Symptoms of Deteriorating Preamp Tubes

pshep68 · #1 01-23-2013, 08:09 AM

What are some signs that your preamp tubes are deteriorating? I've been noticing that my Ampeg 3 Pro isn't as robust and full sounding as of late.

Bassmec · #2 01-23-2013, 09:28 AM

Quote:

Originally Posted by pshep68

What are some signs that your preamp tubes are deteriorating? I've been noticing that my Ampeg 3 Pro isn't as robust and full sounding as of late.

There are all sorts of different performance losses with failing pre amp tubes.
Microphonics causing feedback, increased level of general noise, loss of MU as in amplification factor and so forth, the only thing to do is get a known good tube and substitute it to see if it improves anything.

Sid Fang · #3 01-23-2013, 09:40 AM

Pre-amp tubes usually have VERY long lifetimes, unless they are wildly overdriven, as happens in some guitar "lead" circuits, but rarely in a bass rig. I suppose you might have a defective one. The effect on sound would depend on the stage of the tube, and yes, one failure mode could be a massive loss of gain. The classic no-tech-tools thing to do to test would be to rotate tubes of the same type between sockets to see if the trouble tracks/changes, but I hesitate to suggest that to anyone who's not already used to working with high voltage electronics.

chaosMK · #4 01-23-2013, 09:45 AM

Fortunately they are cheap. I love JJ's for $10 a piece. Trying out some higher gain long plates ($13) in a few amps I own soon.

I've never had a preamp tube go out me. I have had amps come with generic chinese tubes I had to replace.

If your tone is feeling weak, it could be anything.

craig.p · #5 01-23-2013, 10:09 AM

Quote:

Originally Posted by chaosMK

If your tone is feeling weak, it could be anything.

Actually I think that's the best way to describe the symptoms of an off-spec preamp tube. It feels like the amp is flat-sounding, 2-dimensional, like it's struggling to do anything. Not distortion like you'd get with a failing power section, but instead this sort of "meh" quality about the sound.

B-string · #6 01-23-2013, 10:22 AM

The symptoms you describe could also be drifted bias on the MOSFET output. The adjustment is VERY touchy and requires great care to perform. Jerrold Tiers has covered this many times, a search or PM him?

seamonkey · #7 01-23-2013, 10:22 AM

Look on CL for a hobbyist that collects and works on tube stuff. HAMM radio boards are full of them also although the tube gear is getting rare.

Ask if they have a tube tester and ask them to test your tubes.

You may in fact like the sound of tubes that FAIL testing. At least you'll know. And you'll know what side of fail you like.

JimmyM · #8 01-23-2013, 10:32 AM

Tube gear is getting rare...BWAHAHAHAHAHA!!!

Mr. Foxen · #9 01-23-2013, 10:43 AM

How old is the amp? Seeing failing caps in some 90s Ampeg stuff.

beans-on-toast · #10 01-23-2013, 10:52 AM

The Nature Of Tubes: Why (And When) To Replace Them: ...according to Ampeg's SVT-VR user's manual
Tubes are made up of a number of fragile mechanical components that are vacuum-sealed in a glass envelope or bubble. The tube's longevity is based on a number of factors which include how hard and often the amplifier is played, vibration from the speakers, road travel, repeated set up and tear down, etc. Any time you notice a change in your amplifier's performance, check the tubes first. If it's been a while since the tubes were replaced and the sound from your amplifier lacks punch, fades in and out, loses highs or lows or produces unusual sounds, the power tubes probably need to be replaced. If your amplifier squeals, makes noise, loses gain, starts to hum, lacks "sensitivity", or feels as if it is working against you, the preamplifier tubes may need to be replaced.

With time, your jacks and tube socket can develop a layer of oxidization or accumulate dirt. Cleaning them periodically with a contact cleaner such as Deoxit helps.

I like to keep what is called a gold standard tube in reserve. This is a known good tube that you only use for testing purposes. If you feel that something is lacking in the way that your amp is performing, plug in the gold standard tube and see if it clears up the problem. If it does, replace your old tube. If not, it is something else causing the problem. This is an easy way to check out your amp and it, along with your ears, serve as a tube tester.

B-string · #11 01-23-2013, 11:13 AM

Quote:

Originally Posted by beans-on-toast

The Nature Of Tubes: Why (And When) To Replace Them: ...according to Ampeg's SVT-VR user's manual
Tubes are made up of a number of fragile mechanical components that are vacuum-sealed in a glass envelope or bubble. The tube's longevity is based on a number of factors which include how hard and often the amplifier is played, vibration from the speakers, road travel, repeated set up and tear down, etc. Any time you notice a change in your amplifier's performance, check the tubes first. If it's been a while since the tubes were replaced and the sound from your amplifier lacks punch, fades in and out, loses highs or lows or produces unusual sounds, the power tubes probably need to be replaced. If your amplifier squeals, makes noise, loses gain, starts to hum, lacks "sensitivity", or feels as if it is working against you, the preamplifier tubes may need to be replaced.

With time, your jacks and tube socket can develop a layer of oxidization or accumulate dirt. Cleaning them periodically with a contact cleaner such as Deoxit helps.

I like to keep what is called a gold standard tube in reserve. This is a known good tube that you only use for testing purposes. If you feel that something is lacking in the way that your amp is performing, plug in the gold standard tube and see if it clears up the problem. If it does, replace your old tube. If not, it is something else causing the problem. This is an easy way to check out your amp and it, along with your ears, serve as a tube tester.

Actually the BEST kind of tube tester available anywhere.

pshep68 · #12 01-23-2013, 11:31 AM

Quote:

Originally Posted by Mr. Foxen

How old is the amp? Seeing failing caps in some 90s Ampeg stuff.

It about 8-9 years old.

pshep68 · #13 01-23-2013, 11:37 AM

Quote:

Originally Posted by craig.p

Actually I think that's the best way to describe the symptoms of an off-spec preamp tube. It feels like the amp is flat-sounding, 2-dimensional, like it's struggling to do anything. Not distortion like you'd get with a failing power section, but instead this sort of "meh" quality about the sound.

The amp is sterile sounding. I find EQing much more difficult. For example the bass knob used to require only a little tweaking to achieve a desired tone (I would usually set it at 12:00 or 1:00), but now 3:00 is hardly enough.

pshep68 · #14 01-23-2013, 11:40 AM

Occasionally, the green EQ light come on and goes off accompanied by a loss of volume ... and the EQ button is not engaged. I don't know if this is a part of the problem.

JimmyM · #15 01-23-2013, 11:49 AM

I'd guess you have at least one tube on the way out. Preamp tubes are pretty hardy but they're not magical and they do go out sometimes.

Sid Fang · #16 01-23-2013, 02:17 PM

Have you at least tried wiggling the tubes in their sockets and reseating them?

Seans · #17 01-23-2013, 02:42 PM

There's some very good advice here, (usually comes from a good thread topic).
A valve tester would be my first, or a clean, Deoxit with a brass brush on the valve pins and the small round tooth cleaner in the valve base , (remember though, there is no such thing as a de oxidising spray over the counter, it's clever marketing), then tube swapping, all good fun.

Bassmec · #18 01-23-2013, 03:00 PM

Quote:

Originally Posted by Mr. Foxen

How old is the amp? Seeing failing caps in some 90s Ampeg stuff.

Yes I have pulled those crappy Samwah caps out of marshall's that are less than 5 years old.
A good many cheap Chinese capacitors are in fact what the trade describes as poisoned.
This is because they tried industrial espionage from the likes of United Chemicon and Nicicon and got somewhat of a bum steer about what sort of juice to actually put in them.

pshep68 · #19 01-27-2013, 07:30 PM

Loss of volume happening more frequently. The green EQ lighth dosen't seem to com on when it happens. Volume just decrease to "no sound" or "very faint sound". I'n not really tech savy. I can change cables, but pretty incompetent when it comes to tube work, etc. I just a bit concerned that since it doesn't happen all the time the tech at the local music store will have a problem finding the source of the problem.

Mr. Foxen · #20 01-27-2013, 07:35 PM

Quote:

Originally Posted by Bassmec

Yes I have pulled those crappy Samwah caps out of marshall's that are less than 5 years old.
A good many cheap Chinese capacitors are in fact what the trade describes as poisoned.
This is because they tried industrial espionage from the likes of United Chemicon and Nicicon and got somewhat of a bum steer about what sort of juice to actually put in them.

I think a bunch of it it more to do with manufacturers cheaping out on the voltage tolerances, caps that are just good enough for the expected voltage in the amp running normally. Except people don't run valve amps as they are designed, they overdrive them so they distort, and everything gets non linear, so any caps on the low side of their tolerance are subjected to more than they can deal with and die.