That’s really interesting.
So really all classic guitar amps designs just using “audio” components because they were what was available and presumably cheap, maybe came with anti-microphonic designs. But electronically the tone could be achieved is with other tubes.
So really all classic guitar amps designs just using “audio” components because they were what was available and presumably cheap, maybe came with anti-microphonic designs. But electronically the tone could be achieved is with other tubes.
In the real world there are "audio tubes" and other tubes. Much of those designations come from the world of "that's the way it has always been done" which was before many modern high gain tubes were developed for TV and industrial, military, and specialized communications (usually long distance telephone).
Here on this forum you will find many builders like myself who have used these "other tube" for very good HiFi and guitar amp designs.
There is also another constraint given to modern guitar amp designers. Sure it is possible to make a killer guitar amp that uses tubes from TV sets, or other old equipment. These tubes are no longer being made, and will eventually get scarce and expensive. This is already happening with many TV tubes that have found use in commercial audio amps. Then someone, usually in the oriental countries, buys up all the remaining stock and holds them until the price rises. This is happening in the semiconductor world too.
Joe Rocker buys a "magic" guitar amp from a small supplier, then goes on tour with it. He is in small town nowhere when a tube blows. If that tube is not available in a larger music store, or from a supplier that can quickly Fedex him one, he is out of luck.
So, anyone designing a guitar or HiFi amp for possible commercial sale must use tubes from the list of about 30 tubes that are being made today.
If you are building an amp for yourself, or a few for your friends, use any tube you want, just buy a few spares.
I started making tube guitar amps out of scrap TV sets as a kid in the 1960's because the parts were free. I still make amps with TV tubes today, so in my opinion there are very few tubes that can't be used in audio.
I have found the limit for absolute stage gain is about 1000 V/V (60 dB). Even at this level most tubes will be so microphonic that the head can't sit on top of the speaker cabinet without feedback.
There have been channel switching amps where two or three complete, but different preamp sections get the same guitar signal, but one at a time can be switched to the output (power) stage.
I made a dual channel amp where one channel was the typical 12AX7 - tone stack - 12AX7 design and the other used two 6EJ7 TV pentodes with the tone stack between them. I used a pot to blend from one preamp to the other. Somewhere in the middle all sorts of unique tonal structure can be obtained with 6 active tone pots.
I am an engineer who used to be a good guitar player. The text book approach can only go so far with guitar amps, especially those designed for operation in the nonlinear (distortion) region.
Getting more gain than 50 to 70 V/V requires a pentode. A pentode with a very high load impedance can have near infinite gain.....and be horribly microphonic. Some pentodes are much worse than others, and two of the same kind can be quite different.
Here on this forum you will find many builders like myself who have used these "other tube" for very good HiFi and guitar amp designs.
There is also another constraint given to modern guitar amp designers. Sure it is possible to make a killer guitar amp that uses tubes from TV sets, or other old equipment. These tubes are no longer being made, and will eventually get scarce and expensive. This is already happening with many TV tubes that have found use in commercial audio amps. Then someone, usually in the oriental countries, buys up all the remaining stock and holds them until the price rises. This is happening in the semiconductor world too.
Joe Rocker buys a "magic" guitar amp from a small supplier, then goes on tour with it. He is in small town nowhere when a tube blows. If that tube is not available in a larger music store, or from a supplier that can quickly Fedex him one, he is out of luck.
So, anyone designing a guitar or HiFi amp for possible commercial sale must use tubes from the list of about 30 tubes that are being made today.
If you are building an amp for yourself, or a few for your friends, use any tube you want, just buy a few spares.
I started making tube guitar amps out of scrap TV sets as a kid in the 1960's because the parts were free. I still make amps with TV tubes today, so in my opinion there are very few tubes that can't be used in audio.
I have found the limit for absolute stage gain is about 1000 V/V (60 dB). Even at this level most tubes will be so microphonic that the head can't sit on top of the speaker cabinet without feedback.
There have been channel switching amps where two or three complete, but different preamp sections get the same guitar signal, but one at a time can be switched to the output (power) stage.
I made a dual channel amp where one channel was the typical 12AX7 - tone stack - 12AX7 design and the other used two 6EJ7 TV pentodes with the tone stack between them. I used a pot to blend from one preamp to the other. Somewhere in the middle all sorts of unique tonal structure can be obtained with 6 active tone pots.
I am an engineer who used to be a good guitar player. The text book approach can only go so far with guitar amps, especially those designed for operation in the nonlinear (distortion) region.
Getting more gain than 50 to 70 V/V requires a pentode. A pentode with a very high load impedance can have near infinite gain.....and be horribly microphonic. Some pentodes are much worse than others, and two of the same kind can be quite different.
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Perhaps. Look at the record. There was a pre-Octal type with Mu~~100. It was clearly aimed at the "1st Audio" socket in an AM radio. And considered a "cheap trick" because if your RF and IF stages were good you did not need a bunch of gain at audio.
In Octal (with slight improvement) this was 6F5 in 1935.
The idea of using two of these in one chassis was absurd. It would howl. And as George says, acoustic feedback too. Note that soon after release, the 6F5's grid was moved to top-cap, much like high-gain RF/IF tubes. The no-top-cap became 6SF5.
But we needed analog computers for some important projects, and a dual high-Mu tube could be tamed. 12AX7.
The octal 6Q7 had a mu of 70. The metal 6SQ7 brought that number up to 100 and ditched the grid cap. Two triodes with a Mu of 70 got stuffed into a metal envelope with the 6SC7, they do share a common cathode. I don't know of any octal twin triodes with a Mu of 100.
I got a big box full of metal tubes for free at a hamfest several years ago, so I have been experimenting with metal tubes for a guitar amp I will call "MetallicAmp, all tube, no glass."
Some metal 6SJ7 pentodes (think 5C1 Fender champ) are dead quiet, while some are pretty microphonic. The glass 6SJ7's are similar though. I only have a couple 6SF5's, one is quiet, the other is not. I have a pair of NOS RCA "Special Red" 5693's, (expensive 6SJ7's) they are dead quiet. The 6SC7's will be used for the PI, and maybe for some gain stages, depending on how many good ones I have. Ditto the 6SQ7's.
In general pentodes are more microphonic than triodes just because there are more grid wires inside to vibrate. G2 and G3 create less of an electrical disturbance than G1 though. I have used the 6BE6 pentagrid converter as a triode before, bit have not tried the 6SA7.
I want to make a modern screamer with parts that were available before I was born (1952), hence MetallicAmp. There will be multiple signal paths controlled by photoresistors. The CdS cell existed in the 1930's, but LED's did not, so my Soldano style channel switching will use incandescent or neon light.
I got a big box full of metal tubes for free at a hamfest several years ago, so I have been experimenting with metal tubes for a guitar amp I will call "MetallicAmp, all tube, no glass."
Some metal 6SJ7 pentodes (think 5C1 Fender champ) are dead quiet, while some are pretty microphonic. The glass 6SJ7's are similar though. I only have a couple 6SF5's, one is quiet, the other is not. I have a pair of NOS RCA "Special Red" 5693's, (expensive 6SJ7's) they are dead quiet. The 6SC7's will be used for the PI, and maybe for some gain stages, depending on how many good ones I have. Ditto the 6SQ7's.
In general pentodes are more microphonic than triodes just because there are more grid wires inside to vibrate. G2 and G3 create less of an electrical disturbance than G1 though. I have used the 6BE6 pentagrid converter as a triode before, bit have not tried the 6SA7.
I want to make a modern screamer with parts that were available before I was born (1952), hence MetallicAmp. There will be multiple signal paths controlled by photoresistors. The CdS cell existed in the 1930's, but LED's did not, so my Soldano style channel switching will use incandescent or neon light.
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The '75 DDT is the gran-daddy of the hi-Mu single triodes; actually older than RMA.
Pentode 6J7 (topcap) was a mainstay of broadcasting audio; often used as a medium-Mu triode (which would be a 6J5, but it was rational to minimize the number of types in the spares closet). The nominally similar capless 6SJ7 never caught on in this field (perhaps too many 6J7 spares).
No, there is no Mu=100 twin triode in the US Octal line. Mu is not a constant and 6SL7 can exceed nominal 70 while 6Q7 et al sure can drop below 100 at low current.
Pentode 6J7 (topcap) was a mainstay of broadcasting audio; often used as a medium-Mu triode (which would be a 6J5, but it was rational to minimize the number of types in the spares closet). The nominally similar capless 6SJ7 never caught on in this field (perhaps too many 6J7 spares).
No, there is no Mu=100 twin triode in the US Octal line. Mu is not a constant and 6SL7 can exceed nominal 70 while 6Q7 et al sure can drop below 100 at low current.
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But that's the beauty of the 12AX7, isn't it? Adds a free pole to the circuit to tame out oscillation tendencies at high gain and keeps that pole above the audio band to get rid of any HF EMI.
Also, given the gain, it's quite linear. It's not a bad tube like the 12AT7...
One thing to think about is how amplifiers are designed. You want to be able to get a clean 15 W out of a pair of 6V6's. So you need to select the right voltages and load to get your target. Then you need the preceding stage to be able to give the signal voltage needed to drive the 6V6 to full output. Then you need the stage before that one to pass enough clean signal to it, and on and on.
The higher the supply voltages used on the output tubes can produce more power (Hello George) but that requires a greater grid bias and therefore a greater clean signal from the preceding stage(s). You can play around with the supply voltage, the load line (AC and dc). Changing the operating parameters is a fun way of making a tube do different things. Ultimately your ears have to decide. As said, you can plug different values of components into a circuit.
I messed around with a 6AU6 and had pots rather than resistors on the plate cathode and screen. It is much faster than changing resistors and you can hear the change in real time. I found, for the particular voltage as I did not have that adjustable, there was a very obvious sweet spot with the three values adjusted. If I did not mess around to find it I would have thought the tube did its job but was nothing special. While on the tube, I will mention I used it in the first stage of a 5F6 Bassman styled circuit in place of the triode. Sounded good, the gain was something. But don't stop playing otherwise the 6AU6 started singing on its own.
There are a few ways of messing with tube types to see what they may do, datasheets have the curves for you to draw upon. There are spice models of many tubes, one day I will get into that. In the mean time there is an online load line calculator that has wasted many hours for me.
Universal loadline calculator for vacuum tubes - Vacuum Tube Amplifiers - DIY
You can do output duty or as signal amplification. In this mode it will tell you the second, third and forth harmonic distortion components. you put the voltage you want out (in the bottom Output Headroom box). At the Next Stage AC Impedance line it tells the 2,3,4th harmonic distortion component. Shame it does not have a cathode resistor function, you could calculate the value and adjust the bias voltage. Anyway, you get an idea of a few different tubes listed.
The higher the supply voltages used on the output tubes can produce more power (Hello George) but that requires a greater grid bias and therefore a greater clean signal from the preceding stage(s). You can play around with the supply voltage, the load line (AC and dc). Changing the operating parameters is a fun way of making a tube do different things. Ultimately your ears have to decide. As said, you can plug different values of components into a circuit.
I messed around with a 6AU6 and had pots rather than resistors on the plate cathode and screen. It is much faster than changing resistors and you can hear the change in real time. I found, for the particular voltage as I did not have that adjustable, there was a very obvious sweet spot with the three values adjusted. If I did not mess around to find it I would have thought the tube did its job but was nothing special. While on the tube, I will mention I used it in the first stage of a 5F6 Bassman styled circuit in place of the triode. Sounded good, the gain was something. But don't stop playing otherwise the 6AU6 started singing on its own.
There are a few ways of messing with tube types to see what they may do, datasheets have the curves for you to draw upon. There are spice models of many tubes, one day I will get into that. In the mean time there is an online load line calculator that has wasted many hours for me.
Universal loadline calculator for vacuum tubes - Vacuum Tube Amplifiers - DIY
You can do output duty or as signal amplification. In this mode it will tell you the second, third and forth harmonic distortion components. you put the voltage you want out (in the bottom Output Headroom box). At the Next Stage AC Impedance line it tells the 2,3,4th harmonic distortion component. Shame it does not have a cathode resistor function, you could calculate the value and adjust the bias voltage. Anyway, you get an idea of a few different tubes listed.