Does valve watts relate to amp output watts?

pickerg

I've been learning about bias, plate voltages and rectifiers etc. over the last couple of days and have possibly a silly question. Does the valve wattage (heat dissipation?) relate to the actual wattage the amp is putting out, or is it more complicated than that, and about the circuit and output transformer?
Thanks.

ICBM

Very roughly, in Class A the output power is limited to half the total maximum plate dissipation of all the valves, and in Class AB to about the total maximum dissipation. ie a single EL84 (12W max) can put out up to 6W in Class A, or two EL34s (25W max each) can put out 50W in Class AB.

They don't need to be operated at these limits though, you can easily have them put out less - or a bit more if you're willing to push the ratings a bit hard... or be creative with how you define and measure it .

ESchap

No, most 40-50 watt valve amps will use around 150 - 250 watts or so.

I think most of the power use is by the valves and converted to heat.  Transformers are very efficient devices, though there is a small loss there too, circuit components will not use much, speaker a little if your running very loud. 

NervousJohn

Simply, DC input power - output power = dissipation.

Dissipation is generally heat mostly from the valve dissipation + heaters. There is a tiny amount from the transformer but seeing as they are pretty much the most efficient things humans have made you can safely ignore it.

From memory each output valve of an AC-30 draws about 18W at full chuff, to deliver about 7W audio. The other 11W is heat. Multiply this by 4 and you can see why they get so toasty.

You can break many rules in rock and roll but not the laws of thermodynamics.

ecc83

pickerg said:

I've been learning about bias, plate voltages and rectifiers etc. over the last couple of days and have possibly a silly question. Does the valve wattage (heat dissipation?) relate to the actual wattage the amp is putting out, or is it more complicated than that, and about the circuit and output transformer?
Thanks.

Complicated only in the sense that you cannot just look at an amp and say "Ah" two EL34s, that'll be a 50 watter then!"

Also complicated by the fact that SOME amp makers tell porkies or at least make the name suggest a power level the amplifier is unlikely to achieve in practice such as the "BogRock 25" when it contains but two EL84s. Such a pair in the vastly more common cathode biased mode (for 84s) will kick up to about 20W with a high HT and the mains on top but 15-18W is a fairer description.

A good set of examples (if I might be spared accusations of spam?) is the HT range?

The HT-20 uses 34s but with a fairly low HT and part cathode bias. The amp will only deliver around 20 watts before the sine wave gets nasty.

Up the HT a bit and you have the HT-40 which with fixed bias now easily hits 40W and quite a bit beyond.

The HT-60, still only 2 EL34s will easily deliver 80watts but you run into power supply cost issues (I guess!) because the HT-100 only puts out a bit over its honest specc'.

Note too that all these figures are for a PROPER nominal 230V mains input, most folks get around 240 so win-win!

Mind you, these numbers are also for clean power out. Start to push amps into heavy distortion and the actual "rms" power climbs and as the output gets ever "squarer" its destructive power increases. This is why I always want a speaker  to be rated at least 50% higher than the amp driving it.

In my last days at B's I was asked to check the PO of some chassis at 10% THD. Is the guitar amp industry starting to get some decent standards?

Dave.

GuyBoden

IMHO, you hear sound pressure in decibels SPL, electrical power is in watts. You can't hear watts, but you can hear sound pressure.

ICBM

ecc83 said:

Complicated only in the sense that you cannot just look at an amp and say "Ah" two EL34s, that'll be a 50 watter then!"

It is a reasonable guide with most amps, but I agree that you definitely can't tell for sure.

ecc83 said:

Also complicated by the fact that SOME amp makers tell porkies or at least make the name suggest a power level the amplifier is unlikely to achieve in practice such as the "BogRock 25" when it contains but two EL84s.

I spoke to the designer of the "Fictional-30" (2-EL84) some time ago and he assured me it was because they used a patented circuit called "DYouKno What" or something like that which magically delivers more power than the circuit would normally be capable of. As politely as possible I pointed out that I'd actually measured it and found it only gave 22W, to which he replied that it "sounds like 30 though"!! They're still using this patented magic technology on some of their amps too.

Not as good as Roland though, who have found a way to get around the laws of physics much more spectacularly with their new GA-112 and GA-212 amps, which put out more audio power than they draw from the mains . (45W draw for the 100W model, 75W draw for the 200-watter!)

ecc83 said:

The HT-60, still only 2 EL34s will easily deliver 80watts

That's really going some… are they using very high HT, and/or running in near Class B like the old MusicMans did?

NervousJohn

ICBM;246201" said:

ecc83 said:Also complicated by the f
Not as good as Roland though, who have found a way to get around the laws of physics much more spectacularly with their new GA-112 and GA-212 amps, which put out more audio power than they draw from the mains . (45W draw for the 100W model, 75W draw for the 200-watter!)

Stuffed full of unicorns. They have the correct paperwork to exempt themselves from physics.

pickerg

Thanks for the replies, this has been a big help.  So potentially, in something like a 5F6-A Bassman clone, going from GZ34/6L6 to something like 5V4/5881 (as long as the plate voltage was low enough for the 5881) could get you down from around 40 watts to around 30 watts(60% of the maximum combined 46 watts from the 5881s)?  I realise that the difference to the ear will probably not be much if anything but the brake-up sweet spot would be lower and it would influence available speaker choice?

 

ICBM

It won't have much effect on either the volume or the speaker choice, but the tone character at the point of breakup might be quite a lot different, which will be more noticeable probably.

5881s should be OK anyhow, but a 5V4 will help. You do need to get the power down a bit even for a 5V4 to take the current though - it will be at the upper limit (175mA). Not sure it will drop to as low as 60% of full power though.

For illustration, in my Mesa Trem-o-verb the plate voltage is 445V in solid-state diode mode and the power output is 100W - I didn't measure it with GZ34s but it will only be a bit less than this. With 5V4s (which are right at their limit, at 350mA with two valves) I get 420V and 80W.

jpfamps

The power dissipation rating of the valves is simply that: the maximum dissipation that the valve can withstand.

For valves (like many parameters) this is quite a "soft" limit, ie exceeding the maximum dissipation won't automatically result in the immediate destruction of the valve (unlike solid state parts.....).

Whilst the power output relates in part to the maximum power output of the valves, as pointed out above, it's not the only factor, although all things being equal (which they usually aren't), the higher the maximum plate dissipation the greater the potential power output, however maximum anode voltage is also important.

The EL34 for example has a maximum anode dissipation of 25W, however it has a maximum anode voltage of 800 VDC, so can get 100W from a pair when run at maximum voltage.

I once worked on a Dynachord amp that I measured about 95W from a pair of EL34s that had nearly 800 VDC on the plate.

The 6L6-GC has a maximum dissipation of 30W, however it's got a maximum anode voltage of 500 VDC, and although several manufacturers have pushed this a bit, you rarely see amps with 2 6L6s in that can generate more than 55 W.

Increasing the anode voltage will increase maximum power output, however you will need to bias the valves more towards class B loading to avoid over dissipation. You also need to increase the anode loading as you increase anode voltage to ensure that the valves are operating optimally.

jpfamps

ecc83 said:

pickerg said:

I've been learning about bias, plate voltages and rectifiers etc. over the last couple of days and have possibly a silly question. Does the valve wattage (heat dissipation?) relate to the actual wattage the amp is putting out, or is it more complicated than that, and about the circuit and output transformer?
Thanks.

Complicated only in the sense that you cannot just look at an amp and say "Ah" two EL34s, that'll be a 50 watter then!"

Also complicated by the fact that SOME amp makers tell porkies or at least make the name suggest a power level the amplifier is unlikely to achieve in practice such as the "BogRock 25" when it contains but two EL84s. Such a pair in the vastly more common cathode biased mode (for 84s) will kick up to about 20W with a high HT and the mains on top but 15-18W is a fairer description.

A good set of examples (if I might be spared accusations of spam?) is the HT range?

The HT-20 uses 34s but with a fairly low HT and part cathode bias. The amp will only deliver around 20 watts before the sine wave gets nasty.

Up the HT a bit and you have the HT-40 which with fixed bias now easily hits 40W and quite a bit beyond.

The HT-60, still only 2 EL34s will easily deliver 80watts but you run into power supply cost issues (I guess!) because the HT-100 only puts out a bit over its honest specc'.

Note too that all these figures are for a PROPER nominal 230V mains input, most folks get around 240 so win-win!

Mind you, these numbers are also for clean power out. Start to push amps into heavy distortion and the actual "rms" power climbs and as the output gets ever "squarer" its destructive power increases. This is why I always want a speaker  to be rated at least 50% higher than the amp driving it.

In my last days at B's I was asked to check the PO of some chassis at 10% THD. Is the guitar amp industry starting to get some decent standards?

Dave.

The couple of HT40's I've had in have both failed to make 40W.

Furthermore there was evidence of saturation at 80 Hz, which is hardly surprising given that the OT is the smallest I've seen in a 40W amp; indeed it's the same size as used in the Marshall studio 15W, and smaller than that used in the Orange Tiny Terror.

pickerg

Thanks for taking the time to explain guys.

ecc83

"The couple of HT40's I've had in have both failed to make 40W.

Furthermore there was evidence of saturation at 80 Hz, which is hardly surprising given that the OT is the smallest I've seen in a 40W amp; indeed it's the same size as used in the Marshall studio 15W, and smaller than that used in the Orange Tiny Terror.

Yes, you have mentioned the 80Hz issue before. I confess it has been quite some time since I have specc'ed a 40 and now no longer have the opportunity but I HAVE tested them in the past and I am sure I would have called the low power delivery to a director's attention. Blackstar are very keen that their products do what it says on the tin! They would not perhaps be concerned by a fall off at 80Hz since power is tested at 1kHz and unless the 80Hz distortion was causing a bad sound it would pass muster.

Should you (or ICBM) get across another HT-40 and it is low in power I am sure Blackstar would be very interested to have the figures. Maybe later version mains traff ratios have gone awry? ***T 'appen!

Dave.

ecc83

Further to the valve specc'/power output thread  (and the HT-40 debacle).

I mentioned testing output power to the 10%THD point?

This is the only fair way IMHO to test valve guitar amps. Some of you will be familiar with "hi fi" specifications will look at 10%THD with horror but in fact it is only just detectable on a sine wave and hardly at all on music and speech . Therefore a guitar amp hitting that figure will be judged to still be "clean" and  10% is handily the level that most valve makers quote for their valve power output point, e.g. the EL84 delivers about 5W at 10%THD.

Power output is often quoted at "clipping"? This can be very difficult to call on a scope and in fact if you are looking at a waveform and reading THD the trace often barely changes from 1 to 10% distortion.

Dave.

ChrisMusic

Thanks for the clarification ecc83, I grew up on solid state so the quotation of 10% THD made my face approximate to :-O  .  I appreciate your explanation, it makes sense when put like that.

I am enjoying all the discussions on valves on here, although they are few and far between this year.  These are filling in the pitifully inadequate knowledge I have on valve technology.  Keep up the good work chaps  

ecc83

ChrisMusic said:

Thanks for the clarification ecc83, I grew up on solid state so the quotation of 10% THD made my face approximate to :-O  .  I appreciate your explanation, it makes sense when put like that.

I am enjoying all the discussions on valves on here, although they are few and far between this year.  These are filling in the pitifully inadequate knowledge I have on valve technology.  Keep up the good work chaps  

Ah well! A decent solid state amp would probably clip/go into protection mode way before THD reached 10%! Valves just keep on giving!* BTW there is a rule of thumb that you can extract about 2 times the rated anode dissipation from a valve. The tables show that two EL34s could give you 100watts but I do not know of any commercial amplifier produced that has ever claimed this?

*Not QUITE true! A valve amp with a high level of feedback will have a very steep power to distortion curve . The venerable Mullard 20-20 hits only 0.1% at 27W but at just 35W it has reached 5% and is climbing fast!

Dave.

imalone

ecc83 said:

ChrisMusic said:

Thanks for the clarification ecc83, I grew up on solid state so the quotation of 10% THD made my face approximate to :-O  .  I appreciate your explanation, it makes sense when put like that.

I am enjoying all the discussions on valves on here, although they are few and far between this year.  These are filling in the pitifully inadequate knowledge I have on valve technology.  Keep up the good work chaps  

Ah well! A decent solid state amp would probably clip/go into protection mode way before THD reached 10%! Valves just keep on giving!* BTW there is a rule of thumb that you can extract about 2 times the rated anode dissipation from a valve. The tables show that two EL34s could give you 100watts but I do not know of any commercial amplifier produced that has ever claimed this?

*Not QUITE true! A valve amp with a high level of feedback will have a very steep power to distortion curve . The venerable Mullard 20-20 hits only 0.1% at 27W but at just 35W it has reached 5% and is climbing fast!

Dave.

Surely the THD goes up as you approach clipping, i.e. by the time you're clipping THD is high (by definition)? If I understand correctly (which may not be the case), valve power will continue to go up as THD increases while solid state designs typically are operating near their ceiling power before THD starts to rise (and then it rises fast), due to the higher level of negative feedback in most solid state designs. Or do you mean clipping because they've hit protection?

jpfamps

ecc83 said:

ChrisMusic said:

Thanks for the clarification ecc83, I grew up on solid state so the quotation of 10% THD made my face approximate to :-O  .  I appreciate your explanation, it makes sense when put like that.

I am enjoying all the discussions on valves on here, although they are few and far between this year.  These are filling in the pitifully inadequate knowledge I have on valve technology.  Keep up the good work chaps  

Ah well! A decent solid state amp would probably clip/go into protection mode way before THD reached 10%! Valves just keep on giving!* BTW there is a rule of thumb that you can extract about 2 times the rated anode dissipation from a valve. The tables show that two EL34s could give you 100watts but I do not know of any commercial amplifier produced that has ever claimed this?

*Not QUITE true! A valve amp with a high level of feedback will have a very steep power to distortion curve . The venerable Mullard 20-20 hits only 0.1% at 27W but at just 35W it has reached 5% and is climbing fast!

Dave.

As stated above I had a Dynachord amp that used 2 x EL34 at around 800 VDC on the anode and this measured at around 95W at clipping.

http://bmamps.com/Schematics/Dynacord/dynacord_eminent_2.pdf

ecc83

"Surely the THD goes up as you approach clipping, i.e. by the time you're clipping THD is high (by definition)? If I understand correctly (which may not be the case), valve power will continue to go up as THD increases while solid state designs typically are operating near their ceiling power before THD starts to rise (and then it rises fast), due to the higher level of negative feedback in most solid state designs. Or do you mean clipping because they've hit protection?"

Yes, sorry! My point should have been that it might be difficult to FIND a 10% THD point for many sstate designs. They tend to produce buggerall distortion then whammo! Off the scale. A valve amp with little or no feedback can be pushed progressively, 1,2,3,....10% and so on.

Err? What did that Service man Terminator SOUND  like J?

Dave.

imalone

ecc83 said:

"Surely the THD goes up as you approach clipping, i.e. by the time you're clipping THD is high (by definition)? If I understand correctly (which may not be the case), valve power will continue to go up as THD increases while solid state designs typically are operating near their ceiling power before THD starts to rise (and then it rises fast), due to the higher level of negative feedback in most solid state designs. Or do you mean clipping because they've hit protection?"

Yes, sorry! My point should have been that it might be difficult to FIND a 10% THD point for many sstate designs. They tend to produce buggerall distortion then whammo! Off the scale. A valve amp with little or no feedback can be pushed progressively, 1,2,3,....10% and so on.

Err? What did that Service man Terminator SOUND  like J?

Dave.

Makes sense, thought I might be missing something subtle.