Does class A burn through tubes faster??

[jsabo]

Does it? not really sure about class a to be honest.

 

[Mark 63]

Does it? not really sure about class a to be honest.

It should. The difference between Class A operation and Class A/B is more voltage runs through at idle.

 

[jsabo]

hm, just talked to a dealer near me who claims it will only be a little hotter but not enough to literally reduce tube life. interesting.

 

[J.J]

It all depends on how an amp is set up as to if it will wear out its tubes faster. The operating class doesn't tell the whole storry.

In the case of the Express amps it really won't make any difference. The operating parameters of the 5:25 in 5 watt mode are quite gental and is not really working any harder than 25 watt mode.

Running tubes hot is not a big deal for tubes, they have to run hot or they don't work :shock: As long as the plate is not glowing red (and a red plate is always well past max ratings) a well made tube will not suffer. A pooly made tube can start to have mecahnical problems over time.

What does effect tube life is plate current. The higher the current the shorter the tube life.

In general most class a tube amps are run at the limits to get the most power output since class A amps tend to be fairly low on power. High plate current = shorter life. For the Express amps the idea was to be low in power in the 5 watt mode to keep volume low. Despite its rating, the 5:25 can only deliver 2 watts in watt 5 mode.

 

[EtherealWidow]

As long as we're talking about tube life here, class A in blues mode with the gain and master dimed sounds really really cool.

 

[J.J]

Couldn't agree more. All my favorite tones in the express come from the 5 watt mode with the master way up. I always wished I could get the same from 25 watt mode.

Clean mode with the master way up is my favorite and it gets that great Freddie king screaming cat tone.

 

[EtherealWidow]

Clean mode is ridiculously dynamic, I've come to find. It's a lot louder than crunch mode. A LOT louder. I don't know why. Maybe it's because those 2 12AU7's in the pre. I can't remember it being that way with all 12AX7's.

 

[J.J]

I'd have to check, but clean mode is loud from memory. I think Mesa used a different tapper for the either the clean gain or master. It definately comes on much faster than the other modes, but may be deceptive in that it probably runs out of head room much lower on the dial.

Its interesting that you mention dynamics. It took me a long time to realise that all the modes bar clean are quite compressed. Crunch mode in particular seems to have way more compression even with the gain at 11:00 than you would expect. After playing the Express for a few years now I have to keep reminding myself when I play other amps with more dynamic crunch modes that I can use those dynamics :mrgreen: Compression makes me lazy.

12AU7 tubes probably do make quite a difference. Id guess dynamics would improve a lot at reasonable gain levels.

 

[EtherealWidow]

The amp is ridiculously compressed. I didn't even realize it until the 12AU7's. I just had a really hard time getting good note definition. Messed around with the 12AU7's for fun and thought it sounded awesome. Decided to throw one 12AX7 back in V3 and all the sudden the master volume was way too sensitive again and the definition went out the window. Then when you turned it up real loud, the power tube compression sounded nasty. Like a 300 pound girl trying to fit into size 2 jeans. Way too tight and ready to burst at any moment. Didn't like it. That being said, I still use a compressor in front of the amp, though.

 

[BobR]

Class A or cathode biased? Most amps that claim "class A" are really just cathode biased.
An amp designed to primarily run as cathode biased will run thru tubes quicker. They are idling at 100% dissapation all the time. Bob

 

[J.J]

Like a 300 pound girl trying to fit into size 2 jeans. Way too tight and ready to burst at any moment. Didn't like it.

Mental image go away :lol:

Class A or cathode biased? Most amps that claim "class A" are really just cathode biased.
An amp designed to primarily run as cathode biased will run thru tubes quicker. They are idling at 100% dissapation all the time. Bob

5 watt mode is cathode biased for the Express 5:25, but it doesn't run anywhere near 100% I'd have to look at my notes, but from memory it is around 70%. A cathode biased amp can still have it's operating point set anywhere the designer wants. The higher the idle current (and dissapation) the higher the output power will be for a single ended class A amp. Push pull is a little different though as they just push into class B operation at higher power if the bais is set lower.

 

[BobR]

Like a 300 pound girl trying to fit into size 2 jeans. Way too tight and ready to burst at any moment. Didn't like it.

Mental image go away :lol:

Class A or cathode biased? Most amps that claim "class A" are really just cathode biased.
An amp designed to primarily run as cathode biased will run thru tubes quicker. They are idling at 100% dissapation all the time. Bob

5 watt mode is cathode biased for the Express 5:25, but it doesn't run anywhere near 100% I'd have to look at my notes, but from memory it is around 70%. A cathode biased amp can still have it's operating point set anywhere the designer wants. The higher the idle current (and dissapation) the higher the output power will be for a single ended class A amp. Push pull is a little different though as they just push into class B operation at higher power if the bais is set lower.

Hence why I said "designed primarily to run" cathode biased trying to exclude some of Mesa's innovative designs. I should have just said "most cathode biased amps" as I didnt think the Mesa fell into this catagory .

Again most amps it seems that are touted as Class A are in fact cathode biased push-pull designs operating in AB. (Not that design matters since you could have a single ended or push pull amp operating in class A .) They also tend to set the idle current very high-over 100%. Sure they could be set anywhere the designer wants but for some reason its usually very hot. The bias will shift some with signal applied so perhaps this accounts for this partly but not nearly enough to justify it IMO. Bob

 

[J.J]

I hear lots of people talking about how they like the tone of tubes with the bias set to melting point and then backed off a tiny bit :lol:

For a single ended amp, you have to set the bias as high as you can since power output before clipping is directly related and it can't run in class B. 5:25 5 watt mode really is only a bit more than 1 watt RMS before clipping. So Mesa have not chosen the operating point based on the output power, is it to do with tone ?

I'd expect the nature of the overdrive to change quite a lot depending on where bias is set. Asymetircal clipping (5:25 5 watt mode) will introduce more even harmonics which tend to sound nice and symetrical clipping will tend to add the odd harmonics which tend to sound nasty (in a good way). Changing the bias will change the harmonic blend when the power stage is overdriving.

While we are talking about strange things, I'd still like to know how Mesa rate the output power of their amps. I really don't care that 5 watt mode on my Express does not make 5 watts, it sounds great, but why is it called 5 watt mode. Marketing perhaps ? Would we still like it if it was called a 1:25 or 2:25 ?

 

[BobR]

Well,5:25 does sound better! ...Mesa has done its share of strange things for sure. Why bias all thier early EL84 amps(.22,DC-2,DC-3) at the meltdown point? My DC-3 was running around 150% dissapation with most tubes stock. Bob

 

[J.J]

That is very interesting. And they were Mesa Tubes ? How long do they last at 150% ?

 

[BobR]

That is very interesting. And they were Mesa Tubes ? How long do they last at 150% ?

It didnt matter what tubes you put in it with only -11 neg bias voltage and 400 VDC on the plates. The Mesa (Sovtek EL84/6BQ5's) seemed to tolerate it the best very slight redplating. Before researching a fix for it I tried very low current draw tubes and that wouldnt work either. The lower current (tubes) draw allowed the plate voltage to rise even higher leaving you where you stated-very hot bias.

Evidently the Sovtek EL84/6BQ5 is pretty tough. Most guys who own this amp would complaint that it would eat thru tubes fast in a few months. Its amazing they lasted a few days let alone months!

Ive got the bias voltage down to around -15.5 on mine and its idling at around 80% now. I had it at 70% and I thought it sounded better at 80% though I'd never set any of my other amps that hot! I figured if Mesa can run them at 150% I can run them at 80! lol! Bob