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Post subject: Troubleshooting a '63 Fender Concert
Posted: Mon Mar 31, 2008 2:07 pm
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I need help guys. I changed out the power supply caps and changed the powertube sockets. Both were shot. I'm getting crackling especially on startup. Playing at low volume its OK. As soon as I get up to 4-5 it starts cracking. I pulled the PI tube and it stops. I put the PI tube back in and pulled all the rest of the preamp tubes and it still cracks. I'm stumped.

Heres a layout of the amp.

http://www.ampwares.com/ffg/schem/conce ... layout.gif


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Post subject:
Posted: Mon Mar 31, 2008 4:26 pm
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I changed the PI plate resistors(82K and 100K)
I changed the PI grid resistors (1M 820 1M)
Changed both PI caps (.1's)
Changed the PI tail cap (.1)
Hasn't fixed the problem. Crackles on startup and can't play past 4 without it crackin out. :cry:


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Posted: Wed Apr 02, 2008 12:22 pm
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...Try changing the pots.


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Posted: Wed Apr 02, 2008 4:12 pm
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hey it could be a million things, you check your bias circuit for AC leakage? since you isolated it to the PI, could be AC volts on your B- to the output tube grids. take your problem to HoffmanAmps forum, you'll get excellent help. be prepared to post more information. Voltages, etc., usually always tell the story.....


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Posted: Wed Apr 02, 2008 4:31 pm
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Last edited by tinker tech on Wed Jul 29, 2009 10:02 am, edited 1 time in total.

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Post subject:
Posted: Thu Apr 03, 2008 6:57 am
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I have heard a sound like this, it was arcing power tubes. If you have or could borrow spares, it could be a quick test.

However, you just changed the sockets. I would check for a "cold" solder joint at all the power tube socket connections ... or, just touch them up with a hot soldering iron. And, actually, check/retouch all the connection to the caps as well. You say it "crackles at startup" (large current demand to charge caps) ... and at higher volume levels (also large current demand to discharge caps). Cold solder joints can act like that, ... that is conduct smaller amounts of current but then the connection breaks down under higher loads. Also, check to see if it is "microphonic" ... by gently rapping the cabinet. this. Could point to a cold solder connection or an arcing tube.

And ... in trouble shooting you always start with "what was the last thing(s) you did."

I would also undo your other mods. You don't want to change things further. change only one thing at a time and reset. Next, unsolder one new cap at a time and bridge in the old one with a temporary solder connection. Sometimes new components fail early. I have had the happen fairly often. But, certainly do all the other things first. The previous suggestions of scratchy pots and isolating channels is also a good idea.

When you change power supply components you always run the risk of raising the B+ supply voltage ... which can affect things that were sort of set and working. Changing rectifier tubes, or especially switching to solid state rectifiers OFTEN does this as well. Tubes and other caps can fail. I have seen this Many times. Good luck and keep the feedback going. We'll get `er. :lol:


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Posted: Thu Apr 03, 2008 8:14 am
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Thanks for the replys.

Well I just took some voltage readings and somethings screwy.
V1 1 202
2 0
3 1.4
6 212
7 0
8 1.5
V2 1 207
2 0
3 1.5
6 209
7 0
8 1.5
V3 1 Irratic reading up and down continously between 150 and 215. Pins 1 and 6 are tied together with a resistor and pins 1 and 7 are jumped.
2 0
3 1.5
6 334
7 same as 1
8 same as 1
V4 1 2 3 not used
6 282
7 24
8 65
V5 1 317
2 0
3 3.3
6 311
7 0
8 3.3
V6 1 285
2 18
3 29
6 282
7 20
8 29
V7 3 443
4 443
5 -39
6 443
V8 3 443
4 443
5 -39
6 443


Last edited by War Admiral on Thu Apr 03, 2008 1:55 pm, edited 1 time in total.

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Post subject:
Posted: Thu Apr 03, 2008 12:50 pm
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Last edited by tinker tech on Wed Jul 29, 2009 10:03 am, edited 2 times in total.

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Post subject:
Posted: Thu Apr 03, 2008 1:11 pm
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Last edited by tinker tech on Wed Jul 29, 2009 10:04 am, edited 1 time in total.

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Post subject:
Posted: Thu Apr 03, 2008 2:01 pm
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That varying voltage is at the tremolo circuit. It is not a problem.

I don't think you will find anything with a meter. The problem is not static enough (no pun intended). I would take a non- metallic probe, like a chop stick, and touch/put-some-pressure-on the tube socket pins one at a time while the amp is hot. Also, do the same to the new cap connections. Be very careful, as I am sure you are aware since you took voltage measurements. I still think it is a cold solder joint. Cost nothing but time t


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Post subject:
Posted: Thu Apr 03, 2008 4:26 pm
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I was taking measurements of the plate resistors and I found one that was overload and pulled it out and it crumbled in my hands. I found another 100K and put it in and fired it up and it did the same thing. I tried taking a measurement of the resistor I just place and it was like a meg or something???? How can I check if the cap is short?

Could the bypass cap be short and fried the first resistor? Heres a pic of the resistor that was fried and I replaced along with its bypass cap.

Image


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Post subject:
Posted: Thu Apr 03, 2008 8:48 pm
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:shock: It appears to me the ONLY way that you can smoke that resistor is for the tube to be shorted, or conducting like crazy, ( OR there is a ground on the plate ... like where your probe seems to be. I presume your meter probe isn't grounded through the chassis. Some, actually many, VTVMs do not have a bridge mode and the only way to make measurements across things in a circuit are by subtraction/math with respect to ground.) Shorting any one of the caps attached that resistor would most likely not cause it to overheat.

If you think it is the bypass cap on the cathode that is biasing the tube incorrectly which in turn causes it to overcurrent, just pull the cap off. The cathode resistor is there to DC bias the tube, the cap bypasses the AC signal. IF you ain't playin', it don't matter. The resistor should not smoke ... (if the tube isn't shot.) and the plate voltage should then be as the drawing indicates.


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Post subject:
Posted: Fri Apr 04, 2008 6:39 am
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Last edited by tinker tech on Wed Jul 29, 2009 10:04 am, edited 2 times in total.

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Post subject:
Posted: Fri Apr 04, 2008 8:06 am
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I agree with Tinker, if you are looking for noises in the preamp section, just pull the tubes ... one at a time while using the other section. My suggestion, however, is "don't go there" . You have other things to look at first. Again, I suggest, look at things you did last while the amp was good ... then the things that making the changes might have affected.



Tinker - Factory Certified?, Naw! I am an old EE who learned electronics on tubes in the mid sixties. I have had my hands in over 100 Fenders plus dozens of others over the years. Also do solid states. Mostly for friends who are amp and guitar collectors like me. Got a few black faces, some silvers and some odd unique ones like an Oliver. (Also have a couple of solid state amps I keep around to remember how bad they sound next to my tubies.) Also built a few while thinking about getting into the boutique amp business. Since retiring from Bell Labs/AT&T/Lucent I do management consulting.


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Post subject:
Posted: Fri Apr 04, 2008 12:18 pm
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:idea:


Last edited by tinker tech on Mon Apr 07, 2008 10:18 am, edited 1 time in total.

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