A correctly setup Class A output stage will cut off (in the negative half cycle) at the same signal level as it saturates at low anode voltage (during the positive half cycle). It is important to set it up this way as it maximises the power output.
However, even if it isn't set up to cut off and saturate at the same signal level, it doesn't make a lot of difference.
Many PA and guitar amps are partly or wholey Class B anyway, to get more power output than what the tube dissipation permits in Class A. So the tubes are biassed further toward or completely on cut-off.
As I said, a Class A amp will be, or certainly should be, biased so that it cuts off and saturates at the same signal level. That's why my comments apply to SE, which must be Class A.
Actually, when a Class A SE stage is overdriven, it will draw grid current if the driver is still going strong. The grid current charges the grid coupling capacitor and overbiases itself, so the time spent saturated becomes less than 180 degrees or 50% of the time.
Last edited:
A correctly set up Class A amp never goes into cutoff or overdrive unless the drive signal is larger than the bias voltage. When that happens... turn it down. ...Guitarists...
It's not completely true. A guitar amplifier can be used in quite clean mode and differences in frequency response (I should say sound which is more appropriate) are REAL when swapping tubes. A good number of guitar amps when run clean, being push-pull type, can have lower distortion of many acclaimed HiFi single-ended amplifiers!
If you have one amplifier that can have both 6L6 and EL34's, biased properly, you can have quite different frequency response mainly because of the different plate resistance of these two types might display.
Why?
Most guitar amps use 10-12" cones which, especially the 12" ones, don't seem to be able to go beyond 4-5 KHz decently looking at datasheets. The trick is that in reality they do because most guitar amps don't use feedback and run in pentode mode and thus work as current amplifiers with rather high Zout. The effect on the big driver high frequency response is rather "profound". If you consider that the EL34 has a typical plate resistance in the 10-16K range while the 6L6 has 20-30K you can get quite some difference, especially when swapping tubes you fall into the extremes (i.e. 10K for the EL34 and 30K for the 6L6).
To prove the pentode mode without feedback ALONE, regardless of the tube you use, has a profound effect on the high frequency response:
1) bypass the tone controls
2) add a switch (that works with the amplifier OFF only) that moves the connection of the screen grid from pentode to triode ( i.e. screen grid tied to the plate by means of a 220-470R resistor, for example).
You will see that once you switch to triode all the higher trebles from your 12" cone will "magically" disappear! You will get some sort of warm dark sound and even if you play with the bias of the tube it will not change too much....
Tubes amplify from DC to several hundred kilohertz's, if not MHz's, they don't have a "frequency response".
However, tubes naturally do have parameters like gain, internal impedance, internal capacitances, etc. and these will naturally interact with the surrounding circuit.
e.g.
If you swap a tube to another with different internal impedance this interacts with RC circuits within the coupling circuit, and will therefore have effects on frequency response. It's not tube's frequency response though, it's circuit's.
If you swap a tube to another with different gain it interacts with feedback circuits (e.g. degenerative feedback through cathode) and this will have effect on frequency response. It's not tube's frequency response though, it's circuit's.
If you swap tube to another with different internal capacitances they will interact with all RC circuits within. This again affects frequency response. It's not tube's frequency response though, it's circuit's.
There's a pattern in this, which should be obvious. Need I continue....?
Well yes there is. You only need to glance at the characteristic plate curves to see it. These differences will affect overall distortion and overdrive characteristics, though. They don't have anything to do with "frequency response" of the tube but generally different overdrive characteristics (e.g, hard vs. soft clipping) can give an impression of having a different response.
Last edited:
What nonsense!
If you have ONE amplifier, and try 6L6 and EL34, the amp cannot be correctly set up or optimised for both. At best it must be a compromise set up - and this type of compromise can onlt shift the difference, it cannot reduce it.
In Class A recommended operating conditions, for example, the anode resistance of a 6L6 is 22 kohm; and EL34 has 15 kohm. The difference is approximately in proportion to the higher plate current (100 mA vs 72 mA) and greater power output the EL34 is designed for. So, if the amplifier is optimised for the tube in each case the output transformer will be different and the damping on the loudspeaker will be about the same.
I said right at the start triodes are different.
Last edited:
Most of what you wrote is correct. However:-
The tube capacitances are so small that in audio of guitar amp service they are entirely negligible. It's the transformer that sets the bandwidth.
But the gain of an EL34 is nearly twice that of a 6L6 - that will certainly make a difference in any amp using feedback, and may make the amp unstable, even without feedback.
Your bit about hard and soft clipping needs to come back to what I said right at the start: An amplifier designed/optimised for 6L6 and fitted with a 6L6 will sound the same as an amp designed/optimised for EL34 and fitted with EL34.
People who swap tube types in the one amplifier hear a difference in sound. Of course they do! It's about as dumb as taking the engine management computer out of a Mercedes car and sticking it in Ford car. Of course there will be a difference!
Last edited:
As you usual here he is Mr "I know everything without knowing what I am talking about"
This is nonsense. This is not HiFi!
Guitar amps GENERALLY don't run in class A except if they are SE where class A is the only mandatory choice. Generally they are class AB or even class B. Swapping tubes can move the amp from AB to B, alone. Already the Zout has changed! The plate resistance will also depend on the specs of the ACTUAL tubes which can deviate quite a lot from the average! The Zout is not simply the plate resistance. A factor of 2 in Zout CAN give already an audible effect. A factor of 3 can happen quite likely taking into account the above differences. As usual you show you have no experience and I can bet you don't even play the guitar. I do and I know enough to state that optimization for most guitar players has only to do with the sound they listen to!
Last edited:
But will an amp optimized for EL34 pentode sound the same as amp optimized for 6L6 beam tetrode....? Differences are pretty well documented and while differences in characteristic curves are slight they are still as obvious to spot as those between pentodes and triodes. This will most undoubtedly effect something that is audible.
That something, however, is not frequency response and will manifest itself prominently only when the stage with said tubes gets ovedriven and is not operating "clean". I guess those issues are kinda off-topic then given the thread's overall idea... Anyway, my point was that in cases of things like soft vs. hard clipping people may perceive one as "brighter" tone than the other and mistake the effect as higher bandwidth towards higher frequencies while in essence they are really just hearing effects of more upper order harmonic distortion in the signal.
I wouldn't count on that. The overall impedances in guitar amp -type circuits tend to be rather high-ish so even very low capacitances may have a great importance. One good example is the input stage of a typical guitar amp, which in combination with the pickup works as a resonant circuit and changes its response even due to guitar coord's capacitance.
Last edited:
Don't be ridiculous.
Whether an amp using tetrode or pentodes clips hard or soft depends on the circuit, not the tubes. Negative feedback causes hard clipping. Co-incidence or non-coinicidence of driving stage clipping affects it. Power supply regulation and choice of screen feed type affects it. So does unintentionally or intentionally undersizing teh transformer core. Not the tube.
But, as I said, if you go dropping a high gain pentode like the EL34/6CA7 into an amp designed for 6L6, you will get a different sound becasue the gain is different, and because 6L6 optimum operating conditions are not optimum for an EL34
You can indeed count on it.
Tube capacities, grid to gathode, anode to cathode are of the order of a few picofarads. Wiring strays are of the same order to pehaps a bit more. But the internal capacitances of the output transformer are of the order hundreds of picofarads, and completely swamp out the effect of tube capacitances.
Bandwidth is also limitted by output transformer leakage inductance.
The same tubes as used in audio amplifcation have been used in ham radio HF radio transmitters working efficiently at 28 MHz.
That's your strategy, not mine! You pass judgements like "What nonsense" without knowing anything about guitar amplifiers and what people do with them.
The hen was born before the egg or the contrary? A tube cannot work without a circuit and all they tubes mentioned in the title are not equivalents. I didn't say that each tube has a sound although it might be true to a certain extent, for example, just because they all are more or less microphonic having different sizes, shapes and construction.... a guitar amplifier is a good tool for selecting microphonic 12AX7's. Didn't you know? And until they ring like mad they DO give a different sound (or better the amp sounds different) even if they measure and bias the same! Your problem is that you ignore the reason why people ask questions....
Last edited:
You forget that guitar amps often use very high values of grid stopper (perhaps partly to offset their often appalling layout and wiring) so Miller effect means that 'tube rolling' becomes a form of treble tone control.Keit said:
This thread is about power tubes, not preamp tubes, isn't it? An it is about frequency response, not microphonics or distortion.
Differences in frequency response when plugging in different preamp tubes is much greater than when plugging in different power amp tubes, especially if no global feedback is used. If global feedback is used then differences in gain will indeed affect the frequency response of the overall amplifier, but it would not be correct to blame the frequency response of the tubes for this. e.g. it is not correct to say pentodes 'sound brighter' than tetrodes. Pentodes have more gain and therefore more NFB, making the whole amp seem brighter.
It's not me misquoting other members' posts. The reality is that different tubes produce a different sound because they can be used rather differently in different amplifiers, because people don't care too much about working conditions when the swap them and tubes are subject to a lot of mechanical feedback. Microphonics and distortion play a fundamental role in the sound and the sound IS the only thing that matters!! This is not HiFi as there is nothing to reproduce. You just can't take this out of the equation. In the end with "if" and "then" one gets nowhere. It's just academy.....
Anyway, frequency response isn't just how far it goes. Even without considering microphonics and distortion, the frequency response of a typical guitar amplifier is TOTALLY dominated by the guitar one uses, all tone controls (or sort of) in the path of the signal, the way the various stages work, especially the output stage, and its speaker.
Most guitar amps don't have feedback. I only remember some Marshall's using some feedback and possibly there are others but you will notice that most of them (if not all) only use ALNICO drivers to get back some trebles. If you apply feedback you get less trebles from a big cone. It doesn't work, for example, with my favourite Eminence GB12.
If you swap tubes like MOST people do (i.e. listening to the sound rather than measuring anything) you can go from class A to Class B and that will change the frequency response because the Zout of the amp does change and has a measurable and audible effect on the FR of a 12" driver.
Last edited:
Irrelevant. This thread is about frequency response and power tubes. You're trying to answer a question nobody asked.
Maybe for some other question. This thread is about frequency response and power tubes.
Uh...no... the guitar has nothing to do with the frequency response of the amp.
Yes but this thread is about the frequency response imparted by the power tubes.
Yes, others like Fender (ever heard of them?), who made more amplifiers than anyone else, and were copied by almost everyone. Most Fender amps have feedback and therefore most guitar amps have feedback.
Good, what I and everyone else said was correct then. It's not the frequency response of the tubes, it's the circuit around them that they affect.
It's irrelevant for someone like you who doesn't read carefully! I just answered to a misquoting. Wasn't talking at about microphonics and distortion in the first place.
You will never get a unique solution if you only stick to power tubes. The frequency response will depend on the driver which is part of the amplifier exactly like the output transformer and the power tubes. This is not current HiFi where one has typical voltage amps which enable speaker design independently!
Yes, it doesn't for all those who don't play it and insist on viewing things from HiFi perspective which is totally wrong.
Again....POINTLESS!!! Too generic to get a unique answer.
I don't think so!
Again, read carefully next time. You are confused...
In the attached pictures there is the frequency response in air of the same single 10" cone driven by a current amp and a voltage am. The title says which is what....or you can guess....
Attachments
Last edited:
- Status
- Not open for further replies.