been doing. The driver tube is the one that has the triodes for the plates and cathodes connected. I tried separating the cathodes.
The recovery tube does not have shared cathodes.
You and I are not communicating very well. I have never mentioned the driver tube except when I said "I ain't talking about the driver tube". :icon_biggrin:
The recovery tube (V4) in the original amp absolutely does share a common cathode resistor/cap with the wet/dry mixer. Pins 3 and 8 are tied together on the socket.
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My amp has separate r/c on the recovery tube. I built it (the board)seven years ago so i cant remember why i did it that way. I will go over it again.
I built all seven v-verbs the same and this is the only hummer
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Phsyconoodler, have you figured out the source of your hum yet? I'm chasing down the same kind of thing on my new Princeton Reverb build.
http://el34world.com/Forum/index.php?topic=17846.0 (http://el34world.com/Forum/index.php?topic=17846.0)
Let me know what you come up with and if I get there first I'll do the same.
-MadMax-
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Did you find any solution yet? I'm curious to know what to check if there is an issue in future builds (I might be building a standalone reverb unit in the near future).
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If it helps HBP, the bulk of my hum turned out to be inverted heater wiring between tube sockets. You can follow that link if you want to read my painful search for reverb hum.
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grounding
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No luck yet but using my limited mental capacity i have determined it isn't from the reverb circuit because the signal goes through the reverb circuit before it gets to the pi.
if you remove the recovery tube there is no signal so thats why it isn't noisy when the tube is out
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If it helps HBP, the bulk of my hum turned out to be inverted heater wiring between tube sockets. You can follow that link if you want to read my painful search for reverb hum.
I think it's extremely likely your moving the reverb tranny outside the chassis and away from the heater wires were the magic bullet.
"Heater phasing" is a Gerald Weber myth. Try it for yourself: grab any non-humming amp you have and swap the heater wires at some tube socket. I'll bet you have no increase in hum... I know I never found any difference due to "proper" and "improper" heater phasing when I tried it back when I didn't know nuthin'. :icon_biggrin:
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No luck yet but using my limited mental capacity i have determined it isn't from the reverb circuit because the signal goes through the reverb circuit before it gets to the pi.
if you remove the recovery tube there is no signal so thats why it isn't noisy when the tube is out
But doesn't yanking the reverb recovery tube also remove the gain stage where the dry and reverb signals are mixed? So that would kill the dry path of the Vibrato channel as well?
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Use Doug`s listening device to pin point where the hum comes into the circuit. Stop guessing
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Use Doug`s listening device to pin point where the hum comes into the circuit. Stop guessing
That makes waaaayy too much sense!!! Minus 25 points!! :l2:
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Right on Punky. Gotta break down and make one!
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Phsyconoodler did you solve the reverb issue? I've got a similar problem with a Twin II.
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No i haven't. I dont think its the reverb at all. Im going to examine the preamp stage grounding and lead dress more closely but not for a while as i have tons of other stuff to catch up on.
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The 6G16 Vibroverb uses a split plate resistor (82K in series with a 22K) in order to balance the reverb send higher than the dry signal. (This is in place of the 3.3m / 10PF mixing resistor used in the blackface amps, to balance down the dry signal. If your amp uses this arrangement, I'm wondering if you perhaps have the resistors backward or miswired such that your dry signal is not attenuated sufficiently relative to the reverb return. This would cause you to turn up the reverb mix control higher than normal to compete with the dry signal, and bring in hum. My reissue 6G16 does hum when I turn the reverb way up, i think that's inherent to the circuit. Have you checked those resistors for correct value / placement?
-Tom
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Interesting. That's a pretty savvy observation/theory, whatever you want to call it. Thanks for sharing that.
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Any solution? I have same issue.
/Leevi
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Try grounding the cathode resistor and bypass cap somewhere else. I have had many many many reverb recovery tubes with ground loops at the cathode for no reason I can figure out. Sometimes they are not happy until you find that one place on the chassis somewhere you would have never expected.
Dave
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Thanks Dave,
I have tried :it to same point with 220K leak resistor but maybe I should do more tries to unexpected points.
/Leevi
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the recovery circuit is very small signal, it should ground with other very small signals, so at the input jack like a pre-amp tube.
the drive side is big signal, it should be grounded with big signal stuff like PI and PA
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Come back after Midsummer celebration
Thanks
Leevi
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I got it fixed. The best grounding point was at the grounding lug of the RCA jack. I had tried to connect the 220K leak resistor to same point with the cathode ground but never at the jack.
Now there is not hum at all. The only noise is coming from the tank/cable where the tank placement plays a remarkable role.
Thank you Dave and Shooter for answering to this old thread.
Leevi
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Yay! I am tickled to have been able to help. I have taken a lot more information than I have, thus far, been able to give back.
Dave
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I have met this issue some times before and fixed those by doing different kind of rewirings/changes in the circuit but never noticed this simple rule. I have learned it now and hopefully I will remember it as well.
Thanks
Leevi
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Here's a direct link to Hoffman's amp grounding scheme. It can also be easily accessed through the Tube Amp Library link at the bottom of this page...
https://el34world.com/charts/grounds.htm
Well worth reading. Maybe even save the link for future reference.
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Thanks for the link sluckey
/Leevi
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Want to share here another case I met recently.The amp was a point-to-point converted DRRI where the reverb was noisy even if the recovery input was grounded.I spent lot of time with it trying to move the grounding of the recovery cathode. By moving the 220K grid leak resistor to the grid and grounding it to the ground bus helped a bit. The biggest impact was when I replaced the foot switch jack to an isolated one and grounded it to the ground bus. The noise decreased more than 50% and it is now on acceptable level.
/Leevi