jbennett
Joined: Mar 29, 2011
Posts: 110
Brooklyn, New York
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Posted on Mar 13 2012 10:42 AM
Bought a square of brass yesterday for 3.50. Cut and drilled it last night and installed it at at the pots/jacks. Soldered my grounds to the brass.
Sounds great, unit operates with almost no noise BUT, I still have the BUZZZZZZZZ when I turn the Dwell knob all the way off. I guess I'll investigate that pot. Maybe i toasted it for too long when soldering the ground to it?
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tubeswell
Joined: Sep 24, 2011
Posts: 1424
Wellington, NZ
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Posted on Mar 13 2012 01:14 PM
jbennett wrote:
... I'll investigate that pot. Maybe i toasted it for too long when soldering the ground to it?
I'm guessing its either the pot or the way its grounded. Either way you need to check that pot
— He who dies with the most tubes... wins
Surf Daddies
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jbennett
Joined: Mar 29, 2011
Posts: 110
Brooklyn, New York
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Posted on Mar 13 2012 10:52 PM
Tubeswell... I checked the pot, it seemed to be okay when I desoldered it and tested it at all points along the wiper.
I was just experimenting. Using my meter I poked around solder points with the amp on. Black wire to ground, red probe touching certain points. I noticed that my DWELL BUZZ disappeared when I touched the tail of the 1500 ohm resistor farthest to the left on the board, and, naturally got the same reaction when I touched pin 8 of the 12AT7 tube socket where that line terminates. (see diagram at red dots)
So, I clipped a jumper from pin 8 to the chassis and fired it up. The Dwell problem is gone!
Now, can you tell me exactly what it is that I've discovered here? Is this a potentially dangerous or damaging option. Assuming I run a short bus wire from pin 8 to the chassis?
I measured 2vdc off pin 8. When I jump it to ground I get 0vdc off the chassis. So it seems that no voltage is running to the chassis. And 2 volts is nothing anyway.
Please weigh in as I respect your opinion. (anyone else reading should chime in too...)
Last edited: Mar 13, 2012 22:53:11
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jbennett
Joined: Mar 29, 2011
Posts: 110
Brooklyn, New York
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Posted on Mar 13 2012 11:37 PM
And here are the last photos, I hope... of the brass plate installed. (those ground joints are solid, I don't have a great iron so they are a bit funky looking but they are secure and conduct.)
And my experimental jumper from pin-8 to the ground tab of the RCA output. Tested it out. No hum, no buzz with the dwell knob. I am victorious!
Last edited: Mar 13, 2012 23:38:01
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tubeswell
Joined: Sep 24, 2011
Posts: 1424
Wellington, NZ
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Posted on Mar 14 2012 12:23 AM
jbennett wrote:
Tubeswell... I checked the pot, it seemed to be okay when I desoldered it and tested it at all points along the wiper.
I was just experimenting. Using my meter I poked around solder points with the amp on. Black wire to ground, red probe touching certain points. I noticed that my DWELL BUZZ disappeared when I touched the tail of the 1500 ohm resistor farthest to the left on the board, and, naturally got the same reaction when I touched pin 8 of the 12AT7 tube socket where that line terminates. (see diagram at red dots)
So, I clipped a jumper from pin 8 to the chassis and fired it up. The Dwell problem is gone!
Good troubleshooting skills! Could be the solder joints around that CC need reflowing, or it could be that the CC resistor is kaputt. CCs don't respond very well to heat and its easy to overcook them when soldering them in - Try soldering in a replacement one.
BTW you shouldn't jumper pin 8 of the 12AT7 to ground because you need that 1k5 resistor for cathode-biasing that triode. If there is no cathode resistor that triode won't be biased properly (It will be relying on insufficient grid-leak biasing from the 250k dwell pot - which come to think of it could be what's causing the buzzing when the pot is cut) and the triode will likely heat up very quickly.
— He who dies with the most tubes... wins
Surf Daddies
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jbennett
Joined: Mar 29, 2011
Posts: 110
Brooklyn, New York
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Posted on Mar 14 2012 06:51 AM
Interesting. I'll check the resistor. Thanks for the info. I'll let you know how it goes.
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jbennett
Joined: Mar 29, 2011
Posts: 110
Brooklyn, New York
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Posted on Mar 14 2012 07:56 AM
What exactly do you mean when you say "CC"? I'm assuming you are just referring to the Carbon Composition resistor but I want to make sure I follow you.
I just tested the 1500 resistor and it went to zero, so I clipped it out. Stupidly realizing that of course it read zero across, I grounded it on both sides! Anyway. I tested it once removed and it was still okay. I replaced it with a new 1500 ohm resistor already anyway so... yeah. I guess that was not the issue. If my 1k5 resistor is still going strong, does that mean my pin8 to chassis jumper is still going to cause trouble with grid bias leakage?
Is there another point in the circuit that you think would resolve the dwell buzz? For now I've still got pin8 grounded.
Again, thank you for your thoughts.
Last edited: Mar 14, 2012 07:58:35
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jbennett
Joined: Mar 29, 2011
Posts: 110
Brooklyn, New York
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Posted on Mar 14 2012 08:55 AM
Just a thought. If I keep the pin#8 to ground scheme... could I use a 1k5 resistor instead of a plain bus wire and get the same quieting effect with the proper grid bias? Even if the cathode is already running through that 1k5 resistor on the board to ground? I guess part of the mystery here is... pin#8 is already grounded through that 1k5 resistor on the board, right? So... what gives?
Is this a case of the signal taking the path of least resistance and instead of heading up to the board 1k5 resistor it's running via the plain bus wire to ground?
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vintagesurfdude
Joined: Nov 28, 2011
Posts: 795
Prescott Valley, AZ
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Posted on Mar 14 2012 09:40 AM
I hate it when I sound stupid, but, looking at the wiring diagram which you've highlight with the red dots. That's what's called for, a 1.5K resistor between pin 8 and ground. Any chance we missed the lead under the fiber board between the pin wire and the resistor?
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jbennett
Joined: Mar 29, 2011
Posts: 110
Brooklyn, New York
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Posted on Mar 14 2012 10:07 AM
VSD. That's the first thing I checked. Using my meter I got a zero measure from the pin to the 1500. That yellow wire that enters the board through a hole from pin-8 comes up in the same eyelet hole as the tail of the 1500 and you can actually see the tip of that wire sticking up through the hole.
I'll triple check that connection tonight anyway, just to be sure.
Thanks.
Last edited: Mar 14, 2012 10:09:59
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vintagesurfdude
Joined: Nov 28, 2011
Posts: 795
Prescott Valley, AZ
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Posted on Mar 14 2012 03:34 PM
If you remove the tube from the socket in question and measure pin 8 to ground, what do you get? Should be 1.5K. Like tubeswell mentioned earlier, maybe a blown resistor or cold solder joint. (Though I must say, your soldering looks pretty good. Cold joints can look fine, always reheat when in doubt.)
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tubeswell
Joined: Sep 24, 2011
Posts: 1424
Wellington, NZ
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Posted on Mar 15 2012 11:32 AM
Ok so its not the resistor and it doesn't appear to be the pot itself.
Apart from reflowing the solder joints (as vintagesurfdude suggests) for those 12AT7 pins and on the pot and resistor and where these ground to the brass plate, maybe also check the socket pins for that 12AT7. They may need re-tensioning (with a small (tiny) screwdriver). I don't suppose you have tried a different 12AT7 just in case its the tube itself?
Another thing could be where the dwell pot is grounded - try taking the ground lug off the pot shield and instead putting it onto the brass plate?
Without putting a scope on it, troubleshooting is a process of elimination, so you just have to keep on trying things.
— He who dies with the most tubes... wins
Surf Daddies
Last edited: Mar 15, 2012 11:34:40
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jbennett
Joined: Mar 29, 2011
Posts: 110
Brooklyn, New York
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Posted on Mar 15 2012 11:50 AM
I don't have a spare 12AT7 but maybe I'll pick one up to test. First I'll re-tension to make sure that it's making contact.
I can try to re-ground the pot to the plate.
My dad has a scope and is a proper engineer. maybe I'll have him dust off the old scope and we can look at things that way sometime.
For now, I have pin-8 grounded to the chassis and it sounds great. Hopefully I'm not frying things.
Sidenote: Not sure if I mentioned it, but since I put in the brass plate and redid those ground points there is no more sporadic popping/zapping. Those must have been a product of those crap flux connections to the chassis.
Last edited: Mar 15, 2012 12:23:18
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tubeswell
Joined: Sep 24, 2011
Posts: 1424
Wellington, NZ
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Posted on Mar 15 2012 01:28 PM
jbennett wrote:
For now, I have pin-8 grounded to the chassis and it sounds great. Hopefully I'm not frying things.
Hmmm... that just still doesn't compute. Something is not wired right or the tube is bad or something. If you have the cathode of a cathode-biased stage grounded to the chassis, it definitely shouldn't be working.
— He who dies with the most tubes... wins
Surf Daddies
Last edited: Mar 15, 2012 13:30:53
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jbennett
Joined: Mar 29, 2011
Posts: 110
Brooklyn, New York
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Posted on Mar 15 2012 02:03 PM
It is a mystery. And the only thing it seems to effect is the dwell knob buzzing when turned to zero. I'm stumped but I'll continue to investigate.
It does work quite well right now.
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jbennett
Joined: Mar 29, 2011
Posts: 110
Brooklyn, New York
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Posted on Mar 15 2012 10:32 PM
I clipped the bus wire from pin-8. Measured through the 1k5 resistor to ground and It is connected and measuring correctly.
I guess I'll mess with other grounding issues this weekend. Luckily, of all the problems I have to deal with my issue only comes up in a situation I never have to worry about since I use the foot switch and would never have the dwell knob all the way off. If I want reverb off, I step on it.
Still, it's curious. I measured a lot of other components tonight and couldn't find any oddities anywhere.
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tubeswell
Joined: Sep 24, 2011
Posts: 1424
Wellington, NZ
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Posted on Mar 16 2012 02:39 AM
How well are the ground wires soldered to the brass plate? Did you use a really hot iron to get a smooth strong glassy solder finish? Or have you still got blobs? (I still wonder whether its grounding related)
— He who dies with the most tubes... wins
Surf Daddies
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jbennett
Joined: Mar 29, 2011
Posts: 110
Brooklyn, New York
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Posted on Mar 16 2012 07:01 AM
Still got blobs, the brass seems to work as a heat sink and makes it hard to flow the solder. I only have a 30 watt iron. Maybe that is the problem, though all my ground connections are reading well on my meter. The solder seemed to have the same issue when I tried to get it on to the chassis directly, but at least this connection seems very solid. I can't get it to loosen by pushing at it with a tool or my finger like when I was trying to get it on to the chassis.
Still, I'd like a better connection.
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tubeswell
Joined: Sep 24, 2011
Posts: 1424
Wellington, NZ
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Posted on Mar 16 2012 10:46 AM
jbennett wrote:
Still got blobs, the brass seems to work as a heat sink and makes it hard to flow the solder. I only have a 30 watt iron.
You need a 60-80W iron to solder onto large bits of metal because of the heat sink effect. (The more powerful the better. Its about having the right tool for the job).
— He who dies with the most tubes... wins
Surf Daddies
Last edited: Mar 16, 2012 10:47:48
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vintagesurfdude
Joined: Nov 28, 2011
Posts: 795
Prescott Valley, AZ
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Posted on Mar 16 2012 11:13 AM
These joints justify having a high wattage soldering gun.
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