Calling amp experts!

[ElNumero]

Plugged into my Peavey Delta Blues last night. Everything sounded normal for about 2 minutes, then the sound diminished to a faint whisper.  I attacked the strings with heavy picking and there was a loud crackling followed by a momentary restoration of sound, but it didn’t last.  It again faded down to a whispering level.  Ruled out bad cable.  Thinking bad tube(s)?   Where to begin………? Tubes are 3 12ax7 and 4 EL84
 

 

[kbp810]

Sounds like a tubes dead or dying

Could be power or pre… but I’d start with preamps; if you’ve got a spare or one you can borrow, I’d start by swapping preamp tube 3 first, if that doesn’t resolve, then try 2, then 1. 
 

While you’re testing/swapping; if issue does persist, look at the tubes as you try striking a note; see if any seem to red plate or if the glow flickers in line with the crackle; that could possible show you right off bat which the bad tube is. 

[212Mavguy]

KBP went the front end route...great.  He knows.

From the back end, the power section...  Make sure all four of the power tubes are straight in their sockets.  Look for a bit of blue glow in all of them when taken from standby to operate.  A lightning show with loud crackling means shut it off NOW.  Could be a short inside the tube.  If you got to see the light show remove the tube(s) that did the lightning show to the exhibition cabinet or safe disposal.  

Could be that a tube went out and took out a screen resistor.  Should not be a horribly difficult repair with that amp in worst case.  If it were mine and it went silent or nearly so I'd yank the power tube glass and measure voltages at the pins of each tube socket in the power section.  Tubes die.  Power sections get hurt more often than the preamp, power tubes are more delicate as a tendency than pre's are.

[ElNumero]

55 minutes ago, kbp810 said:

Sounds like a tubes dead or dying

Could be power or pre… but I’d start with preamps; if you’ve got a spare or one you can borrow, I’d start by swapping preamp tube 3 first, if that doesn’t resolve, then try 2, then 1. 
 

While you’re testing/swapping; if issue does persist, look at the tubes as you try striking a note; see if any seem to red plate or if the glow flickers in line with the crackle; that could possible show you right off bat which the bad tube is. 

Thanks!

[ElNumero]

44 minutes ago, 212Mavguy said:

KBP went the front end route...great.  He knows.

From the back end, the power section...  Make sure all four of the power tubes are straight in their sockets.  Look for a bit of blue glow in all of them when taken from standby to operate.  A lightning show with loud crackling means shut it off NOW.  Could be a short inside the tube.  If you got to see the light show remove the tube(s) that did the lightning show to the exhibition cabinet or safe disposal.  

Could be that a tube went out and took out a screen resistor.  Should not be a horribly difficult repair with that amp in worst case.  If it were mine and it went silent or nearly so I'd yank the power tube glass and measure voltages at the pins of each tube socket in the power section.  Tubes die.  Power sections get hurt more often than the preamp, power tubes are more delicate as a tendency than pre's are.

Thanks!

[yoslate]

Will, you called "the amp experts," and you got 'em.  A forum couldn't ask for two better resources!

[ElNumero]

So I removed the tube protector cage to get a better look.  All the tubes are glowing orange and there does not seem to be any difference in intensity.  However I noticed that the two panel button switches, one for channel and one for boost seem to cause the amp to go quiet intermittently.  Wondering if I should spray something in them. 

[Steiner]

1 hour ago, ElNumero said:

So I removed the tube protector cage to get a better look.  All the tubes are glowing orange and there does not seem to be any difference in intensity.  However I noticed that the two panel button switches, one for channel and one for boost seem to cause the amp to go quiet intermittently.  Wondering if I should spray something in them. 

Do you have a garden hose?

[ElNumero]

2 hours ago, Steiner said:

Do you have a garden hose?

Greg, is Deoxit the recommended spray for amplifier controls and switches? 

[Steiner]

1 hour ago, ElNumero said:

Greg, is Deoxit the recommended spray for amplifier controls and switches? 

I don't use it in controls.  They're supposed to have enough lube to last a lifetime.  Deoxit is good to clean tube contacts.

[High Flying Bird]

5 hours ago, Steiner said:

Do you have a garden hose?

Yeah Will, keep a hose handy for clean up after you pour some alcohol in the switch and light a match to it.  We use contact cleaner on switches.  That does wonders. 

[Gianluca]

not an amp expert so I'd take my amp to a tech

that said, could it be a capacitor?

[kbp810]

14 hours ago, ElNumero said:

So I removed the tube protector cage to get a better look.  All the tubes are glowing orange and there does not seem to be any difference in intensity.  However I noticed that the two panel button switches, one for channel and one for boost seem to cause the amp to go quiet intermittently.  Wondering if I should spray something in them. 

I still feel like it might be a preamp tube. I'd still want to borrow a known good working preamp tube from another amp and try swapping it around in the peavy. At the very least it will rule them out. Quick/easy check before diving down the troubleshooting rabbit hole any further. If you have another set of El84's you could temporarily borrow, worth a quick swap/trial there too. 

Anything beyond that, next step would be opening it up to start inspecting the circuit; not sure what your comfort level is here, but the ever important precursor to this is draining the caps before you do any form of poking around... if you're not feeling good about it, I'd stop here and take it to a tech. 

Else... 

For visual inspection, I'd start by looking at the components around the power tube socket; as Mav said above, a blown screen resistor is a possible suspect, and isn't all that uncommon to happen - you'll notice the burn marks around the resistor (if it hasn't popped like a fuse entirely). Next, I'd look at the large capacitors and see if any have swelled or appear to be leaking. 

If nothing is visually obvious; well, then it's multimeter and measuring time. Maybe we'll cross that path again with a deeper dive if/when it comes to it, lol. 

[212Mavguy]

+1 for preceding post... times many.  Careful approach with safety is correct.

On those switches, yes, contact cleaner...wiggle them when wet then let them dry a good hour.  

[DetroitBlues]

Try the tubes first, first thing I'd do.    

That's an old dusty amp you have there.  Probably just too much dust in them controls......

https://www.stewmac.com/electronics/amps/components-and-parts/cleaners-and-lubricants/caig-deoxit-pot--switch-cleaner/

 

[Spectrum13]

Check the infinite flux capacitor. Set to 88 MPH use a variac powered to 1 Gigawatt.

 

See you back in 1955! 

[HANGAR18]

https://robrobinette.com/Tube_Guitar_Amp_Troubleshooting.htm

[ElChoad]

If it were me, I would start by putting all new tubes in. Tubes are pretty cheap, all things considered, unless you are a snob like me that goes for NOS '60s tubes...

[Kuz]

When that same issue happened to me, it was the rectifier tube.  Replaced it and proceeded to rock on, completely fixed.  

[pressure]

I have had a power tube and rectifier tube go out together. I think one tube took out the other but I don't know which one went first.

[ElNumero]

On 1/18/2022 at 10:33 AM, 212Mavguy said:

+1 for preceding post... times many.  Careful approach with safety is correct.

On those switches, yes, contact cleaner...wiggle them when wet then let them dry a good hour.  

WIggle when wet, I ain't gonna touch that one..............

[ElNumero]

UPDATE: I worked the knobs out really good, and waited a day, then plugged it back in and problem seems to be solved. I need to road test it a few more times before I reinstall the tube protector cage. With my luck, as soon as I reinstall it, BAM the problem will happen again!

[212Mavguy]

On 1/24/2022 at 1:13 PM, ElNumero said:

WIggle when wet, I ain't gonna touch that one..............

For once I wasn't misbehaving.  Glad someone took up the slack!  Gladder that you got it going well.  If you used an appropriate cleaner on those pots go ahead and put it back together, if just rotating them back and forth with no cleaner  put it back together, but it will get noisy again sooner than if you used an appropriate cleaner.

[Spectrum13]

Alls well that ends well