Why not construct a small test circuit of a power supply, variable grid voltage, and resistors/caps to see whether it is the tube or not? A higher voltage on the plate will just give you a larger current. Kinda stinks that it's a $22 tube, dunno what it costs you on that side of the pond, but you could always get a second tube and check that way as well.
As for the broken reverb pedal, does it just need to electrolytic caps? The output caps do go bad eventually as well, a lot of the time replacing them gets rid of a DC issue.
Increasing the resistance will decrease the cutoff frequency.
If we're assuming 3pF of capacitance, and another ten added on manually.. F=1/(2*pi*R*C)=1/(2*pi*13*10^-12*68*10^3)=1.8x15^5 or 180kHz (I used a calculator on my computer so I may have made an error) Multiplying the resistance by ten divides the frequency by ten, meaning using a 680k will give you a cutoff of 18kHz. You still want that higher, so divide that 680k by 1.5, giving you 450k. 470k is the closest usual standard.
This doesn't account for the anode-cathode cap, but that would be a second filter for most of our purposes, that 1uF isn't gonna impede higher frequencies much. You could just as soon hook up the anode cap to ground or the B+ feeding that stage. Many times, that second resistor from the plate to another place (be it B+, ground, or cathode) is to make the amp more stable and reduce the chances of oscillations (and hiss of course).
You should invest in his preamp book, quite good for an intro to designing preamp stages. It includes the PI and even global feedback. Also check out Graphical Construction for Vacuum Tube Circuits by Albert Preisman, marvelous. I think you can find the latter on PMillett's site in .pdf form, but the book is old and can be picked up pretty cheap.
That's a pretty neat looking wah, how do you like having the controls towards the front of the wah? I usually prefer them back towards the heel.
There's no need to bump 4 hours later. The top switch is SPDT, the bottom SPST. In series, the capacitance divides. You'll need a SP3T switch if wanting to simply select between three different capacitors. In series, you can short across capacitors to then increase the capacitance, it's a little harder to calculate, but you eliminate switching using this method. So you would have two caps in series, and using a SPDT ON-OFF-ON again, you would short out one cap, then in the center short none, and in the third position short out the other one. The OFF in between the switch throws is simply a no-contact setting, it is not another throw. For instances like this, it allows you to design a third setting. You only need a SPDT.
One of my favorites to do with wahs it to use an ON-OFF-ON switch, so have your .01, then parallel it with a .022/.033 and .056 caps. It's been a little while since I've experimented with wahs, so I don't quite remember what values I preferred. By all means, hook up a couple caps with wires attached and find your favorite.
Looks interesting Jim, the differential input to the power amp is neat. Is it taking the signal at the speaker output and feeding it back to the other input? Are you going to match the power trannies and D1/D2 to reduce the error? No worries about a matching transformer, that's always good.
Jason, Penn State Industries sells relatively cheap lathes that hold up fine if your only wanting to do pens on it. You're not an idiot when it comes to tools like kids in shop class, I don't see why it wouldn't last quite a bit of time in your can. The little miniature lathes work quite well. If you buy any pen materials, avoid horns/antlers, I made a couple pens with river buffalo horn, and My God it smelled something awful. They're in the U.S. though, dunno about shipping up your way.
Don't get a Toshiba, mine aggravates the crap out of me at times.
The cathode resistor is the resistor from the cathode of the power tube to ground. Find where the cathode of the power tube is, find the resistor that connects it to ground. Doesn't matter, you'll just get a negative value if using a digital meter. If using analog; however, you need the + to be at the plate, and the - to be at the cathode.
The resistance can be determined by the colored bands on the resistor. If you measure the voltage across it, you can use Ohm's Law to find the current. Same rule applies, doesn't matter if using digital, if analog, you want the + to be at the more positive voltage, which in this case is the cathode rather than ground.
It essentially controls the current. If you lower the resistance, more current flows, but more current means a larger voltage across the same resistance. This counter-action is why the cathode resistor bias method is called self-bias. Sounds like you've got it about right. If it's a class AB Push pull amp, you can go upwards or 80% at idle without too much concern.
It's is very important to understand that the voltages within your amp are highly dangerous. You should spend an hour looking up tube amp safety practices before you go poking inside of your amp.
You only need the voltage from plate to cathode and the current through the tube (the same as the current through the cathode resistor. The voltage across the tube, which is the same as the high voltage minus the cathode-ground voltage. Using the voltage across, and the current through, you have the idle power dissipation. Connect the voltmeter across the cathode resistor, I=E/R, that's your idling current. The voltage across the resistor is the cathode-ground voltage, you can measure the high voltage, or if it's known, just use the schematic approximation. This voltage, minus the cathode-ground voltage is the voltage across the tube. You only need to use a voltmeter and the cathode resistor.
You can; however, each tube will draw a different current based upon the grid-cathode voltage, so the power dissipated through each tube is really not able to be determined without the current. With vacuum tube amplifiers, they are typically loaded with an output transformer, that said, the plate rests at the high voltage. P=IV, the current through the tube idling, and the voltage across it while idling is the quiescent power dissipation. To return to the concept of measuring the voltage, voltage is a measurement taken with a certain reference, for instance, there is no such thing as a voltage by itself, it must be in reference to another. Therefore, to measure the grid voltage, you must also measure another voltage (in this case the cathode) to arrive at the measurement you desire.
Yes, any and or all of the 4 tubes may interest you. If they are matched (as the should be) you need only to measure one of the tubes.
My method will tell you the voltage from grid to cathode, due to variations in the tubes themselves, most people use a current method rather than voltage as each tube will pull a different current for a given grid-cathode voltage. Measuring the voltage across the cathode resistor tells you the current through ohms law. I=V/R=E/R depending on how you learned ohms law you'll either use V or E, they both stand for the voltage in this equation.
The resistor 6k8 combo is where it'd go, you could even go down to 4k7 or 5k1 (5k1 would be a safer choice) which would give you variability from 5.1 thousand to 15 thousand ohms. The 5k1 sets a minimum resistance value so that the output stage can self-bias. The 10k pots allows you to play with the value to find one that you prefer, ideally, you'd replace this pot with a resistor of the closest value to whatever value you liked best on the pot. So if you toyed with the pot and found that you liked the sound best when it was at 7400 ohms, you might replace it with a 7k5 resistor, or, if ordering from a place like Mouser (check the resource thread for other places) you could find a value even closer. The reason is that like all components, potentiometers will drift, but it will most likely drift in value more than an equivalent resistor will.
You can remove the legs of the 12k and solder into the holes where the component legs were, or you can solder onto the legs of that cap. Electrically, they're about the same, personal choice. Soldering onto the cap will be less of a headache for you though. Be sure that you are following safety practices, while the voltages of the bias circuit may or may not be of concern, they should be treated with the same fear as the HV. And just as you could damage yourself and your amp from the bias, you could make a mistake and handle the higher voltage wires and be even worse off. The voltages in any amp are not laughable.
30-50W iron is fine, you can spend 50 bucks and get an XYTronics and have a digital temperature setting, spend another 20 and you can get one with a built in fume extractor if you plan on soldering a lot. If just doing this mod, a 30W rat shack iron will suffice. 60/40 solder is standard is works just fine.
To check the bias, take a volt meter with the + side at the cathode, and the - side at the grid of the tube.
Are you able to get crisp photos? If that 12k is what is setting the bias currently, then yes, replace it with a new bias setup.
It depends on the internal guts of the wah. I never opened mine up back when I had one. The other option is to create a true bypass box, which would consist of an input and output jack, and a loop with send and receive jacks. The signal enters the input, and is either sent to the output directly, or sent to the send of the loop, the return of the loop would be switched to connect to the output when the effect is desired. This option is nice because it will certainly not ruin your wah if you make a mistake. I glanced this over, looks decent, and premier guitar is a reputable source to my knowledge. If you're uncomfortable with building the electronics I'm sure that one of the major pedal companies has made a true bypass box for sale.
You can serve in some places, just not partake, but I dunno the laws there. Plus, there's good money in it if you are able to do it.
I agree with the not lying. My boss and I live a two or three minutes walk from one another including getting shoes on and waiting for a traffic light. I always joke that we live too close to one another for either of us to screw over the other. I believe that over the winter he'll be paying me to learn to create a website in .html for the store. Kinda neat, being payed to learn a skill I had every intention of learning at only the cost of my time, I'll take it though.
Found out that the state gave me a tuition grant today. It's just shy of 1200 dollars, I hope I get 1200 per semester, which means the next half of the school year that would have been out of my pocket is paid for. Plus I have my savings for it, and if I get a cash refund for the $1173 for this half, I'll be one happy mother..
Have you been checking classifieds? Hell, just go up and down blocks walking in asking if they're hiring, that's the best way to get the seasonal work down here. I've never really jammed, I barely play anymore, it's been over a month since I've played..
If you're not kiln drying, it's going to take quite a long time to mature, you'll want to research the process of aging green lumber for use. Worry about that before tone; however, plenty of guitars are made from walnut and have sounded fantastic.
The reflected impedance on the tube plates varies with the transformer. For instance, if you hook up a 4 ohm load to a 6600:8 transformer, it will appear as a 3300:4, and the tube will be loaded based on the 3300 primary. Transformers have impedance ratios. The given values on the datasheet for P-P and SE are recommendations, not necessarily specific values that must be used.
The output will still be for 8 ohms, however, the impedance on the tube plates will force different currents based upon the bias and the tubes used.
More simply, look at a resistor loaded triode. Say 22kohms plate resistor, put a 12AX7 in, swap it for a 12AU7, the currents will be different as well as the bias points and available grid swings. The output stage is more complicated, but follows this rule, the load will be the same, but how the tubes react will be different. And that is what needs to be taken into account.
The output impedance is the load in parallel with the tube's impedance. However, if you are interested in designing an output stage based off of a tube's dynamic characteristics, then the Valve Wizard's site is a good place to check out. The suggestions for which tubes and the number are just that suggestions, you're limited to the devices at hand. For instance, an EL34 tube will draw such a current and have such a limited dynamic grid swing available, the KT88 will draw a different current and will have different curves for the same grid voltage swing. If you would like me to clarify that some I'd be happy to.
It would be pure amp distortion, that's the beauty of it. You use a clean booster to merely drive the tubes harder. If you wanted more distortion, you lower the voltage to decrease the headroom. Adding a tube sounds all and well, however, if the power transformer isn't designed to handle the additional load for the filaments, it's more complicated. Whereas a MosFET stage can use hardly any current and your PT won't even know.
You may want to just build a booster pedal to push the tubes harder, rather than creating a new tube stage, just incorporate a MosFET booster before the preamp. Makes the tubes distort more and offers little distortion itself.
Oh, okay. Yeah if it's the same circuit then you've got no worries. It's hard to patent/copyright schematics, layouts, however, are almost always protected. 150 is a lot to pay for a couple opamps diodes resistors and capacitors in a dirt box from a major company.
It's the same circuit, the difference will be in the layout, quality of components etc. It will by in large sound the same, you could even experiment with different clipping diodes to get cool alternate tones once you've built it. If you don't have a soldering iron already, the cheap radioshack irons will do for getting starting, but be sure to use sockets for transistors and ICs. Note, DIY is typically not cheaper for the same product as production in most cases, but can lead down a couple interesting roads after you've played a bit. Don't forget to check out Geofex.com for all sorts of effect pedals information and he even has an article on the Tube Screamer if you'd like to learn a little more.
It's much easier to use a fabricated PCB than it is a piece of perf board and create the layout yourself, much less to worry about, so it should largely be a successful first build. Don't forget to check all of your solder joints and switch wiring before trying the effect, getting discouraged after testing is makes diagnosing harder than just looking over the circuit to double check.