What makes a good pre-amp valve for a guitar amplifer?
What I'm interested in here is approaching the question from the perspective of a designer.
I know that selecting the operating point to have some clean headroom and also allow for significant harmonic distortion via cascading gain stages is typical for a guitar preamp, and, they are predominately ECC83 based.
I'm interested in playing/designing with different triodes and pentodes, but when I read the data sheet they all look very similar.
In the attachment I am comparing ECC83 with EF80 and EF86.
I see that the "non-audio" tube here (the EF80) has higher current on the anode and much larger current swing for a given change in grid voltage (the mutual conductance).
Why do the "audio" tubes prefer a lower mutual conductance? Or maybe I'm generalising too much.
What are the characteristics of a "good" pre-amp valve for a guitar amp and why?
What I'm interested in here is approaching the question from the perspective of a designer.
I know that selecting the operating point to have some clean headroom and also allow for significant harmonic distortion via cascading gain stages is typical for a guitar preamp, and, they are predominately ECC83 based.
I'm interested in playing/designing with different triodes and pentodes, but when I read the data sheet they all look very similar.
In the attachment I am comparing ECC83 with EF80 and EF86.
- The anode voltages are all similar ~250V
- The grid voltage ranges are similar
- ECC83 and EF86 have lower plate current
- The EF86 has much lower (x6 smaller) mutual conductance than the EF80
- The EF80 has much high screen and voltage and currents and much lower anode impedance than the EF86
I see that the "non-audio" tube here (the EF80) has higher current on the anode and much larger current swing for a given change in grid voltage (the mutual conductance).
Why do the "audio" tubes prefer a lower mutual conductance? Or maybe I'm generalising too much.
What are the characteristics of a "good" pre-amp valve for a guitar amp and why?
Attachments
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Moved to Instruments and Amps.
A good front end for a guitar valve amplifier is soft limiter.
It can be varied from clean to mild distortion.
It can be varied from clean to mild distortion.
Seems interesting, I have to admit that goes a bit above my head.
Is the idea of soft limiting to keep the valve grid current in a nice range?
I support my question is more about, "how can I read a valve data sheet and know if this is an "ok" audio tube given the characteristics".
Is the idea of soft limiting to keep the valve grid current in a nice range?
I support my question is more about, "how can I read a valve data sheet and know if this is an "ok" audio tube given the characteristics".
Keep in mind this is just a guitar amp, not precision lab gear. Guitar amps have sounds of their own, they are not hifi reproducers of audio. 12AX7 is popular because it is readily available, it has useful gain. The pentode has more gain if gain is all you want. And that is part of it, a tube that might be a poor choice for a hifi might be a GREAT tube for the guitar amp. SOme smaller amp like a Champ comes to mind.
A Marshall and a Fender using the same tubes sound nothing alike. And yet both have MANY fans. Just my opinion, but you may find tubes that are not OK for audio but shine here.
Look at the curves for tubes, now imagine moving the active region up or down those curves. What would result in unwanted distortion in a hifi might result in a very pleasing distortion in a guitar amp. COmpare teh curves for a triode with those for a pentode.
A Marshall and a Fender using the same tubes sound nothing alike. And yet both have MANY fans. Just my opinion, but you may find tubes that are not OK for audio but shine here.
Look at the curves for tubes, now imagine moving the active region up or down those curves. What would result in unwanted distortion in a hifi might result in a very pleasing distortion in a guitar amp. COmpare teh curves for a triode with those for a pentode.
RF (radio) circuits (with coil loading) can run HIGH currents. 10mA.
Audio circuits with resistor loading usually run lower current. 1mA.
Gm rises with current. About as square root. So expect a 10mA tube to show about 3X the Gm of a 1mA tube. Pretty much even if they are the same tube under different bias.
If you do not know better, just steal Fender's stages.
If you must experiment, build a breadboard where all parts can be changed quickly and *securely*. (Not a big sprawling bench-size breadboard; it would hum and squeal.)
If you're really into it, you may want to build up a Fender stage or two and make it modular. Then you could swap in the (5 or so component) circuits built around your own tube choice and compare how it sounds quickly as part of your final amp design.
This has been tried before commercially and never really took off as an idea. I thought it was a great idea, but that's because I'm an EE - not a guitarist.
This has been tried before commercially and never really took off as an idea. I thought it was a great idea, but that's because I'm an EE - not a guitarist.
Enzo, I completely agree. It’s all about the circuit the tube is in. It’s a great point about it not being a precision product and that being a part of the charm and therefore non standard tubes might work well. I can imaging moving the operating point around and getting a feel for the harmonic distortion. Thanks.
PRR, why do audio tubes tend to have lower currents? Is it because for audio (at least in the preamp) one wants voltage amplification rather than current amplification? But… can’t we just use the load resistor to “convert” the large current swing to a large voltage swing? This in fact would be great for a guitar amplifier. Sorry of the basic questions. I’m still trying to understand the pros and cons. about why when valve manufacturers saw a gap in the market for audio tubes they designed them with the characteristics they have. Yes maybe it is easy to start with a really simple schematic and just try!
jjasniew, PPR, yeah I will do this at some point, will help understand how they sound. I guess I’m interested more in the electronic engineering/physics of why audio tube have the characteristics they have; they must have been designed that way for a purpose.
Thanks all!
PRR, why do audio tubes tend to have lower currents? Is it because for audio (at least in the preamp) one wants voltage amplification rather than current amplification? But… can’t we just use the load resistor to “convert” the large current swing to a large voltage swing? This in fact would be great for a guitar amplifier. Sorry of the basic questions. I’m still trying to understand the pros and cons. about why when valve manufacturers saw a gap in the market for audio tubes they designed them with the characteristics they have. Yes maybe it is easy to start with a really simple schematic and just try!
jjasniew, PPR, yeah I will do this at some point, will help understand how they sound. I guess I’m interested more in the electronic engineering/physics of why audio tube have the characteristics they have; they must have been designed that way for a purpose.
Thanks all!
It really depends on what sound you are designing for. Clean, overdriven, or distorted? OMG-Loud or home-friendly?
I find that the tone stack and the gain structure design for a specific volume are far more important in defining a distinct and recognizable tone.
Other than that, you can find differences and preferences between anything, and that includes iterations of the same schematic and amp.
I find that the tone stack and the gain structure design for a specific volume are far more important in defining a distinct and recognizable tone.
Other than that, you can find differences and preferences between anything, and that includes iterations of the same schematic and amp.
Hi dimkasta,
Hehe OMG-Loud not heard that before.
What I’m asking, which might be a stupid question, so please do tell me if so, is this: when I look at data sheet for a valve - see my attachment showing EL80 (radio) and EL86 (audio) - can I tell from that if the the valve has good attributes to be used with audio or not.
I think what you are saying is this, any valve is fine, just adapt the schematic? It that correct?
I love the sound of JCM800 for example and love the sound of Boogie clean, I know that will sound weird because they are better known for high-gain, but I guess is similar to Fender clean (just that I have never owned a Fender).
Hehe OMG-Loud not heard that before.
What I’m asking, which might be a stupid question, so please do tell me if so, is this: when I look at data sheet for a valve - see my attachment showing EL80 (radio) and EL86 (audio) - can I tell from that if the the valve has good attributes to be used with audio or not.
I think what you are saying is this, any valve is fine, just adapt the schematic? It that correct?
I love the sound of JCM800 for example and love the sound of Boogie clean, I know that will sound weird because they are better known for high-gain, but I guess is similar to Fender clean (just that I have never owned a Fender).
IF there is in fact a large resistor. There are capacitances hanging off of everything, and at RF they can dominate the impedance. You can resonate them out, but the Q of the tank will only let you get the impedance so high. If the working impedances are low, you want high current tubes. At audio where a few pF of capacitance won’t bother anything, you can have 100k or higher plate resistors so low currents work well. High resistances also generate more noise than low ones, so in very low noise situations the currents will creep up, even at audio. Guitar amps tend not be be concerned with noise as much as gain, so you’ll see lower currents and larger resistors than even in hi-if.
P=UI. Power of a component is physically limited. So if you have a big V you can have a low I and the opposite.
V of valve is high so current is low @ P = constant
P=10W :
If V = 250V then I = 40mA
If V = 20V then I = 2A
So for the same output power a valve need less current than a BJT or AOP because voltage is bigger.
V of valve is high so current is low @ P = constant
P=10W :
If V = 250V then I = 40mA
If V = 20V then I = 2A
So for the same output power a valve need less current than a BJT or AOP because voltage is bigger.
And as the current is more little, you need bigger resistors to have the voltage drop necessary for your circuit 😉
Thanks WG and Kartapus,
This really made things click. I will try and summaries what you said so check my understanding.
At radio frequency the capacitance of the tube can dominate the circuit. This can be mitigated by using lower value resistors, which means that currents will naturally increase because I=V/R. Moreover, if R goes down then I goes up.
At audio frequency the capacitance of tubes are not really a problem so it's possible to use lower currents. Lower current tubes require higher resistance values for setting the bias. However, high resistance increase noise in the circuit, but for guitar amplifiers this is generally not much of problem.
So the reason ECC83 tends to be used in guitar amps is that it gives the most amount of gain (mu=100 for that tube), acceptable frequency response and introduces acceptable noise, and because the current is low the power rating of the components can be less.
I hope I followed!
I think I have some follow up question but I will wait a little to let things settle in my brain.
This really made things click. I will try and summaries what you said so check my understanding.
At radio frequency the capacitance of the tube can dominate the circuit. This can be mitigated by using lower value resistors, which means that currents will naturally increase because I=V/R. Moreover, if R goes down then I goes up.
At audio frequency the capacitance of tubes are not really a problem so it's possible to use lower currents. Lower current tubes require higher resistance values for setting the bias. However, high resistance increase noise in the circuit, but for guitar amplifiers this is generally not much of problem.
So the reason ECC83 tends to be used in guitar amps is that it gives the most amount of gain (mu=100 for that tube), acceptable frequency response and introduces acceptable noise, and because the current is low the power rating of the components can be less.
I hope I followed!
I think I have some follow up question but I will wait a little to let things settle in my brain.
Guitar valve amps are often open loop as in no feedback.
This keeps the 2nd harmonic distortion needed for a warm valve sound.
This keeps the 2nd harmonic distortion needed for a warm valve sound.
ECC83 is the highest gain of the commonly available “preamp” tubes. To get higher gain in a single stage you have to resort to using pentodes. That results in a far different distortion profile. Many are horribly microphonic - not what you want in an amplifier with a built-in speaker. And they use more power in general - higher plate and heater currents, and wasting some in the screen. Sometimes you do want that much gain, especially if feedback is used to linearize things and keep them quiet. But thats not the typical way to design a guitar amp.
70 mu triodes often do provide sufficient gain, especially if 5 stages are used instead of 4, for instance. Depending on the circuit it may be a wash with the 100 mu since that isn’t the only factor involved.
Hi WG,
Ok then I think I “got it”.
Yes I was wondering if it would be possible to make a pre-amp with ECC81 tubes. Is the extra stage needed to give enough gain to crunch the power tubes?
But I think tonality wise, by setting the correct load lines, an ECC81 and ECC83 pre-amp could be made to sound more or less identical.
Ok then I think I “got it”.
Yes I was wondering if it would be possible to make a pre-amp with ECC81 tubes. Is the extra stage needed to give enough gain to crunch the power tubes?
But I think tonality wise, by setting the correct load lines, an ECC81 and ECC83 pre-amp could be made to sound more or less identical.
IIRC, 12AX7 was categorized as "audio" tube merely because its (rather high) interelectrode capacitances prevented using it in RF applications in typical high impedance circuits.
For "audio" in general you would probably be looking for most linear operation (leading to smallest percentage of distortion) as possible. That would be quantifiable objectively. ...But it's easier to just introduce enough of negative feedback to linearise circuits.
For an effect processor that deliberately distorts anything goes and there are no objectively quantifiable guidelines.
For "audio" in general you would probably be looking for most linear operation (leading to smallest percentage of distortion) as possible. That would be quantifiable objectively. ...But it's easier to just introduce enough of negative feedback to linearise circuits.
For an effect processor that deliberately distorts anything goes and there are no objectively quantifiable guidelines.