Ideas on where to start with diagnosing intermittent static?

[markmalin]

I just got this amp back - it's basically a Hoffman style AC30 clone with a Master Volume and a 1/2 power switch.  I can post a schematic if that helps, but initially just wanted to ask some advice on how to reproduce and diagnose the issue. 



I've attached a link to a video below. Intermittently it starts emitting static like in the video, the owner said most recently after it was on about 2 hours.  It will do this for a minute or two and then stop and be fine. Doesn't always happen.  Before I dig in, I'd really value anyone's expert advice on debugging this - or just curious to know if anyone's ever had this happen before.  I've never seen this, but many (most) of you have diagnosed way more amps than I have!


Mark


About 1 minute into the vid, it stops and was fine from that time until they finished this gig.  Next it happened a couple weeks later.


https://www.youtube.com/watch?v=S9HNu31IgZA

[John]

I seem to recall someone, Tubenit maybe? had a similar problem he talked about. Turned out it was a bad resistor lead, it usually made contact with the body of the resistor,but not always. Just a wild guess of something to think about. Those intermittent problems are da woist, Jerry, da woist!

[labb]

Wiggle the pre amp tubes and see if it affects the static.

[tubenit]

The most difficult problem I have had on an amp was the first tube amp I worked on.

It was a perfectly fine looking resistor (brand new) that would test fine with voltmeter. 

However, it had a break in the resistor wire inside the resistor body that would open and shut with only certain bass notes playing.  I could not reproduce it chopsticking the amp either.

Because it tested continuity/resistance most of the time,  I had a terrible time finding the problem.  Replacing the resistor resolved the issue permanently. I am not thinking this is the problem with your amp however.

I have had amps with oscillation problems stop working and no sound.  Replacing the power tubes resolved those issues. The tubes were JJ EL84's  & replacing them with Hoffman's  Sovtek EL84's resolved the issue

With respect, Tubenit

[markmalin]

Thanks, guys.  It should be an adventure tracking it down, to say the least.  My only encouragement is they were having it happen more often.  I think I'll let it idle a couple hours and see if it exhibits the problem.  If so, play around with things like wiggling tubes, messing with the volume(s) to see if it changes the static volume, mess with the switches, etc.  Then pull the chassis and start poking around with a chopstick.

[sluckey]

Starting with the tubes is a good idea. I would pull a tube (one at a time) and reseat it. Do this several times to kinda wipe the tube sockets. Spray some deoxit (or other electronics cleaner) on the tube pins if you have any. Repeat for every tube. Then wait. The waiting is the hard part.

[PRR]

Chopstick.

[markmalin]

I appreciate the input, guys.  I get the feeling this is going to be a tough one.


Mark

[Paul1453]

Intermittent problems are often connection issues.

Sometimes they make good contact and then they don't.

Like Tubenit's intermittent resistor, they can be frustratingly hard to identify.

Consider reflowing solder connections, among the other suggestions.   

[PRR]

After chopstick--

Long ago we bought strong Freon in a can. Spurt a connection and if it just-touches at room temp, it may contract and not-touch when very cold. Oh the pounds of Freon we used to spray!! Maybe there is some cold-in-a-can which is still acceptable.

Never wrong to take it out in good sun with a magnifier. Most solder joints should be inspectable-- you can SEE that the solder WET both/all conductors. Or not. Or if in doubt, suck the solder off, scrub the surfaces, flux, and solder carefully, watching the wetting action.

[John]

Oh the pounds of Freon we used to spray!!

That kept global warming at bay.

[markmalin]

After chopstick--

Long ago we bought strong Freon in a can. Spurt a connection and if it just-touches at room temp, it may contract and not-touch when very cold. Oh the pounds of Freon we used to spray!! Maybe there is some cold-in-a-can which is still acceptable.

Never wrong to take it out in good sun with a magnifier. Most solder joints should be inspectable-- you can SEE that the solder WET both/all conductors. Or not. Or if in doubt, suck the solder off, scrub the surfaces, flux, and solder carefully, watching the wetting action.

I actually do have a can of freeze spray.  So are you saying, inspect the solders and if one looks questionable try to freeze it?


Incidentally...back in the day I worked in the Space Science and Engineering Center at the University of WI...soldering space flight hardware no less.  I used to "wash down" the wiring harnesses with freon, which when I mentioned my concerns to my then supervisor he said "nonsense...back when I worked at Lockheed in the 70's we used to fill entire hangers with freon for testing, and when we were done, just opened the hanger door and let it all out".

[shooter]

looks questionable try to freeze it?

Yup, works good for Iffy R's, transistors, IC's etc.

If you buy a can of *canned air* for keyboard cleaning, shake it real good, hold it up-side down, you can get a pretty cold shot of ...CO2?

[PRR]

"Many gas dusters contain HFC-134a...", the stuff which replaced Freon R-12 in air conditioning. It is far from harmless, though much less ozone damage than R-12. Inhalation abuse exists. Some keyboard cleaners are dosed with nasty to reduce this. The main effect of inhalation is simple asphyxiation, though there are some effects on tissues.

The amount of keyboard duster you will use diagnosing an amplifier is far less than the total release of HFC-134 from such uses as airsoft guns and foam blowing.

And yes, bad old Freon was *heavily* used in electronics cleaning before it went out of fashion. I dunno about hangers full, but "dishwasher" use was standard process. My 1970s A/C book suggests disposing of large amounts of dirty/oily Freon in a hole in the ground.

Refrigeration has also used Ammonia (pure, not the dilute water in your kitchen), carbon tet, and several Sulfur compound gasses. Freon got us away from such noxious things.

[shooter]

HFC-134a

ahh, thanks.

That kept global warming at bay.

, I was hoping for the good 'ol days in MI, pre Ice-age, giant ferns, temperate climate, not like the 200ft burg that sat on my home awhile back  

[jojokeo]

Don't forget the input jack wiring to V1 as I had this happen recently. I'd perform a quick mod/change and unsolder a couple of shielded wires and such to get them out of the way so they weren't able to get hit accidently by the iron's shaft. And since they were already soldered with solder left on it's socket or jack I just melt it back on. So I quickly go back and forth to make changes. But it's always a good idea to just touch a new bit of solder back to the joints when doing this. I know better but when trying to be working quick - things like this can happen fairly easily. But they're easy to remedy and after I've finished with things I'll usually go back and re-touch everything with solder again anyway - always a good thing to do especially on new builds anyways.

Also, open the jack up (or others in the amp) with un-chorded plugs in places to see if you imitate the sound you are hearing. The switching jacks can be notorious for not making good connections especially when they're being used (Murphy's law). I've got an amp's inputs based on a JCM800's that does this intermittent thing all the time and it bugs the heck out of me. I've been too lazy to take the amp apart to replace the jack but I know what it is and how it affects things and eventually I'll get around to it. I seem to notice this happening to Cliff jacks more than Switchcraft types?

If you are working with new tubes in an amp then always suspect these also as said by others in this thread. New tubes can always have things happen to them in today's world & manufacturing. Hit them or snap them with a pencil and see if you can jar anything to happen like your video. Replicate the scene of the crime, meaning heat the amp up - 15 minutes should be fine and them snap them a few times each from different angles. If you're lucky and the amp begins doing the noise in front of you I'd immediately start doing this first before any chop sticking. I'm wondering you or the owner have tried just slapping the side or top of the cab and it's done anything to stop it from happening rather than just talking and looking at it?

[Paul1453]

So just slapping the gear can be considered a troubleshooting method?

I've always considered it a frustration venting method.   

But I have seen gear straighten up a fly right after a good slap a number of times.   

[markmalin]

I didn't start slapping gear (ala The Fonze), but I started tapping the tubes rather hard with a chopstick. Pre-amp tube 1 will crackle or pop sometimes. I can imagine that static sound being generated by that.  I'm going to be pulling the chassis this week, but thought it was interesting.  Unless it would be common for a tube to crackle and pop when you flick it with a chopstick.  I've never seen that before.

[John]

Did you try Sluckey's advice of removing and re-inserting the tubes? Contact cleaner on the pins would be good too.

[dude]

I've had that noise before, it was the 1st tube, 12AX7. When I tapped it, I heard all kinds of noise like the tap was amplified. The tube was "microphonic". Some microphonic tubes will make noise (like you have) and not be a problem...? But the louder the noise, the better chance the tube is bad.


Just replace the first preamp tube with known good one if it's microphonic. Another good thing is to recondition the tube sockets with an ice pick, just close the small pin gaps to make the tube fit tighter. Just make sure the caps are drained or a bleeding R is used.


Just to mention most noises like you have are tubes, either bad connection or bad tube.


Good luck,
al

[markmalin]

Ok, so far I've swapped out V1 with a new JJ (not sure I like the sound as well), and re-tensioned the V1 tube socket. Incidentally, these are ceramic sockets.


The other thing I did was blast around with some freeze spray in the first gain stage area.  When I hit the brite cap (silver mica 120pf) it was majorly noisy, which may be normal, but I swapped it with a ceramic and spraying it doesn't make anywhere near as much noise.  I then sprayed the treble cap (500pf mica) and again, pretty noisy. I may swap that with a ceramic as well.


I played the amp loud (painfully loud!  had my earplugs in) for about an hour with no issues, which may mean nothing because it's so intermittent.  I want to let it soak for maybe 4 hours tomorrow to see if I have the noise issue

[jojokeo]

Ok, so far I've swapped out V1 with a new JJ (not sure I like the sound as well), and re-tensioned the V1 tube socket. Incidentally, these are ceramic sockets.

Try the new Tung Sol or Doug's Electro Harmonix if placing an order from him? Ceramic sockets are good, likely not your issue.

The other thing I did was blast around with some freeze spray in the first gain stage area.  When I hit the brite cap (silver mica 120pf) it was majorly noisy, which may be normal, but I swapped it with a ceramic and spraying it doesn't make anywhere near as much noise.  I then sprayed the treble cap (500pf mica) and again, pretty noisy. I may swap that with a ceramic as well.

I personally have had no issues with where I've bought silver mica's but others definitely have here. Not sure where they got them from, but these have been known to cause problems with others here in the past. There's been threads and discussions about them you might try searching for to read for yourself?

I want to let it soak for maybe 4 hours tomorrow to see if I have the noise issue

good idea - "try to repeat the scene of the crime" and even if nothing occurs in front of you try the pencil tapping method again to rule out the tube internal connections deal. Then if you had the silver mica's in place I do more that chopstick and actually grab them with insulated needle nose pliers and tweak the crap out of them and the connections. Sometimes this is what's needed over the chopsticks anyway cause the chopsticks only do so much in this regard.

Lastly - double check your switches for the circuits, especially the pentode/triode switch for intermittent contact. It may be best to do it with your meter either while in standby or with the amp off. Not good to do this while amp is in full operation as you would suspect/know? The reason is that especially with mini-toggles if they are over tightened even slightly (which is very easy with the mini types) they can & will have intermittent problems. It's the way they are constructed, designed, and made which is affected by over tightening.

[markmalin]

Thanks for the ideas on the switches.  And the insulated pliers for pulling on components.  I was torquing pretty hard with the chopstick.  I forgot to mention, I reflowed the solder on all V1 connections since I was in there.  And resolidered a couple suspect looking turrets, though spraying them didn't make any difference, just thought I'd re-solder those few to be sure.


Again, I really appreciate all the input :)

[markmalin]

Ok, I was able to reproduce it once. 


Previously I swapped out the first pre-amp tube with a good one and re-flowed the solder connections around V1.  After that powered it on.
I let the amp sit idle, power on, speaker plugged in, master volume switched "out" (see schematic) all controls on 5, nothing plugged into the input.  I had two lamps shining on the opened chassis on my bench to heat it up as it sat there.


About 1 1/2 hours in the amp started making the noise.  Sounded like putting your mouth up to a mic and blowing in it. This continued about 10 minutes then stopped. I observed the following:
- volume on Top Boost channel affected loudness (i.e. the rushing wind sound still there, quieter as you turn volume down)
- volume on Normal channel did not affect it if I remember correclty, but I'm just recalling that as I'm typing!
- tone controls shapped it (brighter as you turn up treble, etc.) as did the cut control
- switching 1/2 power switch made brought total amp volume down is it should and the noise followed (quieter with the rest of the amp at 1/2 power)
- wiggling tubes did nothing
- tapping tubes with chopstick did nothing
- poking around in the chassis pressing components and wires did nothing
- plugging in a guitar I could play it just fine with the sound happening, but in/out didn't matter
- blasting (briefly) components in the pre-amp section with freeze spray didn't do anything


I measured voltages while this was happening, and remeasured after it stopped.   See below...the parenth's are after it stopped:


V1
pin 1  209 (211)
pin 3   1.4 (1.4)
pin 6  130 (130)


V2
pin 1 303 (303)
pin 2 178 (182)
pin 6 178 (182)
pin 3 179 (182)


V3
pin 1 246 (246)
pin 2 43  (44)
pin 3 64  (64)
pin 8 64  (64)
pin 7 ??   (44)  - forgot to measure ??


Filter caps: 
305 (305)
302 (302)
315 (315)
344 (344)
352 (352)


I did not get the power tubes.  If I can get it to happen again I will measure those.


Here's the thing I don't understand.  I touched my DVM probe to pin 2 of the Phase Inverter (V3) to measure voltage, it went quiet.  Took the probe off and it came back on.  I actually did this after the sound stopped, and the general background hiss of the amp does the same thing.  Touch (+) probe to pin 2 and (-) probe to chassis, and it gets quiet (any background buzz or hiss stops).  Why is this?


Anyhow, I don't know if this is helpful or not, but it's really a puzzle and I need to get this silly thing figured it.  Bad component in the bridge section of V3??  Just general badness in the preamp?  Bad V2 or V3?


Thanks for any help, fellas.

Mark

[sluckey]

Here's the thing I don't understand.  I touched my DVM probe to pin 2 of the Phase Inverter (V3) to measure voltage, it went quiet.  Took the probe off and it came back on.  I actually did this after the sound stopped, and the general background hiss of the amp does the same thing.  Touch (+) probe to pin 2 and (-) probe to chassis, and it gets quiet (any background buzz or hiss stops).  Why is this?

That's normal. Your meter is loading the very high input impedance on that bootstrap biased LTP PI. Take a look at the voltages you posted for V3. Both grids measured about 20v less than the cathodes. If that were really true, V3 would likely be cut off hard and not pass much of any signal. But the grid readings are bogus. I don't even bother taking grid voltage reading on a LTP anymore. It's better to measure the voltage at the common node of those four resistors in the cathode circuit. That is the voltage that is really on the grids and it will be a couple volts less than the voltage on the cathodes for a properly working LTP PI.

Make a note of this fact and nail it on the wall above your workbench (It also applies to bootstrap biased cathodynes or bootstrap biased cathode followers). But don't get side tracked on chasing your noise. This most likely ain't the cause.

[markmalin]

update : I replace V2 and V3, re-installed V1 because the issue happened with a different tube in V1.  I just powered it on and about 45 minutes later my wife yells "...Thar' she BLOWS!"  This time I observed the following:


Power tube voltages:
pin 1  0  (0)
pin 2  0  (0)
pin 3  12 (12)
pin 7 351 (353)
pin 9 344 (347)


measured the voltages on the filter caps again


307
307
320
351
358


Now this time while it was happening I flicked the 1/2 power switch on and off.  fine
Then I flicked the Standby switch on and off a  couple times and the noise stopped and didn't come back  (i.e. I think it was the standby switch that stopped the rush of air noise)

[sluckey]

Power tube voltages:
pin 1  0  (0)
pin 2  0  (0)
pin 3  12 (12)
pin 6 351 (353)
pin 8 344 (347)

You probably meant pins 7 and 9 rather than pins 6 and 8?

[markmalin]

Power tube voltages:
pin 1  0  (0)
pin 2  0  (0)
pin 3  12 (12)
pin 6 351 (353)
pin 8 344 (347)

You probably meant pins 7 and 9 rather than pins 6 and 8?

Thanks, Sluckey.  I edited the message.

[Planobilly]

I had this type of issue with a 18 watt Fender reissue last week. Drove me crazy. I replaced V1 and the noise stopped then it came back. I replaced V3 and it stopped and then it came back. I then took all the pre amp tubes out and cleaned  and retighten ALL the tube sockets and reflowed the tube sockets and replaced ALL the old 12AX7s. I told the amp I would run over it with my truck next...lol I guess it got the message cause there is no more issues.

In general the kind of noise I heard on your video is almost always tube issues. For sure other things are possible. Small changes in voltages at the tubes will not produce that type of noise in my experience.

Just a side note: I use to buy tubes from my wholesale supplier. I got tired of good stuff one day and crap the next. I now buy almost all my tubes from Eurotubes and have not had one bad tube yet from those guys. I have ordered over a hundred tubes from Eurotubes to date. I repair guitar amps every day so I got no time for BS with tubes. Every tube MFG makes crap and good stuff, China included. It is not so much about the MFG but how well they are tested. ALL the tubes I get from Eurotube are tested and matched for what (I) am looking for...ALL even the 12AX7s.

And of course different brands of tubes are different in some cases. For example a Solvtek GZ34 will put out more voltage than a JJ. Install a Solvtek and the amp will sound more like a silcon diode based amp because there is less sag.

I have good scopes and other test equipment but for noise I normally use a simple probe with a .001 cap hooked up to a small amp to listen to what is going on in the amp. This is a very cheep tool to make and very useful.

Cheers,

Billy

[tubeswell]

dodgy intermittent plate resistor maybe?

[markmalin]

Thanks for the info  guys.  Tubeswell, I had thought of that. Planobilly, thanks for the tip on Eurotubes.  I will take this advice in the future - just bought a set of JJ EL84's and a new rectifier from CE Dist (which is just down the street so I don't have to wait)


Last night I set up the experiment again, but first replaced the rectifier with a solid state plug in (Yellowjacket), and have different tubes V2 and V3.  I retensioned the pre-amp sockets, and retensioned the rectifier socket.  I left the power tubes alone and plan to repeat with the new set this evening.


53 minutes in and "thar she blows!".  While it was happening I removed the clips on the power tubes and moved them around a lot.  Anyhow, at least I know it's not the pre amp tubes and sockets, and not the rectifier and socket.  I'd like to re-do the experiement with new power tubes tonight and if it goes off again 1 hour in, I will try grounding the pre-amp tube grids one at a time to see if I can isolate it more.  I'm open to other suggestions guys :)


Mark

[tubeswell]

The only other things I can think of (apart from a dodgy plate resistor) are either:


random static/crackling can be symptomatic of bad/old output tubes


A bad presence pot on your LTP (if you have a presence pot on a LTP)

[markmalin]

The only other things I can think of (apart from a dodgy plate resistor) are either:


random static/crackling can be symptomatic of bad/old output tubes


A bad presence pot on your LTP (if you have a presence pot on a LTP)

There is a presence pot on the circuit.  The tubes are new, though.  I can look at the pot

[jojokeo]

I've been suspecting the power tubes for a while now. I thought you would be giving them the pencil test also a while ago? When you have that noise going and your tone stack was exhibiting normal behavior it told me your preamp was functioning properly. This left your power section.
Just recently I was playing a custom amp I made a while back and out of nowhere it began acting weird. I don't remember it doing what yours is and sorry I can't remember exactly what it was doing - but I look behind to find one of the newer Tungsol 6V6 tubes red-plating. There was nothing wrong with bias (on the colder side so not being pushed or stressed) or parts or anything else - just the tube ended up giving it up. New Groove Tubes on hand and things are back to normal.

[markmalin]

I've been suspecting the power tubes for a while now. I thought you would be giving them the pencil test also a while ago? When you have that noise going and your tone stack was exhibiting normal behavior it told me your preamp was functioning properly. This left your power section.
Just recently I was playing a custom amp I made a while back and out of nowhere it began acting weird. I don't remember it doing what yours is and sorry I can't remember exactly what it was doing - but I look behind to find one of the newer Tungsol 6V6 tubes red-plating. There was nothing wrong with bias (on the colder side so not being pushed or stressed) or parts or anything else - just the tube ended up giving it up. New Groove Tubes on hand and things are back to normal.

I did whack them pretty good with a chopstick while it was happening.  Also, last time I unlatched them from the tube holders and wiggled them around pretty good.  I'll try a new set, though.  I wonder about them as well...  Thanks for the tip!

[jojokeo]

I did whack them pretty good with a chopstick while it was happening.  Also, last time I unlatched them from the tube holders and wiggled them around pretty good.  I'll try a new set, though.  I wonder about them as well...  Thanks for the tip!

It's interesting that you have to let it bake for around that 1 hour time frame until it wants to get your attention? Can you be sure that all tubes have a grid return resistor on them? I don't know since I haven't seen your internals but sometimes it can be overlooked especially when modifying and customizing things. The tube and amp will usually initially perform fine but then it will being to drift and lose bias. Then you could lose sound altogether or take on a behavior of going in & out like it's trying to keep it's signal flowing.
On power tubes you don't want to exceed recommendations depending on fixed or self biased conditions. Many people install 1M PPIMVs without thinking about this and some tubes/amps could be more sensitive to this being out of the requirement spec?

[markmalin]

I did whack them pretty good with a chopstick while it was happening.  Also, last time I unlatched them from the tube holders and wiggled them around pretty good.  I'll try a new set, though.  I wonder about them as well...  Thanks for the tip!

It's interesting that you have to let it bake for around that 1 hour time frame until it wants to get your attention? Can you be sure that all tubes have a grid return resistor on them? I don't know since I haven't seen your internals but sometimes it can be overlooked especially when modifying and customizing things. The tube and amp will usually initially perform fine but then it will being to drift and lose bias. Then you could lose sound altogether or take on a behavior of going in & out like it's trying to keep it's signal flowing.
On power tubes you don't want to exceed recommendations depending on fixed or self biased conditions. Many people install 1M PPIMVs without thinking about this and some tubes/amps could be more sensitive to this being out of the requirement spec?

Have a look at the schematic I posted.  I know I was messing around with different cathode bypass caps and resistor values to tame down the gain, but the rest of the circuit should be close.  I will double check tonight (this schematic and my chassis).  This amp (and schematic) has a pre PI MV...not a Post PI MV...and there is a switch to bypass it.  I have been testing it with the MV switched out of the circuit (i.e. bypassed) because the amp was being used that way in the field so I wanted to try to re-create the situation as best I could.


As far as grid return resistors, does the 33K 1M bridge on the input jack act as a grid return?  jojokeo, I'm going to have to be honest and say I haven't deviated much from the stock AC 30 because I'm no E.E. by any stretch, so not adept at circuit design.  That said, I'm trying to learn whatever I can...and this is presenting an obvious learning experience!

There are currently 2 of these amps in the field and this one particular player was having the issues, the other guy records with his and plays live with no issues.  However...perhaps using the MV vs not using the MV is affecting the first section of the LTPI?

(Geeze....I'm looking at the schematic I posted and I see one thing that's not per the final circuit.  The Cut control has been modded on the amp such that the wiper of that pot is connected to lower half of the pot...the lower terminal on the pot is not floating per the posted schematic.)

[jojokeo]

The cut control is fine either way and your grid resistors are fine also.  For some reason I didn't see that schematic earlier? However the schematic and the persons build don't always match up being the same thing and that's where problems lie which is why it's always a good thing to be able to get close-up pictures of the internal areas when helping with troubleshooting. Hopefully it's only a power tube and you can be done with this one?!

[markmalin]

I think I may have found the issue.  A buddy of mine with ions more experience than me came over and as we talked he figured since the Top Boost volume controlled the noise along with the overall volume, it must be in the pre-amp section.  So we let the amp idle with the lamps on it and while it was heating up he suggested we put a scope probe on the wiper of the Top Boost volume pot (between C3 and C4).  33 minutes later the amp acted up and we observed a DC voltage at that point that wasn't there before!  When the amp finally stopped making noise, the DC voltage dropped.  It looks like C3 is leaking DC.  It's too late to test right now, but tomorrow I'm going to swap out that 500p mica cap for another and repeat my tests to see if that did it. 

[Paul1453]

An intermittent leaky cap?

Nice.  That had to be an elusive little sucker to track down.   

An old domino style silver mica cap?

[markmalin]

An intermittent leaky cap?

Nice.  That had to be an elusive little sucker to track down.   

An old domino style silver mica cap?

It's a silver mica, yes.  The black kind - I'm not sure what brand.  What's bizarre is once it happened, then stopped, I couldn't reproduce the issue until the amp was powered down and cooled down.  I have to credit my friend Bob, though.  He's a retired EE :)  Had he not come over to help, it would have taken me another couple weeks!

[jojokeo]

The other thing I did was blast around with some freeze spray in the first gain stage area.  When I hit the brite cap (silver mica 120pf) it was majorly noisy, which may be normal, but I swapped it with a ceramic and spraying it doesn't make anywhere near as much noise.  I then sprayed the treble cap (500pf mica) and again, pretty noisy. I may swap that with a ceramic as well.

I personally have had no issues with where I've bought silver mica's but others definitely have here. Not sure where they got them from, but these have been known to cause problems with others here in the past. There's been threads and discussions about them you might try searching for to read for yourself?

Man I thought you had changed the silver micah's with ceramics after these posts to eliminate them from the equation? Either way - This is very likely IT!? 

[markmalin]

The other thing I did was blast around with some freeze spray in the first gain stage area.  When I hit the brite cap (silver mica 120pf) it was majorly noisy, which may be normal, but I swapped it with a ceramic and spraying it doesn't make anywhere near as much noise.  I then sprayed the treble cap (500pf mica) and again, pretty noisy. I may swap that with a ceramic as well.

I personally have had no issues with where I've bought silver mica's but others definitely have here. Not sure where they got them from, but these have been known to cause problems with others here in the past. There's been threads and discussions about them you might try searching for to read for yourself?

Man I thought you had changed the silver micah's with ceramics after these posts to eliminate them from the equation? Either way - This is very likely IT!? 

I just changed the one cap, the 120 uf brite cap.  I didn't change the other one.  So fingers crossed, I'm going to swap it out and let it idle for a few hours :)

[jojokeo]

But the bright cap doesn't do anything to keep DC away. It's only a treble bleed cap retaining your high end by allowing the high frequencies to pass the volume pot's resistance as it's turned down. It's the caps in the tone stack circuit which serve the purpose of frequency response and DC blocking. The signal leaving any tube's plate (aside from the power tubes which go directly to the output transformer) must pass entirely through a capacitor or several caps if a resistor is involved (ie the tone stack) so that no part of a signal is allowed to continue further without going through a cap at some point. Or DC will find a way to get to where it's not suposed to be and mess up your biasing. The infamous scratchy pot symptom like certain presence controls have is a dead giveaway.

[markmalin]

But the bright cap doesn't do anything to keep DC away. It's only a treble bleed cap retaining your high end by allowing the high frequencies to pass the volume pot's resistance as it's turned down. It's the caps in the tone stack circuit which serve the purpose of frequency response and DC blocking. The signal leaving any tube's plate (aside from the power tubes which go directly to the output transformer) must pass entirely through a capacitor or several caps if a resistor is involved (ie the tone stack) so that no part of a signal is allowed to continue further without going through a cap at some point. Or DC will find a way to get to where it's not supposed to be and mess up your biasing. The infamous scratchy pot symptom like certain presence controls have is a dead giveaway.

Right.  I understand that now.  The cap that was leaking is 500pf in the tone stack, not the brite cap.  So with the scope on the wiper of the volume pot on the Top Boost channel we were observing a DC shift as soon as the noise started.


I'm not familiar with the scratchy presence pot symptom.  What exactly is that? (shifting in to learning mode...)

[tubeswell]

I'm not familiar with the scratchy presence pot symptom.  What exactly is that? (shifting in to learning mode...)

On some amps (like the classic 5F6A), the presence control is inherently scratchy because the pot isn't DC-isolated.


(But that isn't why I mentioned a dodgy presence control earlier. Sometimes if you have a bad pot, or if you have let solder/hot rosin flow into the pots innards when soldering, it can wreak havoc with random voltage surges through the pot, causing random hiss/static/microphonics, because this pot is in the NFB loop on most amps, and where it is in the LTP tail, that can be a particular mode of grief (if the pot is dodgy) . This happened in one of my builds once. Only way to be sure that the pot isn't a problem is to change it for a fresh pot and see if the problem persists.)

[markmalin]

Ok guys.  I just let the amp idle for 2 hours and 38 minutes and no "raging flood" noise :)  I let it idle probably 2:15 and played through it the last 23 minutes with no issues.  I need to demo it tomorrow to a guy, but this weekend I'll let it run maybe 4 or 5 hours to simulate a typical gig + rehearsal amount of time.  Then I'll exhale....

[John]

Glad you found the problem, and thanks for the follow up!

[sluckey]

A voice activated recorder would come in handy on this. Just turn the amp on and put the recorder near the speaker. Walk away and check the recorder tomorrow.

[markmalin]

A voice activated recorder would come in handy on this. Just turn the amp on and put the recorder near the speaker. Walk away and check the recorder tomorrow.

Interesting.  I didn't know there was such a thing.


I've been letting the amp sit at 1/2 volume (SUPER LOUD with 4 EL84's!) so it just sits there generating background noise until the cap leaks, at which time it sounds like a guy doing a white noise test on a PA system.  Scares the cr@p out of me every time, no matter where I am in the house