when replacing tubes

[MrMarkIII]

I've rolled pre-amp tubes with the amp on, but flipped to standby.
You're probably supposed to turn it off all the way.
I wouldn't try it with power tubes
I agree, by the time I do all the switching on and off and warming up, I don't remember what the old tube sounded like.
A good use for a looper pedal may be to replace your strumming 10 year old.
Not permanently, of course.

 

[94Tremoverb]

It's *better* to put the amp on standby when trying preamp tubes - that way, there's much less thermal stress to the power tubes and the rest of the amp than if you shut it right off. The worst thing for both is to cool down, then heat up again. I never normally shut an amp off if it's going to be played again soon, at least within an hour or so.

For power tubes, it's probably better to shut off - they run pretty hot and there's a good chance you'll burn yourself if you try to change one with bare hands while it's still on, even on standby. But if you use a glove or something, there's still no problem with changing them on standby - the high voltage is off, and as long as you make sure the filaments light up *before* you flip the standby back on (which you should any time you change power tubes) there's no risk to the tubes.

Rectifier tubes *must* have the amp turned off when changing them, in most amps - they're usually upstream of the standby switch, so they're fully live any time the amp is powered up, even on standby.

 

[EtherealWidow]

If you want a good "looper" I have a Boss DD20 that I'm still in love with even if the buffers are not the best. My sound just wouldn't be the same without that pedal.

 

[thunda1216]

The manual for my Heartbreaker says it's OK to swap tubes with the power on - I have done it many times (both pre and power) with this amp and other non- Mesa amps with no problems

Stop fretting over the tubes! Play, swap, enjoy. Good grief. Folks act like tubes only last a few seconds. Get real. Some last a while, some fart out, some last for decades. It is their nature. It's not like there are not any replacements available. You don't worry about the stove going out if you use it every day. You don't stop driving your car because the tires might wear out. If I play my guitar too much, I might have to replace the strings...

I have a Blaupunkt tube radio console from the 1940's with the original tubes in it that still functions and you can't even *get* replacement tubes for it. I will still turn it on and listen to it until they quit. Life is short. Enjoy.

ty

 

[212Mavguy]

Thunda, I sure do like the way you think on this one...

 

[212Mavguy]

ok then

not sure i indicated i was worried about the tubes at all...was more aiming towards how bad it is for the amp, but, ok.

also, on what page does it say that in the Heartbreaker manual?


i'll work on getting real.

You are about as real as it gets, sir. In a good way.

Peace.

 

[MrMarkIII]

All of these can be substituted for one another.
12AX7 = Gain Factor of 100
5751 = Gain Factor of 70
12AT7 = Gain Factor of 60
12AY7 = Gain Factor of 45
12AU7 = Gain Factor of 19
A good place to start is the "V1" tube. This is usually the pre-amp tube closest to the input jack, and the first tube to "see" the guitar, so it can often have the most effect.
Unless you have an Express. Start with "V2" with this one.
Happy rolling!

 

[MrMarkIII]

Five minutes of Google:

(This guy wants to sell you some... Hmmm.)
"The 12Bz7 is the exact pinout as the 12AX7/5751/12AY7/12AU7 family of tubes and can be used as a direct substitute. The 12BZ7 was a television set tube that has the exact same gain as a 12AX7 and is similar in all respects EXCEPT it has a bigger plate with half the plate resistance of a 12AX7 so it has TWICE the plate current which equates to better drive SLAM! It s a little secret amongst tube replacement gurus. It has a bolder more in-your-face tonality about it vs the 12AX7. In fact if you tied the plates of a 12AX7 together to run both triode sections as one you d have one section of the 12Bz7. Great tube that s not well known. The sleepers sleeper! Get them while you can because they will be more well known someday! If you like that more aggressive bad-to-the-bone tone...THIS is the tube you want!"

(Hi-Fi snob guy...)
"The 12BZ7 is NOT a substitute for the 12AX7. The 'BZ7 draws 2X the heater current. If that's not bad enough, the very different electrical character finishes things off.
Pin out and mu are the ONLY things the 2 types share."

(More Hi-Fi guy ex-pur-teez...)
"A 12BZ7 is roughly equivalent of taking a 12ax7a and cascading both sides together, ie, both grids, both plates and both cathodes, so you have twice the transconductance and half the plate resistance. The guy was right when he said the 9 pin base is about the only thing the two have in common. So all the EQ circuits now have the wrong time constants, the feedback if any is now wrong. Everything is now WRONG if you stick a 12BZ7 in a 12AX7 socket."
"You have to completely redesign for the 12bz7. It requires much lower value of Ra and Rk when compared with a 12ax7. If placed in most cathode bias circuits using a 12ax7, it will hum loudly. It has more crude construction than a 12ax7 with taller spacing rods and much fewer mica insulators. It is much more microphonic."

Thanks, but I'll pass on this one. Wouldn't want to risk ruining the time constants in my EQ.
But, of course, YMMV.

 

[94Tremoverb]

I would only worry about the extra current draw if you were replacing several of the preamp tubes, and even then probably not. The proportionate increase from just changing one or two out of five or six preamp tubes when all the preamp tubes put together draw not much more than one power tube means that the amp would have to be an extremely marginal design for this to cause trouble.

However, one thing not mentioned so far - sorry if you've seen me say this several times before, but it's important - is that the 12A*7 family tubes are NOT all safe substitutes for each other in guitar amp circuits. If the tube is configured as a cathode follower, you really have to watch the cathode-to-filament voltage rating - 12AX7s are rated for 250V, and the cathode voltage is often in the 200V range in these positions. *But a 12AT7 or 12AY7 is only rated for 90V.* So you must not try one of these in a cathode-follower slot, which includes V2 in all vintage Marshall-type circuits, V3 and V4 in a Dual Rectifier, but also (oddly) V1 in the Mesa Maverick and Blue Angel. At best it's likely to sound bad, it's quite likely the tube will blow, and at worst it's just possibly that it could do some damage to the amp - about the only time a preamp failure could. Normally it can't because the current draw is always limited by the plate resistor, but in a cathode-follower there's NO plate resistor. (There are some modern 12AX7s, mostly Russian as far as I know, which are incorrectly spec'ed and don't have enough voltage rating for these slots either.)

You also need to be careful with reverb driver positions because they're effectively a very small power stage, and usually the plate voltage and power dissipation are at (or above) the maximum design spec for the tubes. You shouldn't put a 12AX7 in a reverb driver slot that specifically calls for a 12AT7, for example.

 

[MrMarkIII]

It's simple - if the tube doesn't sound good, replace it.

 

[94Tremoverb]

Now what... get hold of a schematic or contact the manufacturer, if you plan on fitting other than the stock tube types. Find out what the tube positions do, and if any of them are cathode-followers.

Or, stick to 12AX7s, with the exception of the 5751 (just a lower gain 12AX7 really, same ratings otherwise), and 12AU7 (same voltage and greater power rating than a 12AX7, but much less gain so not always usable in most positions). Most amps these days are designed around all-12AX7 because they are by far the most commonly made type and the easiest to get in good varieties. 12AT7s are next but are still rarely used outside vintage reissues (eg Fender Blackface) where they were originally fitted. It's actually simpler to design the circuit stage to lower the gain than use a different tube type, if that's required.

At one point in the 1990s it looked like tube manufacture was going to drop down to just three types - 6L6/5881, EL84 and 12AX7. Even good EL34s were difficult to get, which is why Marshall switched to 5881s for a few years, and Mesa recommended (still does I think) 'use vacuum tube rectifier with EL34'... because the only EL34s available in quantity were crap and wouldn't take the full plate voltage with the silicon rectifier. (In fact, the EL34 is supposed to have a *higher* rating than the 6L6.) Luckily things have improved and some of the less common types are available again, and the quality of some others has improved... even if it has a long way to go to equal the old US and European types (it never will, actually).