6G12-A "frying" sound

[Mars-Hall]

My 61 Concert Amp has developed a frying bacon noise along with some popping. The "frying" also resembles thunder rolling in the distance.  The sound is audible with or without an instrument plugged in.  What I've found is by pulling V4 the frying stops. Pulling V5 both frying and popping cease.  Replacing either with known good tubes does not resolve issues.  All voltages are within tolerance at V4 and V5.  The components look normal, no visual evidence of heat or burnt resistor/components in the area.  By chopstick method, I found the .005 cap and the .01 cap, that connect too pin 2 of V5, have an audible thud through the speaker when struck.  Much louder than any other components on the board.  Are either of these caps my likely culprits?

[Mars-Hall]

I would like to add that all electrolytics were replaced several years ago, including the 4uf 25v cap in the vibrato circuit.

[eleventeen]

Them 100K plate resistors. Times 6 qty. For $2, replace them all with metal films (I like to use 2 or 3 watters, though 1 watts are plenty) and enjoy the silence. You'll be (pleasantly) shocked.

[Mars-Hall]

I switched out the two related to V5 without any silencing benefit.  I replaced with gold banded carbon film resistors.

[jojokeo]

You should replace all your load resistors as eleventeen says, but I'd bet this could just as easily be a bad solder connection somewhere along one of these B+ paths. So if you don't want to replace anything I'd first try this as it's the easiest, least expensive, and least invasive.

[eleventeen]

It's not necessarily clear how one labels their "V1" "V5" etc; on Fenders as most schematics do not have such labeling on their schematics nor on their layout dwgs. However, regardless of whether one starts counting at the preamp end or the power end, "V5" is logically "in the middle" of an 8-tube amp. Yes, yes, I understand that if you label based upon the schematic, most probably you would label the first preamp tube of the normal channel "V1". But if you labeled based upon the layout dwg, you would label the 5881 closest to the AC power cord your "V1". Either way, anything reasonably labeled "V5" is not going to be an early preamp tube.  It is in the first preamp tubes that you'll find the greatest sensitivity to the type of hiss that old carbon comps produce. If you don't believe me, pull the first and/or second 12AX7 (both designated 7025-type in the schematic) and observe/listen to the results. Noise produced by the phase inverter, the last 12AX7 next to the power tubes, is likely to be somewhat suppressed by the push-pull action of the output section, and noise produced by the trem oscillator might not matter at all in terms of what you hear on the audio output.


It *could* be a bad solder joint as jojokeo says, but since you'd have to reheat those solder joints to find this out, which is "invasive" if you are talking about what would be observable after you've done the work. Would the plan be to turn off the amp, reheat one solder joint at a time, then power back up & test the result? I suspect not. The very worst Fender factory-made bad solder joints usually require taking them apart and cleaning the eyelet fairly aggressively, in my experience. Not just adding a touch of new solder & flux. You'd see that.

[jojokeo]

Chop sticking around on the parts with amp on while listening to the snap crackle & pop to change for the worse/better usually works well here. Don't forget to wiggle the tubes in their sockets too.

[Mars-Hall]

In my experience, I've always counted valves starting with the first preamp tube, V1 being nearest the inputs.  In this case, pulling each preamp tube, one at a time, V1, V2 and V3 had no benefit of silence.  As stated earlier, pulling V4 relieved the "distant thunder", however the popping persists. Pulling V5 and V6 eliminated both noises.  In this circuit, V6 is the phase inverter, so it follows that there would be no "noise".  I then swapped out the power tubes with known good, without any benefit. 

With my limited experience, I've conclude the noise source should be in the circuit related to either V4 or V5, which in this case is the vibrato section.  This is what makes me question the two caps mentioned in my OP as a possible source.  Noise from these two caps are the only indications I'm getting from the chop stick.  So far, I've replaced the two 100K plate resistors related to V5 and the 56K that is connected to pin 6 (plate) of V4, again without benefit from either.  Also re-tensioned and cleaned the the tube sockets.  I do plan on changing out the two caps but need to source them first.  I will also change the other plate load resistors just in case up stream events are affecting those down the pike.

[sluckey]

V5 is the vibrato modulator and can be a source of various noises. That's why it calls for a 7025 (low noise 12AX7). I would concentrate on V5 circuit. Get a handful of 12AX7s to roll thru that socket. Better yet, get some 5751s. Replace the 4700Ω and 4µF on the cathode. Those caps and resistors (including the two 1M grid leaks) in the grid circuit are suspect also.

Take a look at page 12 of this pdf for an explanation of how that vibrato circuit works. That may help in troubleshooting.

     http://home.comcast.net/~seluckey/amps/misc/Amp_Scrapbook.pdf

[Mars-Hall]

Thanks sluckey! I will look into that and order some parts soon. What you said about V5 being the modulator makes sense.  While turning the intensity control all the way up, when it hits the stop, it induces the popping noise.

[dunner84]

My 18W Plexi/Bluesbreaker was making the same noises.. I spent a lot of time replacing resistors and chopsticking around with no luck. Just for the sake of maintenance, I sprayed and cleaned all my jacks and pots. Voilla.. No more Pop or Bacon.... ;)

For the jacks, I sprayed them and used rolled up plumbers sandpaper (the stuff they use to clean copper connections) and gave it a few turns inside the jack.

[jojokeo]

Interesting about the plumber's sandcloth technique. It's a little maybe "over abrasive" but will definitely do the job...maybe too much? But whatever works sometimes. I have found many a jack on guitars, amps, and effects boxes become dirty and/or with corrosion issues and a common occurance. I will usually start off with the least abrasive by using contact cleaner first. But this won't always get the hardened-on grime. Guitar players commonly have sweaty, acidic, and grimey fingers if not just from the corrosion rubbed off from the string contacts as the strings age. What I've found as a next good step is to use a metal polishing compound along with Q-tips. This does a great job most every time while not grinding away metal that the sandcloth or sand paper will do.
On another note, I've been seeing a frequent amount of issues with switching Cliff type jacks that are fairly new or have not had much use, to have solid contact issues. For example I have a JCM800 type of build where one input jack has the signal go through it when not in use via the contacts and I get intermittent signal issues at times. I'll have to take the guitar chord plug and work it in & out a few times to get a good signal again. Due to Murphy's Law this is usually at a gig or rehearsal. On another amp I will get a humm with no chord plugged in once in a while and using the same "high-tech technique" with the plug it'll be one of the input jacks not making a good ground via the switching contacts. Lastly, I've also had people with supposed amp problems brought to me with no output and it's as simple as this same contact issue with the Send/Return jacks. I have cleaned many a guitar input jack too that get very grimey and corroded when guitars are brought to me too so this not all picking on Cliff Jacks by any means. On the good side, these are quick & easy repairs which makes easy money (beer or any other fair trade) for figuring this stuff out quickly for customers, friends, or band mates.