Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: theundeadelvis on November 20, 2010, 07:59:34 pm
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You may have seen my thread about problems with my revibe. Well I put it on the back burner to avoid too a break from frustration. So, I broke out my standalone reverb unit (built using the weber 5G15 layout), and now it's developed a strange issue. First I noticed if I turn the dwell and mix up high the reverb distorts really bad, and when I turned the dwell all the way down it makes noise too.
I was checking to see if there was any DC on the pot when I noticed something very strange. I had one probe of my DMM grounded, no reverb pan attached, no guitar plugged in, and nothing plugged into the output either. When I would touch the other DMM probe to the center lug of the dwell pot I would get a high pitched squeal? It was most noticeable with the pot in the middle of it's travel (at max and minimum no noise). It's a very weird squeal with some static noise to it also, and very audible. Again, the reverb wasn't plugged into anything else (no amp, guitar, or pan). What could it possibly be?
Thanks!
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The dwell pot on the 6G15 goes directly into the second half of the first triode... you
are in effect touching the grid (input) of that tube with an antenna (your probe). You
will see this from time to time in other amps that use no grid stoppers.
Turning the dwell wide open should overdrive the second half of the first triode somewhat.
In the ones I have built, I never saw "excessive" distortion, as it goes through a voltage
divider, and he is not hitting the 6K6 too hard. As for the mix, that is really a balance pot
between the output of the reverb recovery triode, and turning the other way it takes
output from the other half of the reverb recovery, which is a cathode follower. You should
not see much distortion there.
I suspect a voltage out of whack (caused by something else, tube, resistor, ground, etc)
or a tube going out. Check (carefully) all of your voltages in the unit.
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Thanks triode. I took some measurements. Nothing jumped out at me except pin 6 of v1 seemed high (not sure if it's within the acceptable range or not). Here's what I came up with:
v1
1-179
2-1.03
3-1.58
4/5-3.29
6-284
7-124
8-143
9-3.29
v2
1-129
2-0
3-2.15
4/5-3.29
6-139
7-0
8-2.1
9-3.29
v3
1-
2-3.29
3-329
4-338
5-14mV
6-
7-3.29
8-25.5
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Thanks triode. I took some measurements. Nothing jumped out at me except pin 6 of v1 seemed high (not sure if it's within the acceptable range or not). Here's what I came up with:
v3
1-
2-3.29
3-329
4-338
5-14mV
6-
7-3.29
8-25.5
V3 pins 1 and 6 should be 160V and 250V respectively, if you used the same pinout as the fender schematic.
V3 pin 8 should be 100-130V or so. 25.5 Seems low. V3 pin 3 should be 1.2V or so. Hrmmm. Pins 4 and 5 should have
(nearly) zero V DC (they and 9 are the heaters)... I think you have the B+ hooked to pin 4 for some reason.
Tube is wired incorrectly, for sure. Or you have crossed/shorted B+ to pin 4 or something screwy.
Double check all wiring on V3, all pins.
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V3 is a 6K6, not a 12A?7.
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V3 is a 6K6, not a 12A?7.
Dang, in my head I keep seeing the board layout where I do the 6V6 in the middle.
Sorry about that.
Wait a second. V1 pin 2 should be 0V... Other than that, yes, they look fine.
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Looking at the schematic I would think the tube in the middle would be V2 also. The layout shows differently though.
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My measurements were taken as V1=the 12AX7, V2= the 12AT7, and V3 is a 6v6.
Thanks guys!
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Been away from this guy due to the holidays. Here's a video of the noise I'm getting when I turn the dwell pot all the way down. It's like the pan is getting kicked all of a sudden:
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Since this noise happens as the pot hits 0ohms (give or take a couple ohms), could I eliminate it by adding a small value resistor between lug 3 of the pot and ground?
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Sounds like a band-aid fix. Replacing the pot or fixing a poor ground might be a better fix.
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Yeah, the more I considered the idea, the more I thought the same. I'll try a new pot. Thanks!
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I'm beginning to hate reverb. I've built a number of Deluxe Reverbs, and never had an issue, but these stand-alone units are killing me! Reworked my grounds and got the popping/splash noise to go away, but now when I turn the dwell and mix up to near max, I get a really high pitched feedback. It's worse if I take my hands off my guitar strings, and it goes away if I turn down mix and dwell or when playing. Is this some sort oscillation? I've moved lead dress around without noticing any difference. Bummer.
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v1
...
2-1.03
...
(unless it was your meeter reading 'causing' the voltage reading on pin 2 of V1) the grid of the 1st stage shouldn't have any voltage on it. Check that the .05uF blocking cap in front of the CF/bypass stage isn't leaking DC back to the V1 grid
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Well, I replaced the .047 cap, and I'm showing 0v on pin 2 of V1, but still getting bad, bad feedback with the mix up to 7 or more. The whole unit seems super microphonic too. I can tap anywhere on the chassis and hear it, and tapping the power tube sounds like I'm holding a microphone to the glass. I tried multiple 12AX7s, AT7s, and 6V6s, and it sounds the same. I think I'm just going to start replacing caps (and never build another reverb unit).
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Try shielding the signal grid wires - that can help with that 'microphonic chassis problem
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I'll give that a try. I've got it sounding much better, replaced a few caps, and replaced the cables going from pan to the chassis. I think some shielded cable to the grids will hopefully get it where it needs to be. Thanks for your help.