so.. why do some valve amps chew through tubes quicker than others?

chrissmith

Hi All

I've been a member since forever, but haven't posted in years and years.

I've had a couple of valve amps over the years.  One was a Mesa Boogie 'Studio Calibre'  - sounded great that one.  Right now I'm enjoying a Laney VC30 212. Basically 4 x EL84 class A.  It sounds quite voxy, classic rock tones.

Anyway.
The VC30 212 goes through valves approximately 1 set a year.  It's only 3 x ECC83 and 4 x EL84, but that is about £70 a year in tubes.

A valve tech told me ''it's because it's Class A, and therefore burning through tubes all the time''.  I read somewhere else ''All Vox type amps chew through valves, it's because they're run hot, which is one reason why they sound so good''.


So - what have you found in your experience?

''Class A/B lasts longer''
'' Class A or A/B has nothing to do with it, it's about how hot the tubes are run in the design''
" it's about how long you leave it on standby before you power it up"


I'm essentially trying to get to the bottom of why some people have the same tubes for 10+ years, whereas I would run through £700 in valves in the same period.

Let the opinions / experience commence !   

ICBM

First, the Laney is *not* Class A. It's cathode-biased Class AB, like all amps derived from the classic Vox circuits - the 'Class A' myth is one of the biggest in anything to do with guitars. They are not only Class AB, the characteristic compression of them is *only* because they are, and not Class A.

But this type of design does usually run the valves much hotter than a fixed-bias Class AB - what's typically called Class AB - and this is why they tend to be harder on valves. Often the idle dissipation is more than the maximum rating for the valve, and sometimes much more - and that's even before a signal goes through them. If you run a valve rated for 12W at 16-19W, don't be surprised if it wears out quickly! It *isn't* necessary to run them this hot to sound good or because of the type of circuit though, it's just become part of the 'accepted design characteristics', along with short valve life.

Second, it's highly unlikely that you need to change the preamp valves when the power valves are worn out - they are completely separate, and there is no need to change them until they fail for some other reason - which will usually take many years.

Standby switches don't make much difference, although if you typically leave your amp on but not being used for a long time then they can, especially on cathode-biased Class AB amps - simply by effectively reducing the number of hours the valves run at over the design ratings.

chrissmith

Hi 

Thanks v much for taking such time on a detailed answer.

At the following link - (the user manual)  Laney are describing it as 'Class A' and also '30 watts'.

https://www.laney.co.uk/wp-content/uploads/2016/01/6e5d65fafa037bb79a8cc5080ce11fa6.pdf

I take it that this is incorrect?

If we take it as granted that it is indeed 30watts (from 4 x EL84) - is this ''running them hot''.  Could a valve tech reduce it, and would it then last a few years on a set of output tubes (and presumably it would sound as good, if maybe not quite as powerful (a non-issue)

jpfamps

As ICBM says many amps using EL84s idle them at greater than the maximum static dissipation, and I expect that this is because VOX did this so that is the "correct" way to run these valves.

VOX also used small value screen grid resistors (100 ohm). Larger values such as 1k will increase valve life.

Combos with the valves hanging down near the loud speaker are the worst for the valves.

I'm not convinced that you need to run EL84s "hot" to get the characteristic VOX sound. A key feature of VOX amps is that they don't employ negative feedback; this is often overlooked when searching for the source of the VOX magic.

Additionally I'm not convinced that cathode biasing has that much to do with the VOX sound either. EL84s were designed to run almost as efficiently in cathode bias as fixed bias.

The VOX AC50 with uses 2 x EL34s in (mainly) fixed bias, still sounds VOXy.

chrissmith

You guys are great.  What a mine of information    So potentially a cheap-fix to get more valve life?  Bigger grid resistors, reducing the bias?

I can't do much about the fact it's in a combo unfortunately, without spending big bucks.

ICBM

chrissmith said:

At the following link - (the user manual)  Laney are describing it as 'Class A' and also '30 watts'.

https://www.laney.co.uk/wp-content/uploads/2016/01/6e5d65fafa037bb79a8cc5080ce11fa6.pdf

I take it that this is incorrect?

Yes, and is easy to show with an oscilloscope. Manufacturers persist in calling them Class A, presumably because that makes them seem better than Class AB - which is usually wrongly written as Class A/B as well!

The power output of 30W is correct - or close enough - and isn't related to the valve dissipation except in as much as it provides an upper limit. But you can get more power by running in Class AB, especially fixed-bias.

jpfamps said:

VOX also used small value screen grid resistors (100 ohm). Larger values such as 1k will increase valve life.

Combos with the valves hanging down near the loud speaker are the worst for the valves.

I'm not convinced that you need to run EL84s "hot" to get the characteristic VOX sound. A key feature of VOX amps is that they don't employ negative feedback; this is often overlooked when searching for the source of the VOX magic.

Additionally I'm not convinced that cathode biasing has that much to do with the VOX sound either. EL84s were designed to run almost as efficiently in cathode bias as fixed bias.

The VOX AC50 with uses 2 x EL34s in (mainly) fixed bias, still sounds VOXy.

Exactly.

The characteristic of Class AB cathode-bias that does matter is the rise in bias voltage when the valves are driven hard, which causes compression. This is something that doesn't happen in a true Class A amp, but is in fact written quite clearly on the original AC30 schematic.

But the *sound* is more to do with the lack of negative feedback, the preamp design and the speakers typically used, than anything in the power valve circuit.

jpfamps

chrissmith said:

Hi 

Thanks v much for taking such time on a detailed answer.

At the following link - (the user manual)  Laney are describing it as 'Class A' and also '30 watts'.

https://www.laney.co.uk/wp-content/uploads/2016/01/6e5d65fafa037bb79a8cc5080ce11fa6.pdf

I take it that this is incorrect?

If we take it as granted that it is indeed 30watts (from 4 x EL84) - is this ''running them hot''.  Could a valve tech reduce it, and would it then last a few years on a set of output tubes (and presumably it would sound as good, if maybe not quite as powerful (a non-issue)

They did at least put inverted commas around "class A".

ecc83

You might also live in an area with mains closer to the allowed maximum of 253V and almost nobody gets the EU "harmonized" 230 volts! ICBM often cautions about checking certain amplifier's mains traff setting.

If you can, look at the valves in a dim room and look closely at the 84 anodes. Is there the slightest sign of a dull red glow? If so they are running over their limit. This was found to be the case in the very first batch of Artisan 15s and 30s and increasing the common cathode resistor by about 25% completely fixed the problem. Note, some brands of EL84 would "red plate" others did not. A/B (not a pun!) testing was done, basically ME switching the extra R in and out while experienced guitarists widdled and listened, showed there was no detectable change in the sound.

Dave.

pintspiller

I only had one EL84 amp. It was a Peavey Royal 8. Great sounding amp, not particularly Voxy. Chewed valves. Sold it. Don't regret it.

joetele

My Peavey Valveking II went through its power amp valves within a year, and I was only using it once a month for two or three hours, and the odd silent practice for 20 mins at home. I was surprised, but considering the advice above, it seems like maybe the valves were rated for lower wattage and I was running the amp at 50W constantly. 

JayGee

Always an interesting one this,

I always assumed that my Mesa 5:25 running 2xEL84s for 25 watts (and they’re really, really, really big watts too, it’s astonishingly loud for its size, weight and rating and can fill a remarkably large space against a full band with the help of a 2x12 extension cab) when 4xEL84s for 30 watts is a “standard” configuration would be working the output valves quite hard, but it’s still on the original set after over 10 years of being regularly thrashed with a loud band and/or as a jam night “house amp”.

I’m guessing that what’s actually happening is that it’s biased to run close to pure Class B in order to get the maximum voltage swing out of a pair of valves, has low idle currents, and hence is actually pretty gentle on valves...

impmann

I have the pleasure of playing through an old friend's 1965 Vox AC30 from time to time - he gigged it around the South East every week (near enough) from 1972 until about five years ago when he retired due to arthritis. It still has all its original valves. It still sounds magnificent. Draw your own conclusions.

Another friend has a 1970s HiWatt Custom 100 that has seen a lot of life judging from the wear on its tolex, but that still has original output valves (Mullard). It recently had a recap and it needed one preamp valve as it was a touch microphonic.

However, I have seen some terrible valve wear on certain modern amps after just three months... I have my own opinions on the design of these, which I shall keep to myself for a variety of reasons. But as mentioned above the input voltage can make a massive difference to valve life - if you bought the amp from a European supplier (such as Thomann etc), there may be a jumper on the board set for the 'European' voltage, that may need moved to the 'UK' voltage. It may need rebiasing as a result (depending on the amp/design etc).

nick79

I had a Mesa mini rec that got needed new el84's after about 10 months.. Used once or twice a week, loud but never fully cranked. Seemed a bit excessive to me.. 

jpfamps

JayGee said:

Always an interesting one this,

I always assumed that my Mesa 5:25 running 2xEL84s for 25 watts (and they’re really, really, really big watts too, it’s astonishingly loud for its size, weight and rating and can fill a remarkably large space against a full band with the help of a 2x12 extension cab) when 4xEL84s for 30 watts is a “standard” configuration would be working the output valves quite hard, but it’s still on the original set after over 10 years of being regularly thrashed with a loud band and/or as a jam night “house amp”.

I’m guessing that what’s actually happening is that it’s biased to run close to pure Class B in order to get the maximum voltage swing out of a pair of valves, has low idle currents, and hence is actually pretty gentle on valves...

That's right.

The static dissipation in the MESA is a lot lower than in most VOX-style output stages.

A lot of my customers don't appreciate that even if they haven't "played the amp very loud" the valves are wearing out simply by being on.

Actually if you look at anode dissipation in valve amps, then in a class B amp, plate dissipation initially increases as you increase the signal (typically be maximum around 50-70% of full clean power), and then starts to decrease.

In contrast in a class A amp, the anode dissipation is maximum with no signal and decreases as the signal increases.

Class A/B is somewhere in between this.

Incidentally when amplifying a full power square wave, anode dissipation in theoretically zero.

What does increase markedly with signal, especially when drive to clipping, is screen grid dissipation, and is why higher value screen grid resistors which reduce screen grid current are recommended.

chrissmith

This is a real ''popcorn'' thread for me.

Manufacturer's don't advertise ''valve life'' on their amps (because it varies of course, and who would say if it was only a year?), so if anyone knows ''any way of knowing'' before buying an amp, that would be a really useful resource.

Modulus_Amps

chrissmith said:

This is a real ''popcorn'' thread for me.

Manufacturer's don't advertise ''valve life'' on their amps (because it varies of course, and who would say if it was only a year?), so if anyone knows ''any way of knowing'' before buying an amp, that would be a really useful resource.

Ask good techs who look at these things all the time.

With EL84 there is also a trend to run the plate and screen voltages above max recommended spec. Another quick way to kill valves fast. All that can be sorted by a good tech who understands what is going on

chrissmith

Got to find a good one in Berkshire.  I do know one, but he charges - ''a lot''.  Any recc's v welcome. Send by message if you like.

ecc83

chrissmith said:

This is a real ''popcorn'' thread for me.

Manufacturer's don't advertise ''valve life'' on their amps (because it varies of course, and who would say if it was only a year?), so if anyone knows ''any way of knowing'' before buying an amp, that would be a really useful resource.

Ninety days Chris because that is all most valve mnfctrs give the amp makers and that was always the case.

In the  Radio/TV service trade we bought Mullards (except where they did not have a type, e.g. 6/30l2) and it was on the box!

Of course the valves lasted way longer than that in practice, ten years was nothing exceptional in a radio using octals. Indeed, the 6V6 which WAS run in "class A" cathode biased almost never needed replacing. Not  a Mully of course and in fact their power pentodes weren't nearly so reliable, e.g. EL32.

The best of the bunch was the EL34 but modern samples are less rugged. Dare anyone put 800 volts on those anodes?!

ICBM

ecc83 said:

Indeed, the 6V6 which WAS run in "class A" cathode biased almost never needed replacing.

Yes, but with the HT voltage within the design ratings, so they could be in Class A and not over the max dissipation at the same time, which isn’t possible in most guitar amps.

Guitar amp designers have always sacrificed audio quality (in a hi-fi sense - of course the resulting distortion became desirable for guitar) and valve life in the pursuit of volume.

But it seems that even though maximum volume is no longer the goal mostly, designers still don’t seem to want to run the valves much more conservatively.

ecc83

ICBM said:

ecc83 said:

Indeed, the 6V6 which WAS run in "class A" cathode biased almost never needed replacing.

Yes, but with the HT voltage within the design ratings, so they could be in Class A and not over the max dissipation at the same time, which isn’t possible in most guitar amps.

Guitar amp designers have always sacrificed audio quality (in a hi-fi sense - of course the resulting distortion became desirable for guitar) and valve life in the pursuit of volume.

But it seems that even though maximum volume is no longer the goal mostly, designers still don’t seem to want to run the valves much more conservatively.

Yes IC, I get all that but still, the 6V6 lasted SO long! I maintained a load of identical radios (called "Bradios!) and most used the 6V6 and I cannot recall ever replacing one. A few samples however used the el33 and they did not last more than 5 years tops and rarely survived  a shorted grid coupler.

Then, there were radios in factories and garages that were on for 12hrs+ a day, 6 days a week and usually a top volume and quite distorted. Ok, HT was only about 250V but the 6V6 longevity was still amazing. Lasted well in Bell&Howell projectors as well despite the vibration and heat.

Dave.

jpfamps

ecc83 said:

ICBM said:

ecc83 said:

Indeed, the 6V6 which WAS run in "class A" cathode biased almost never needed replacing.

Yes, but with the HT voltage within the design ratings, so they could be in Class A and not over the max dissipation at the same time, which isn’t possible in most guitar amps.

Guitar amp designers have always sacrificed audio quality (in a hi-fi sense - of course the resulting distortion became desirable for guitar) and valve life in the pursuit of volume.

But it seems that even though maximum volume is no longer the goal mostly, designers still don’t seem to want to run the valves much more conservatively.

Yes IC, I get all that but still, the 6V6 lasted SO long! I maintained a load of identical radios (called "Bradios!) and most used the 6V6 and I cannot recall ever replacing one. A few samples however used the el33 and they did not last more than 5 years tops and rarely survived  a shorted grid coupler.

Then, there were radios in factories and garages that were on for 12hrs+ a day, 6 days a week and usually a top volume and quite distorted. Ok, HT was only about 250V but the 6V6 longevity was still amazing. Lasted well in Bell&Howell projectors as well despite the vibration and heat.

Dave.

Beam tetrodes are inherently more reliable than pentodes as their screen grids are wound concentrically with the control grids.

As the screen grid is in the shadow of the control grid, screen grid current is much lower, and hence more reliable.

6V6s are also larger than EL84s so have a larger thermal mass and larger surface area, and also have lower filament current, so suffer less heat stress.

The 6V6 was designed for use in mobile equipment (hence the relatively low heater current) and in my experience are very rugged. 

I aslo (generally) prefer the sound of 6V6s to EL84s.