# When to change tubes



## bbigsby (Mar 23, 2009)

I have a 78 Deluxe Reverb with the original stamped fender tubes. The amp still sound pretty good (tubes have zero microphonics) however the bass get's mushy when the volume goes past 3 (With a Les Paul). 

On other sites people say don't change the tubes because the old ones will sound better than newer replacemnts. Is this true? I read Aspen Pittman's book and he recommends changing tubes every 2-3 years.

What is your opinion?


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## Wild Bill (May 3, 2006)

bbigsby said:


> I have a 78 Deluxe Reverb with the original stamped fender tubes. The amp still sound pretty good (tubes have zero microphonics) however the bass get's mushy when the volume goes past 3 (With a Les Paul).
> 
> On other sites people say don't change the tubes because the old ones will sound better than newer replacemnts. Is this true? I read Aspen Pittman's book and he recommends changing tubes every 2-3 years.
> 
> What is your opinion?


First off Mr. BB, we have to make a distinction between preamp and power tubes. Preamp tubes are voltage amplifiers and don't see the same stresses. If they haven't gone microphonic or wonky then I'd leave 'em alone! If you suspect one or more have gone weak you might put in a new tube to see if there's a difference. My experience as a professional tech is that preamp tubes work until they die and that can be years and years! I've got tubes pulled from old equipment that are over 50 years old and are still working great!

Power tubes are a different story. They work with big gobs of current and will slowly get weaker and weaker. It's hard to say how long they will last. Obviously it depends on how many hours a week you play them. Playing loud or soft doesn't seem to make any difference. It's just whether or not they're lit up.

A pair of 6V6's from the glory years might last 5-10 years of steady playing. Modern tubes seem not as good. 2-3 years of steady gigging seems more likely. It's important that they were biased properly. Running cold makes for a thin sound. Running too hot means they burn out a LOT faster! A good tech knows how to measure the plate voltage and adjust the idle current with the bias voltage to set the "sweet spot".

If you have access to a tube tester it's easy to see if a tube is still working well. Using your ears is much more difficult. If a preamp tube outright died then you'd hear little or no signal. If the amp seems to not have the power it should then a weak preamp tube could be the culprit. Again, it's easy to take a known good tube and swap it in one at a time from the 1st position and so on down the line.

Given the age of your amp I would not be surprised if your output tubes are weak, especially if you are noticing poor power or a muddy sound. A mushy bass is actually common to a lot of Fender models. I remember back in the early 70's when I started out it was common to see guys buy a new Fender amp and immediately change the speakers for some Celestions. Those British speakers were more upper-mid and bright in tone, which nicely balanced out the Fender tendency to be too strong in the lower-mid and bass bottom end. If you have access to a Marshall cab and plug your amp into it you will know right away if a different speaker might be the solution.

I frankly don't believe much in the idea that different brands of tubes or new vs. old have differences in tone. There are differences in overall quality and some of the newer brands have a bit more or less gain than the specs call for. This can actually cause more problems in some amps! The idea that one brand of 12AX7 has weaker mids and another has stronger bass frankly seems nuts! The tube will amplify up to maybe 50 million cycles per second. To have it operate differently at perhaps 1500 cycles which would be the mid range of a guitar makes no technical sense at all.

I have never heard of anyone actually being blindfolded and being able to tell when 12AX7's are swapped in an amp. Lots of guys make the claim but somehow wuss out on putting on the blindfold.

The same with power tubes. Again, there are a lot of differences in overall quality. Some brands of EL84's are prone to getting mechanically sensitive to vibrations and making rude noises after a year or two in some of those combo amps that trap the heat and bake the crap outta them! Some Chinese EL34's have problems with the high plate voltages found in some vintage Traynors and HiWatts. They either die within a few weeks or months or sometimes go off like fireworks inside the glass as soon as you plug them in! In other amps where the plate voltage is more reasonable they work just fine. Again, I've never heard of anyone passing a blindfold test. Some guys claim they hear differences if you swap in a different brand but when you ask them they never re-biased the new pair to the same idle current, which of course would affect the tone.

When someone says don't change out the old tubes because they sound better than new ones he's totally ignoring the fact that tubes WILL eventually wear out and die! It's like saying never change the tires on a 57 Chevy 'cuz the originals gave a smoother feeling ride. It may be true but the tires will simply wear out, period!

I suggest you find a good local tech. He should be able to test your old tubes and know for sure if they still show good performance. He can also check the bias while he's in there to make sure it's in the proper range. If it were my shop there would be a hour's labour charge to do this. While I had the amp open I would also poke around with a visual inspection to see if any parts look like they've been overheating or whatever. It also only takes a moment to do some quick voltage checks at the pins of the tubes to make sure things are in the ballpark.

Just my .02 Hope it was useful.

:food-smiley-004:


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## bbigsby (Mar 23, 2009)

Thanks for your response I really learned a lot.


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## mkaye (Jan 25, 2009)

Don't overlook the rectifier tube
i have a '66 Super Reverb and with the original rectifier i had ~415V, a new one bumped it up to 465V where it should be
same with a small Magnatone i have
if you have a solid-state rectifier, then it doesn't apply
loss of highs/lows can certainly be the output tubes
as mentioned above, gotta use your ears too

mark


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## Rugburn (Jan 14, 2009)

Wild Bill said:


> First off Mr. BB, we have to make a distinction between preamp and power tubes. Preamp tubes are voltage amplifiers and don't see the same stresses. If they haven't gone microphonic or wonky then I'd leave 'em alone! If you suspect one or more have gone weak you might put in a new tube to see if there's a difference. My experience as a professional tech is that preamp tubes work until they die and that can be years and years! I've got tubes pulled from old equipment that are over 50 years old and are still working great!
> 
> Power tubes are a different story. They work with big gobs of current and will slowly get weaker and weaker. It's hard to say how long they will last. Obviously it depends on how many hours a week you play them. Playing loud or soft doesn't seem to make any difference. It's just whether or not they're lit up.
> 
> ...


Having swapped out an EH 12AX7 in the V1 slot on my PR after a retubing, with a ANOS CBS/Hytron 7025, I can say the difference was clear as day. The EH was "fizzy" at lower volumes . I put the EH in my '60 National and it sounded better than the Amperex/Bugle Boy that was in there!!(try telling that to a vintage amp connoiseur). The 7025s were designed as a low noise high quality 12AX7, and work as advertised. Friends that were familiar with my amp came by and listened/played it, all told me it souned so clear and bell-like, without my mentioning the tube swap. In all my years of playing/tinkering I think some tubes sound better in some amps than others. This doesn't necessarily mean that the way they behave in one amp will translate to another. As Bill said there's only so much going on in a tube, it's _just_ a part of the circuit. I'd say the wrong tube in the wrong amp *is* noticable though..... even blindfolded. The tube chart on my amp calls for a 7025 in the first preamp slot, and is happier now likely because it was designed that way. I take exception to friends and aquaintances reading about rediculously priced NOS tubes and the "qualities they will impart to your amp". This is snake oil that is slathered on people with no technical understanding whatsoever for big bucks. The worst part is I'll *show* them that the greatest difference between tubes (aside from NOS tubes longer life) is that they can sound like ass in one circuit and just what the doctor ordered in another, and they still *want* to believe in the "mojo". I believe in mojo. but not the kind you buy at the store.

Shawn :food-smiley-004:

P.S. Regarding the OP's question on mushiness, I agree that old Fenders have a big bass response. I put a Weber 10A100 speaker in my Princeton Reverb (which is kinda like the deluxes little sister) and with humbuckers it's pretty boomy. Rolling of the bass entirely works Ok, but I'm looking to build a cab that houses an Eminence Red Coat. Haven't decided on which one, but I have it down to either the Private Jack or Wizard. I'm hoping to post some clips when I'm done so you can hear the differences.


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## Guest (Mar 30, 2009)

A good indication that your power tubes are going to need R&R is a sudden loss of amplitude while playing. I have a set of 6ca7 in my cherished JSX doing that right now. They play well for 20 mins. and then all of a sudden the output power dips. 

Another indicator is the popular blowing fuse. The amp cools down. You put another fuse in and some time later poof again. Some filaments will develop a twist with old age, after warming up a bit. They short out against other internals and pop goes the fuse. They cool down and go back into place only to start their dance one more time. This seems to occur to tubes who have undergone thermal transients akin to going form the sauna to the snowbank. Be kind to your amp, and don't let it cool down too quick. That means, don't throw the head in the backseat two minutes after the new years gig is over. It needs to relax and cool down indoors.





"Friends don't let friends play transistor amps"


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