Tube Amp Common Mistakes

[BassyBill]

I'm going to see if I can get an all-tube PC to use for posting here on TB. I'm sure it will make a lot of the comments sound better.

 

[coreyfyfe]

I've heard people who claim to be techs (I wouldn't know) say they have noticed improved tube life with warm-up and cool-down procedures. Especially in extremes

Having cracked a tube this way I can say that yes, you run the risk of increased failure rate, but just as important bringing a cold amp into a warm environment can cause condensation which can cause shorts/failures on any electronic equipment. So something to consider in general.

 

[David Jayne]

I'm going to see if I can get an all-tube PC to use for posting here on TB. I'm sure it will make a lot of the comments sound better.

They'll sound fatter at high volumes, but with increased distortion.

 

[Calaverasgrande]

Not an old wives tale. You can see where finger prints are. Lightbulbs are more susceptible to this damage. You'd see them brown in that spot and then fail.
Anyway for what we pay for these what does it hurt to grab with a soft cloth?
My tube tech uses a rubber gripper designed for the purpose.

I call shenanigans on that.
I've been playing tube amps for decades. I ALWAYS handle my tubes. I check them every time I transport my amps becuae they DO shake loose. So every power tube in all my amps has multiple greasy fingerprints on it. Preamp tubes too, because I like to experiment with different tubes.
And I've had more transformer and speaker failures than tube failures.

 

[walterw]

I'm going to see if I can get an all-tube PC to use for posting here on TB. I'm sure it will make a lot of the comments sound better.

i'm sure rent on the shopping mall you'd need to house the thing (as well as the massive air-conditioning bill) would be a bitch.

hell, given the number of transistors in today's dual-core and quad-core PCs, i wonder if enough tubes to do the job even physically exist?

anyway,

yes, absolutely do not get the speaker impedance load wrong

no, touching the glass won't hurt anything

yes, let them warm up on standby before firing up

no don't worry about waiting to turn them off

yes, pay attention to vibration and cold shocks

yes, when a tube amp you used to love seems just, i dunno, not so awesome anymore, it's time for new tubes

and remember, they used to have tube testers at drugstores for grandma to check the tubes in the TV! don't be scared of the things, they're pretty robust.

 

[MAMMOTHvolume]

If you don't like the sound of your properly performing tube amp, don't replace the power tubes and think it will sound better. Even replacing preamp tubes in a crappy sounding (to you) amp may make it sound different... But IME, a different kind of crappy.

I replace tubes when they die, begin to sound bad or go microphonic. Not just because they are dusty of I have an extra $300-400 lying around.

My 1936 Gibson tube amp still has a few original tubes. They can last a long time if treated properly.

Wives tale question..... Does using a padding layer between amp and cab (gramma pad, sorbothane balls, etc) actually cut vibrations and extend tube life?

Depends how much your cab vibrates, if it's alot it wont do any heads any good!

 

[Downunderwonder]

i'm sure rent on the shopping mall you'd need to house the thing (as well as the massive air-conditioning bill) would be a bitch.

hell, given the number of transistors in today's dual-core and quad-core PCs, i wonder if enough tubes to do the job even physically exist?

anyway,

yes, absolutely do not get the speaker impedance load wrong

no, touching the glass won't hurt anything

yes, let them warm up on standby before firing up

no don't worry about waiting to turn them off

yes, pay attention to vibration and cold shocks

yes, when a tube amp you used to love seems just, i dunno, not so awesome anymore, it's time for new tubes

and remember, they used to have tube testers at drugstores for grandma to check the tubes in the TV! don't be scared of the things, they're pretty robust.

I'm pretty new to tube amps myself . Reading up on all and sundry tube amp threads for the last year or so, this all resonates as good advice.

 

[chadds]

My SS amp's caps went slowly and it slowly lost its wonderful sound. Recap it's great.
So service service.
Since we are debating on the Internet, this touching tubes thing has one detail.
You know if your hands are dry and you're doing some tube rolling. You know if they are sweaty greasy. Like I said, as rare and expensive as some of these are and as rare as it could be to find a good tube in a batch of new tubes why take the chance of hastening their demise?

 

[BurningSkies]

Every time I change out a tube, I carefully make sure to lick off all the dust on the glass and oxidation on the pins...Is this still standard operating procedure like I was taught back in the 70's?

 

[RSNG]

I used to use my all tube SVT with 2 1x15 cabs with 8 ohm JBL E-140s. So, I was running it at 4 ohms, with the switch in the 4 ohm position.

Then, I got into a new band, and used it in rehearsals with one 1x15. So, I was running it at 8 ohms. It started blowing fuses.

I took it to a tech that works exclusively on tube amps. He retubed it. He told me to never, ever, run this amp at anything other than 2 or 4 ohms.

My amp has been fine ever since. I guess I'm lucky that I didn't really screw it up big time! Now I've got a 4 ohm 4x10 to use when I really don't need 2x15s or 8x10s.

 

[Calaverasgrande]

I'm not sure if this is old wives' tale as well, but...

I was always told to set the amp to standby for a couple minutes before shutting off the power. So, whenever I finish a gig, the first thing I do is put the amp on standby while I pack up and unload everything else. My amp and cabs are the last things to come off the stage.

All that standby mode is for is to turn the heaters inside the tubes on. You aren't "cooling it down" by leaving it in standby after practice, even if there is a fan. Because the heaters are still on! Just turn it straight off, I usually let mine sit for a bit before I move it, though quite often I play 5 band shows which expect each band to clear the stage in ten minutes after their set.

 

[Stumbo]

SS amps have a minimum speaker ohm load built in. Tube amps ratings are maximum speaker ohm loads.

Post your good amp techs here/Look up tube amp techs recommendations Lots of good info here.
How to turn on/off a tube amp
http://www.talkbass.com/forum/f15/how-connect-2-67-ohm-load-tube-amp-795074/

 

[eyeballkid]

so just curious... What would happen if you took, say, an SVT set on 2ohm and ran it with four 4ohm cabs? (since it would be set to a 2 ohm maximum)

 

[Fair Warning]

Another thing to watch out for is using your tube head with a loaner cab or other cab which may not have all the speakers working. Each speaker that does not work with change your impedence, regardless of what is printed on the back of the cab. I once had a 4 Ohm 810, which turned out 3 of the speakers did not work. What impedence was the result?

Bottom line.....better be sure about your cabinet..

 

[coreyfyfe]

so just curious... What would happen if you took, say, an SVT set on 2ohm and ran it with four 4ohm cabs? (since it would be set to a 2 ohm maximum)

Something like this:

The power tubes simply refuse to put out all that much more current with a lower-impedance load, so death by overheating with a too-low load is all but impossible - not totally out of the question but extremely unlikely. The power tubes simply get into a loading range where their output power goes down from the mismatched load. At 2:1 lower-than-matched load is not unreasonable at all.

If you do too high a load, the power tubes still limit what they put out, but a second order effect becomes important.

There is magnetic leakage from primary to secondary and between both half-primaries to each other. When the current in the primary is driven to be discontinuous, you get inductive kickback from the leakage inductances in the form of a voltage spike.

This voltage spike can punch through insulation or flash over sockets, and the spike is sitting on top of B+, so it's got a head start for a flashover to ground. If the punchthrough was one time, it wouldn't be a problem, but the burning residues inside the transformer make punchthrough easier at the same point on the next cycle, and eventually erode the insulation to make a conductive path between layers. The sound goes south, and with an intermittent short you can get a permanent short, or the wire can burn though to give you an open there, and now you have a dead transformer.

So how much loading is too high? For a well designed (equals interleaved, tightly coupled, low leakage inductances, like a fine, high quality hifi) OT, you can easily withstand a 2:1 mismatch high.

For a poorly designed (high leakage, poor coupling, not well insulated or potted) transformer, 2:1 may well be marginal. Worse, if you have an intermittent contact in the path to the speaker, you will introduce transients that are sharper and hence cause higher voltages. In that light, the speaker impedance selector switch could kill OT's if two ways - if it's a break befor make, the transients cause punch through; if it's a make before break, the OT is intermittently shorted and the higher currents cause burns on the switch that eventually make it into a break before make. Turning the speaker impedance selector with an amp running is something I would not chance, not once.

For why Marshalls are extra sensitive, could be the transformer design, could be that selector switch. I personally would not worry too much about a 2:1 mismatch too low, but I might not do a mismatch high on Marshalls with the observed data that they are not all that sturdy under that load. In that light, pulling two tubes and leaving the impedance switch alone might not be too bad, as the remaining tubes are running into a too-low rather than too-high load.

Edit - missed the four four ohm bit, thought you were saying 4 ohms on 2 ohm setting.

 

[fdeck]

Don't take the tubes out to fix the amp, and let the tubes roll off the work table onto the floor. That once happened to me. Fortunately I caught all but one of the tubes.

 

[EastCoastThump]

As a distracted young teen, I once unscrewed the chassis of a Traynor tube amp from the wood head casing, figuring that a few fingers on my left hand would be enough to support the works as I loosened the last screw. Turns out that was a flawed theory, and the amp's works crashed to the bottom of the casing--breaking the power tubes in the process. Luckily, the breaking tubes didn't injure my hand, although the metal banged a couple of my fingers pretty memorably.

Don't do that.

 

[walterw]

awesome, especially since distracted young teens usually can't afford new tubes

 

[walterw]

so just curious... What would happen if you took, say, an SVT set on 2ohm and ran it with four 4ohm cabs? (since it would be set to a 2 ohm maximum)

that would actually be a 1Ω load, almost a dead short!

I once had a 4 Ohm 810, which turned out 3 of the speakers did not work. What impedence was the result?

Bottom line.....better be sure about your cabinet..

+1 to that, but the cool thing about the SVT design was the 32Ω speakers all in parallel; each one that died would just raise the overall impedance a little while letting the rest keep going (unlike having a bunch of 8Ω drivers in series/parallel, where one dying would take at least one more out of the game too).

3 dead ones would have been like an 8Ω 4x10 in parallel with one 32Ω speaker, so the result would be a little less than 8Ω; not great, but not a flaming car wreck disaster, either.

 

[EastCoastThump]

awesome, especially since distracted young teens usually can't afford new tubes

Yeah, even though they were cheaper at the time, a set of GE power tubes still set me back pretty hard. I made my money mowing yards and doing seasonal work at the local farms--so each dollar spent couldn't help but make me think how much sweat and blood (literally) it took to earn.