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As per title looking to get a tube tested by someone that knows what they are doing as I just won an auction for some untested tubes as there was one tube I was interested in and that is an old philips 12AX7 (made in holland) seeing I already have one the same in my DSL100 head in phase inverter position and I am keen to chuck the one that is coming in V3 (got old mesa 7025 tubes in V1 and V2).

Can't help but am interested to know the difference between a new and worn out tube. Frustrating that you can't easily identify which are in need of replacement.

Preamp tubes I'm pretty sure you can just chuck em in and if they sound good then you're good to go. Not like power tubes that need biasing etc

Molly wrote:Can't help but am interested to know the difference between a new and worn out tube. Frustrating that you can't easily identify which are in need of replacement.

The amp is actually brand new I am just swapping out the factory tubes with vintage ones and since I started the swap out it has actually improved my amp in terms of sound and ability to crank up the gain to full without turning to mud, I just wanted to get that one tube tested when it arrives to make sure it is still good. Even my Marshall DSL1 Combo got the same treatment that has an old mesa 7025 in V1, stock marshall 12ax7 in PI and an ECC802S in output and even that has become a right royal little monster with its sound especially through a 2x12 lol.

I have a tube tester and am in Hamilton-ish.

Not back in country for a week or so but happy to look at it then.

GrantB wrote:I have a tube tester and am in Hamilton-ish.

Not back in country for a week or so but happy to look at it then.

That would be awesome Grant it will be nearly a week before they arrive anyway and I do not mind waiting

IMO, the main issue with older preamp tubes - especially ones being sold as NOS these days - is microphonics and noise, which a tube tester can't check-for. A tester is good for measuring transconductance of each triode and seeing how 'balanced' the tube is, and transconductance can generally be used as a measure of how 'used-up' a preamp tube is, but variations in these measures don't often hurt performance in a guitar amp, unless the tube is completely dead. If it works in your amp, it's not microphonic or crackly and it sounds good, then that's really all that matters. I've tested old 12AX7s/7025s that have been used in an amp for 40 years and, according to only transconductance, you could still qualify them as NOS - microphonic or not!

Power tubes are a different matter, however.

RectifiedAmps wrote:IMO, the main issue with older preamp tubes - especially ones being sold as NOS these days - is microphonics and noise, which a tube tester can't check-for. A tester is good for measuring transconductance of each triode and seeing how 'balanced' the tube is, and transconductance can generally be used as a measure of how 'used-up' a preamp tube is, but variations in these measures don't often hurt performance in a guitar amp, unless the tube is completely dead. If it works in your amp, it's not microphonic or crackly and it sounds good, then that's really all that matters. I've tested old 12AX7s/7025s that have been used in an amp for 40 years and, according to only transconductance, you could still qualify them as NOS - microphonic or not!

Power tubes are a different matter, however.

That's interesting. Every time I sell an amp someone will ask when the tubes were last replaced but the manual for, for example, my Royal Atlantic states: "Pre-amp tubes don't normally wear out as a rule". I bet loads of folk are changing tubes unnecessarily.

Molly wrote:That's interesting. Every time I sell an amp someone will ask when the tubes were last replaced but the manual for, for example, my Royal Atlantic states: "Pre-amp tubes don't normally wear out as a rule". I bet loads of folk are changing tubes unnecessarily.

Yes, I haven't seen more than maybe one or two preamp tubes that had a transconductance so low I wouldn't bother using it. As they age, they're more likely to develop a fault, like microphony or hum/hiss - probably due more to loose internal elements from expansion/contractions effects from thousands of heat/cool cycles from powering the amp on/off. Power tubes are similar in that respect actually - you usually get an intermittent (and eventually often catastrophic) internal short long before they get 'tired', transconductance-wise. But they do also get partially gassy, unlike preamp tubes which tend to be full-vacuum or none at all (smokey white)

On older Fenders, I always found it was the preamp tubes that run hotter that are most likely to fail - 12AT7 reverb drivers for example. I guess this is due as much to current flow as it is to temperature stress. Most other preamp stages run at low currents/temperatures - probably why they last for decades.

RectifiedAmps wrote:

Molly wrote:That's interesting. Every time I sell an amp someone will ask when the tubes were last replaced but the manual for, for example, my Royal Atlantic states: "Pre-amp tubes don't normally wear out as a rule". I bet loads of folk are changing tubes unnecessarily.

Yes, I haven't seen more than maybe one or two preamp tubes that had a transconductance so low I wouldn't bother using it. As they age, they're more likely to develop a fault, like microphony or hum/hiss - probably due more to loose internal elements from expansion/contractions effects from thousands of heat/cool cycles from powering the amp on/off. Power tubes are similar in that respect actually - you usually get an intermittent (and eventually often catastrophic) internal short long before they get 'tired', transconductance-wise. But they do also get partially gassy, unlike preamp tubes which tend to be full-vacuum or none at all (smokey white)

On older Fenders, I always found it was the preamp tubes that run hotter that are most likely to fail - 12AT7 reverb drivers for example. I guess this is due as much to current flow as it is to temperature stress. Most other preamp stages run at low currents/temperatures - probably why they last for decades.

Cheers for that.

Would you, as a rule, recommend keeping a fan on the tubes?

Molly wrote:Would you, as a rule, recommend keeping a fan on the tubes?

If it's an installed, dedicated cooling fan, like a 12V PC fan, then it couldn't hurt. If we're talking an external fan placed near the amp, I wouldn't bother unless you're so dedicated that you use it every single time you use the amp for years on end - consistency in cooling over the long-run is the key. Whether or not it'll make a difference is something I'm not sure of. A fan will cool the glass envelope, but whether that cooling will be significantly transferred through the internal vacuum 'space' to the important plate/anode and other metal elements at the centre of the tube... I dunno. We need a physicist for that answer!

Thanks. I can see the logic there.

Still wonder why to this day nobody seems to have taken the inconsistency out of tube manufacture. Grades and matched sets an' all that. Where's the boutique guy in his home workshop knocking out 'premium' tubes at reassuringly inflated prices? I might do it. How hard can it be...?

Valves don't mind being hot, the fan is to keep the enclosure / rest of the amp cooler. Smaller, compact enclosures and more sensitive electronics in modern amps are the usual reasons for a fan to be incorporated.

Sorry for the hijack but would there be any harm in temporarily swapping out my loop 12AY7 tube for a 12AX7? Just until I can find another Y.

RectifiedAmps wrote:IMO, the main issue with older preamp tubes - especially ones being sold as NOS these days - is microphonics and noise, which a tube tester can't check-for. A tester is good for measuring transconductance of each triode and seeing how 'balanced' the tube is, and transconductance can generally be used as a measure of how 'used-up' a preamp tube is, but variations in these measures don't often hurt performance in a guitar amp, unless the tube is completely dead. If it works in your amp, it's not microphonic or crackly and it sounds good, then that's really all that matters. I've tested old 12AX7s/7025s that have been used in an amp for 40 years and, according to only transconductance, you could still qualify them as NOS - microphonic or not!

Power tubes are a different matter, however.

Completely agree. Noisy cant be tested, and often noticed after the tester points to “good”.