Hi, all. Quick question I haven't been able to find an answer to via Google. My training is digital, so pls forgive the naive question as I'm an analog newbie. I'd rather not destructively experiment on too many $10 tubes when I can just ask.
I'm looking at ways to get increased distortion effects from a tube guitar amp I'm slowly building.
What's the effect of sending a much larger signal to the grid of a triode tube? Does it have a different impact on the output waveform than dropping the plate voltage?
Inquisitively yours,
I'm looking at ways to get increased distortion effects from a tube guitar amp I'm slowly building.
What's the effect of sending a much larger signal to the grid of a triode tube? Does it have a different impact on the output waveform than dropping the plate voltage?
Inquisitively yours,
Ummm - any way we can think of. I'd expect to be coming off one half of a 12ax7 to the second half, with any necessary coupling in between.
Am I answering the right question?
Am I answering the right question?
RC coupling, vs direct or transformer coupling changes the behavior. Try searching for blocking distortion for more information.
I see. Excellent discussion (aimed at my level) here, as well: http://www.aikenamps.com/BlockingDistortion.html
So, blocking distortion sounds like it's definitely not what I would want.
Sorry, then for the next question, but...
Where does "desirable" distortion come from? Current starvation?
So, blocking distortion sounds like it's definitely not what I would want.
Sorry, then for the next question, but...
Where does "desirable" distortion come from? Current starvation?
MadDave12 said:
Well it depends who exactly is defining desirable distortion. For example, Eric Clapton went through a period where he used Soldano amplifiers; the resulting sound to me and some of my friends resembled a wasp in a jam jar. Soldano sells amplifiers so someone must like it. I think Eric should have listened to the sound he got in 1966!
Overloading the input means that on one half of the cycle, the valve grid may reach 0V which means that a large current will flow, limited of course by the anode load resistor. On the other half of the cycle, the valve will be turned off by the large negative signal. The result is usually copious amounts of second harmonic (with triodes) which most people consider desirable as it is musical. Inevitably as overload increases, odd order harmonics will begin to appear especially when the grid goes positive and grid current flows. This reduces the input resistance dramatically and odd_order harmonics will be generated.
Of course this is just a single stage I am describing. When an amplifier is driven into clipping lots of horrible odd-order harmonics are produced. This of course may be just what you are seeking!
Others hereabouts will be able to provide a greatly superior summary than I have, so hold on!
Best of luck in your search.
7N7
Grid current on a 12AX7
Could anyone tell me how much grid current a 12AX7 can take before getting damaged. Can't seem to find any information about driving a signal tube grid positive (with a 75k grid stopper) by a low impedance source. I am trying to figure out how many volts you could put on the input.
Old thread, should I have started a new one?
Could anyone tell me how much grid current a 12AX7 can take before getting damaged. Can't seem to find any information about driving a signal tube grid positive (with a 75k grid stopper) by a low impedance source. I am trying to figure out how many volts you could put on the input.
Old thread, should I have started a new one?
I can only tell you that I haven't blown one up yet. With a reasonable grid stopper, and the normal 75 to 100K plate load on the previous stage, I don't think anything can be blown up. I currently have 4 stages of 12AX7 chained together and nothing blows up. There is too much gain though. I am now working on modulating the excess gain with LDR's.
Normally I would say it would not harm the tube. But I am in a discussion elsewhere where the question is about feeding one amplifier 20W tube amp into the input of another. Basically the opinion was that you would blow up both amps, as silly as that sounds. While I have fed a large voltage into a triode it usually was done from a source that had a relatively high output impedance. I am not sure what an output of another amplifier would do to an input. I would try it but I am not set up for it right now.
The EAR V20 used 12AX7s in the output and I think they were driven in class-AB2. I seem to recall some older german (Telefunken?) datasheets showing curves with positive grid. If modern 12AX7s are correct clones of the old ones, they too should handle grid current.
The typical gain stages in guitar amps cannot drive the grids going positive b/c each stage presents a high impedance source, and usually there is some high ohmage and/or EQ-stuff between the stages as well. Driving grid current requires a source that can source the current. But then the question is if the distortion is any better since the result will be that the plate is pulled further down than without grid-drive, and ultimately reaches saturation just as normal, just that the amplitude is larger. I'd think driving the grids positive for the sake of tone (guitar usage) will produce more odd harmonics since it will saturate more sharply and symmetrically.
Driving a guitar amp with another will give you noise problems up your ... A high gain guitar tube amp can probably take an input signal > 20V but it will be so 5150 it wont be very useful. It'd probably be impossible to control b/c of feeding.
The typical gain stages in guitar amps cannot drive the grids going positive b/c each stage presents a high impedance source, and usually there is some high ohmage and/or EQ-stuff between the stages as well. Driving grid current requires a source that can source the current. But then the question is if the distortion is any better since the result will be that the plate is pulled further down than without grid-drive, and ultimately reaches saturation just as normal, just that the amplitude is larger. I'd think driving the grids positive for the sake of tone (guitar usage) will produce more odd harmonics since it will saturate more sharply and symmetrically.
Driving a guitar amp with another will give you noise problems up your ... A high gain guitar tube amp can probably take an input signal > 20V but it will be so 5150 it wont be very useful. It'd probably be impossible to control b/c of feeding.
A typical 12AX7 stage with :
- Ub = 300 V,
- Ra = 150 k,
- Rk = 1k (bypassed )
- no grid stopper
At idle state the anode current is some 1 mA and thus the grid voltage is -1 V.
If this stage is driven with 3 Vpp meaning 2 Vpeak overdrive at positive side, the peak grid current is 0,8 mA. But if 75 k grid stopper is added, the peak grid current drops to some 20 µA. With 10 k grid stopper the peak grid current is 0.14 mA.
So no significant grid current is flown if this size of grid stopper is used in spite of what ever the driving voltage would be.
- Ub = 300 V,
- Ra = 150 k,
- Rk = 1k (bypassed )
- no grid stopper
At idle state the anode current is some 1 mA and thus the grid voltage is -1 V.
If this stage is driven with 3 Vpp meaning 2 Vpeak overdrive at positive side, the peak grid current is 0,8 mA. But if 75 k grid stopper is added, the peak grid current drops to some 20 µA. With 10 k grid stopper the peak grid current is 0.14 mA.
So no significant grid current is flown if this size of grid stopper is used in spite of what ever the driving voltage would be.
Been there blown that!
Back in about 1970 the group now known as Chicago, then called the Chicago Transit Authority did a song featuring a long guitar solo of uncontrolled feedback. I am at work so I don't have my records to look at but it was on the first or second album. I think it was called "Free Form Guitar". In the liner notes the technique was explained.The explanation stated that a Bogen PA amp was used as a preamp to feed a Fender guitar amp. Since Terry Kath was revered as a guitar god before he shot himself dead, every wannabe rock star at the time immediately tried plugging two amps together resulting in a lot of dead amps. Since I was fixing amps then, I have autopsy data.
If the first amp was a tube amp it essentially ran with no load. We all know this can lead to fried parts starting with the OPT and often taking out a few other parts. This was the case in the stuff that I saw. A tube amp with no load can put out a hundred volts of voltage spikes before it fries so ....
If the second amp was solid state it can't eat a hundred volts on the input often leading to input transistor death.
If the second amp was a tube amp it usually suffered no harm. Sometimes the amp just died from being cranked to 11 for minutes at a time.
I personally tried this using my little Lafayette solid state combo practice amp fed directly into the tube powered Stromberg Carlson PA that I had made into a guitar amp. The only negative result was my neighbors calling the cops.
I wound up with a solid state Kustom PA amp for free because of this though. The owner was already rather unhappy with the amp because it would randomly start making high level crackling sounds and he was using it for live performances. Coincidently his local band was the opening act at a all day outdoor free concert at Miami Dade College where CTA was the headliner that night. They wound up spending more time whacking the amp than playing that show.
I explained that connecting the output of his Bandmaster to the input of the Kustom would blow it up, but he wanted to do it anyway. At least I convinced him to make sure that the speakers stayed connected to the Bandmaster. It took him several days but he eventually blew up all 4 channels on the Kustom and then gave it to me. He said that it was fun and his band mates all had a good time watching it die.
All 4 input transistors were dead which required removing the PC board to replace. It was then that I discovered a bad solder connection on one of the wires leading to the output board. I fixed the amp, but the original owner wanted nothing to do with it since he couldn'd trust it in a live situation. I used it for my synthesizers for several years until I sold them all off including the amp. It was far too clean for guitar, but plenty loud.
So you can use a power brake as load on the first amp with an output going into the next amp. As long as it's tube-tube and < 20watts should not harm too much.
Makes sense with the first amp not having any load, never considered it being run without a speaker. The second amp I can see having a problem if it was a SS, tube I could see it surviving, if anything just needing a new input tube.
Hard to get much current to flow into a grid. It is just wire suspended in a vacuum. I think you mean voltage. You can't damage a tube be over driving it with to much grid voltage, well as long as you keep the voltage within sane limits, say 200 volts peak to peak.
The grid stopper does not limit current and there is almost zero voltage drop across it. Its purpose is to act as the R in an RC filter to "stop" high frequency. The C being Miller capacitance. Grid current is measured in uA. It is tiny.
The tube will be damaged if to much current flows from plate to cathode. But you limit that with resistors. When you choose the resistors you assume the tube is a dead short.
You are thinking under normal conditions with an input under 0 volts. With the grid biased positive you are now attracting electrons from the cathode. See 'Grid Current'
Tubes 201 - How Vacuum Tubes Really Work
Since we have a source of current from the input of the amp we also have that current flowing through the grid. The grid is no longer at a high impedance and the grid stopper drop voltage due to the current flow through it.
I have blown up enough tubes to know better. Even a 12AX7 will erupt in a spark fest if you put 200 volts positive on the grid. If the 200 volt source can source considerable current the cathode will be instantly stripped of its oxide coating.
A typical guitar amp will have a fairly large grid stopper on the first stage so that it and Mr. Miller will form a low pass filter to keep the guitar cord from acting like a radio antenna. I use 27 to 47K for the grid stopper.
With a grid stopper in this range you can plug the guitar cord into the US wall outlet and nothing will blow up. leave it plugged in long enough and the grid stopper will die of over dissipation, but the tube should not die if it has the usual 100K plate load.
My measurements on class A2 and AB2 operation have seen 200 mA of GRID CURRENT on a working triode output stage. Granted it was a big triode (833A) operating in A2, cranking out 200 watts of my guitar playing.
Back when I knew less I tried to get the anode to swing down to 0V in a triode-connected KT88 push-pull amp. It was a misguided attempt to get more power out of triode-connected tubes than is possible. I was driving them with mosfet source followers so hard that the output looked like a square wave with these soft rounded corners. I don't know how much grid current I drew but eventually the positive regulator for the source followers blew up and the tubes like to run away now, a couple hundred negative volts won't cut them off.
Anyway, what I didn't know then is that once the plate swings down low, the electrons don't like it as much. At some point, they start going pretty much exclusively to the grid since it is at a more positive voltage than the plate. This is called saturation and any more increase in grid voltage will only draw more electrons to the grid. The plate will pretty much stop swinging down and the grid will get hotter and hotter if you continue to pull it up.
For guitar, just put a grid current limiter in series if you are afraid of hurting the tube.
Anyway, what I didn't know then is that once the plate swings down low, the electrons don't like it as much. At some point, they start going pretty much exclusively to the grid since it is at a more positive voltage than the plate. This is called saturation and any more increase in grid voltage will only draw more electrons to the grid. The plate will pretty much stop swinging down and the grid will get hotter and hotter if you continue to pull it up.
For guitar, just put a grid current limiter in series if you are afraid of hurting the tube.
I was experimenting with some 6BQ6GT's when I had a similar experience. Since I have over 100 of them purchased for an average cost of about 70 cents each, I didn't mind blowing them up. I also had no problem whacking them with a hammer to see what fried inside. There were 2 or 3 grid wires with the center sections burned away with tiny balls on the ends. Spots of damage on the cathode were present with the coating burned completely off in one spot.
Don't toss the tubes just yet. I have been working on a mode where G1 is tied to G2. This makes a beam tetrode work like a high Mu triode like an 811A.
In a guitar amp using a large grid stopper has several advantages. I have been tinkering with some super low buck guitar amp circuits for the Hundred Buck Amp Challenge on this forum. I was battling oscillation in a design that had far too much gain when another poster pointed out that Soldano uses some very high stopper values in their high gain amps. I ripped out the 47K stoppers and put in some 270K's. The amp is now totally stable and rocks!
so, distortion-effect. if what you are looking for. Guitar distortion?
Distortion there are two main types of guitar distortion, Symmetrical and Asymmetrical. it involves clipping the signal, one side (asymmetrical) or both peaks (symmetrical). Tube distortion is usually symmetrical. A lot of fuzz type effects "starve" the plate like running them 15 volts or so. the grid is biased negatively usually with a 1.5 AA or even a 9 volt. the stage before and after is just buffers for the stage, but the first stage could be the preamp stage too.
there is a lot of pedal-ology in the form of schematics on the web.
diode-ology can be used in the cathode circuit too, for some typical diode-ology, look at some overdrive pedal schematics out there(on the web). now these days, they manipulate the diode-ology in the feedback circuit, some between two op amps (pre gain or drive and output level amp).
Distortion there are two main types of guitar distortion, Symmetrical and Asymmetrical. it involves clipping the signal, one side (asymmetrical) or both peaks (symmetrical). Tube distortion is usually symmetrical. A lot of fuzz type effects "starve" the plate like running them 15 volts or so. the grid is biased negatively usually with a 1.5 AA or even a 9 volt. the stage before and after is just buffers for the stage, but the first stage could be the preamp stage too.
there is a lot of pedal-ology in the form of schematics on the web.
diode-ology can be used in the cathode circuit too, for some typical diode-ology, look at some overdrive pedal schematics out there(on the web). now these days, they manipulate the diode-ology in the feedback circuit, some between two op amps (pre gain or drive and output level amp).
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