Tube amp goes quiet, then back to normal

hatamoto · #1 07-29-2022, 03:00 AM

My Blackstar Studio 10 is suddenly fluctuating volumes. It would slowly go quiet, then back to normal volume at random intervals.

I doubt tubes are the culprit because they are new, and my amp is not being played much lately. I'm guessing dust somehow got inside and affected the connections, but I don't know where to begin.

Any ideas on where I should start investigating?

rmp · #2 07-29-2022, 04:34 AM

Hmm,, so that does at first glance sound like power tubes, It's entirely possible you got a bad one. How do the power tubes look when it drops volume?

Is it happening ALL the time or now and then?

about the Power tubes, do they still glow consistently when the volume drops?

And of course, there should be kind of a warm "orange" glow. If you see any "blue", that would be something pointing to an incorrect Bias setting.

Start with reseating all the tubes, and when you fire it up, leave it standby for full minute or 2.

When powering it off, drop it to standy for about 30 secs at least before you shut the amp off.

Just best practices really, I don't think not doing so will HARM anything.

I had a GoldTone Class A combo that I learned was kind of picky about this tho and now and then, I'd hear a drop in volume too. So I got in the habit of doing this at power on and power off.

dnf777 · #3 07-29-2022, 05:14 AM

What Ray said. And it could be ANY tube, not just a power tube that is bad.

Id say take them to your local Walgreens and put them on a tube-tester….but that would age me

jonfields45 · #4 07-29-2022, 05:44 AM

If reseating the tubes doesn't fix it, I would next look for a cold solder joint on one of the cathode resistor bypass capacitors. You'll need to take it to a tech if that suggestion needs explanation...

Dru Edwards · #5 07-29-2022, 10:21 AM

Quote:

Originally Posted by dnf777

What Ray said. And it could be ANY tube, not just a power tube that is bad.

Id say take them to your local Walgreens and put them on a tube-tester….but that would age me

LOL Dave! That made me think of the reverse. "Back to the Future" - the Doc said "I'm sure in 1985, plutonium is available at every corner drugstore, but in 1955 it's a little hard to come by."

As mentioned, it may be one of the tubes. It's easier to troubleshoot if you have a spare preamp and a spare power tube.

jseth · #6 07-29-2022, 03:28 PM

I had a very similar occurrence with my 1976 Deluxe Reverb... turned out it was just REALLY dusty and dirty inside!

The "amp savant" I found to fix it said he just cleaned thoroughly one day... and then went back the next day and cleaned thoroughly, all over again!

Although my Deluxe is pretty old, it had really only been about 7 years since someone had been inside of it... and mine was never gigged with or even taken out of the house!

One important thing - if you have to handle the tubes, you do NOT want any fingerprints on them when you put them back in the amp! The oil from your fingerprints can cause tubes to burn out pretty quickly. Clean them off with a little alcohol and a paper towel and you're good to go!

jonfields45 · #7 07-29-2022, 03:40 PM

Quote:

Originally Posted by jseth

I had a very similar occurrence with my 1976 Deluxe Reverb... turned out it was just REALLY dusty and dirty inside!

The "amp savant" I found to fix it said he just cleaned thoroughly one day... and then went back the next day and cleaned thoroughly, all over again!

Although my Deluxe is pretty old, it had really only been about 7 years since someone had been inside of it... and mine was never gigged with or even taken out of the house!

One important thing - if you have to handle the tubes, you do NOT want any fingerprints on them when you put them back in the amp! The oil from your fingerprints can cause tubes to burn out pretty quickly. Clean them off with a little alcohol and a paper towel and you're good to go!

IMHO, plus a graduate EE degree, sometimes things are just intermittent for reasons not determined. In that case, charge for cleaning.

Fingerprints are bad news on a projector bulb but no problem for the vacuum tubes used in guitar amps.

srick · #8 07-29-2022, 04:15 PM

One old trick is to find a pencil with a big eraser and when the problem occurs, lightly tap each tube and see if anything changes.

Rick

Bob Womack · #9 07-29-2022, 04:46 PM

Another easy fix is to turn off the amp and let it cool. Now, one by one, remove the tubes. Remember to pull out the tubes with bases by pulling on the base only - not the glass envelope. Take a Q-tip and swab each of the legs of the tube with Caig DeOxIt and then push the tube in and out of the socket to clean the socket as well. Alternatively, you can put electrical tape over one end of a pipe cleaner, spray the other end with DeOxIt, and swab out the tube sockets. Carefully re-align the tube over the socket and insert.

That might help a wee bit.

Bob

dnf777 · #10 07-29-2022, 04:57 PM

Quote:

Originally Posted by jonfields45

Fingerprints are bad news on a projector bulb but no problem for the vacuum tubes used in guitar amps.

IME, fingerprints on tubes are more a problem for the fingers that put them there!

hatamoto · #11 07-29-2022, 09:18 PM

@ rmp

I took it apart to see and it's indeed a steady, warm orange glow. I reseated both the preamp and the power tube, gently blew on the sockets and just wiped the tubes with cloth. I played it briefly with the preamp out and I haven't experienced any volume drop so far. Hopefully it's just this. Time will tell.

Thanks for the suggestions guys. I do have some spare tubes so I'll replace if really needed. Is it common practice to replace both the preamp and the power tube, or can I just replace one?

rmp · #12 07-30-2022, 08:02 AM

Quote:

Originally Posted by hatamoto

@ rmp

I took it apart to see and it's indeed a steady, warm orange glow. I reseated both the preamp and the power tube, gently blew on the sockets and just wiped the tubes with cloth. I played it briefly with the preamp out and I haven't experienced any volume drop so far. Hopefully it's just this. Time will tell.

Thanks for the suggestions guys. I do have some spare tubes so I'll replace if really needed. Is it common practice to replace both the preamp and the power tube, or can I just replace one?

Sounds good so far hatamoto. maybe something just needed to be reseated

if the amp is moved around lot, there could be something going there where reseating things can help

hope it works out. getting amps repaired and finding a good amp tech can be a bit of a bother.

ChrisN · #13 07-30-2022, 08:46 AM

Quote:

Originally Posted by hatamoto

@ rmp

I took it apart to see and it's indeed a steady, warm orange glow. I reseated both the preamp and the power tube, gently blew on the sockets and just wiped the tubes with cloth. I played it briefly with the preamp out and I haven't experienced any volume drop so far. Hopefully it's just this. Time will tell.

Thanks for the suggestions guys. I do have some spare tubes so I'll replace if really needed. Is it common practice to replace both the preamp and the power tube, or can I just replace one?

Someone has to mention the potential life-threatening voltages (400+) that dwell within the amp, in its capacitors that store juice. Touching the wrong thing (and there are several of them under the hood, not easily recognizable) can kill you. Folks opening the amp should learn about discharging capacitors as a first step to any investigation, verify discharge with a multimeter, and even then, the underinitiated should consider working via plastic/wooden stick when poking/prodding, working with one hand behind their back, etc. Safety first, and amps can be dangerous.

PTony · #14 07-30-2022, 09:37 AM

Quote:

Originally Posted by ChrisN

Someone has to mention the potential life-threatening voltages (400+) that dwell within the amp, in its capacitors that store juice. Touching the wrong thing (and there are several of them under the hood, not easily recognizable) can kill you. Folks opening the amp should learn about discharging capacitors as a first step to any investigation, verify discharge with a multimeter, and even then, the underinitiated should consider working via plastic/wooden stick when poking/prodding, working with one hand behind their back, etc. Safety first, and amps can be dangerous.

+1…nothing to fool with.

clintj · #15 07-31-2022, 08:54 PM

Regarding blue glow in power tubes, a light blue glow on the gas is normal and was considered a sign of excellent vacuum by the likes of Sylvania and other tube makers. It's from high energy stray electrons striking the glass envelope and causing it to fluoresce. A broader violet glow is a sign a tube is becoming gassy. Amps that run larger tubes like EL34 and 6L6 at high plate voltage, like my Super Reverb, show this very prominently in a dark room.