Hi guys , Im building a simple 12ax7 Guitar preamp and need to figure out the optimal Plate and cathode resistors based on my supply voltage ..... I am doing a single tube with a 250k pot in between the triodes .....
My PSU consists of a 100uF Cap with a 10h Choke and another 100uf cap (CLC) , I have about 127vDC coming off of the PSU under a load and have DC heaters with 10,000uF capacitance ......
I have tried it with 100k Plate and 1.5k cathode (and 220k/2.2k and some other values I have seen in guitar amp schematics) but the sound is just a screamin mess , when you aren"t playing a note the sound devolves into feed back and hum even with the pot at a very low setting , but when your playing there is no hum or feedback but is still way too distorted .....
I"m thinking that maybe because I"m running this tube at a bit lower voltage than most tube preamps , that maybe the values I am using aren"t biasing the tube correctly ??.......
Is there a way to calculate the correct Resistor values to get the tube biased correctly ?? (Disclaimer: I have about pre-school abilities in math)
Or maybe an exel sheet or chart showing it ??
Thanx a lot guys .....
My PSU consists of a 100uF Cap with a 10h Choke and another 100uf cap (CLC) , I have about 127vDC coming off of the PSU under a load and have DC heaters with 10,000uF capacitance ......
I have tried it with 100k Plate and 1.5k cathode (and 220k/2.2k and some other values I have seen in guitar amp schematics) but the sound is just a screamin mess , when you aren"t playing a note the sound devolves into feed back and hum even with the pot at a very low setting , but when your playing there is no hum or feedback but is still way too distorted .....
I"m thinking that maybe because I"m running this tube at a bit lower voltage than most tube preamps , that maybe the values I am using aren"t biasing the tube correctly ??.......
Is there a way to calculate the correct Resistor values to get the tube biased correctly ?? (Disclaimer: I have about pre-school abilities in math)
Or maybe an exel sheet or chart showing it ??
Thanx a lot guys .....
If you take some voltage measurements at the tube, you will have numbers for your bias question, which should help. e.g. voltage across the cathode resistor, plate, grid voltages.
This is a preamp which is being used like an effects pedal? ie between your guitar and guitar amp? Is it modeled/copied from an existing design?
Do you have a picture of your wiring, and a schematic? Both would help readers to give you some more suggestions.
What test gear do you have?
Two 12AX7 gain stages will add a lot to your signal, BTW.
This is a preamp which is being used like an effects pedal? ie between your guitar and guitar amp? Is it modeled/copied from an existing design?
Do you have a picture of your wiring, and a schematic? Both would help readers to give you some more suggestions.
What test gear do you have?
Two 12AX7 gain stages will add a lot to your signal, BTW.
Though your bias point may or may not be suboptimal it sounds to me like you have some other problem going on since a poor bias point should not cause oscillation or hum.
If you can supply more information (schematic) of what you have done especially in the area of power supply and layout. What is this preamp stage feeding? Are you using a grid stopper? Are heater wires twisted? Are you using shielded or twisted pair wiring from the input to the first grid? Are the shields grounded only at one end? Do all grounds go back to the same point (star ground)? Is the power supply properly decoupled? Are you using any feedback? Did you take care to route input and output or power leads away from each other and not parallel to each other?
If you can supply more information (schematic) of what you have done especially in the area of power supply and layout. What is this preamp stage feeding? Are you using a grid stopper? Are heater wires twisted? Are you using shielded or twisted pair wiring from the input to the first grid? Are the shields grounded only at one end? Do all grounds go back to the same point (star ground)? Is the power supply properly decoupled? Are you using any feedback? Did you take care to route input and output or power leads away from each other and not parallel to each other?
Well the design is exactly the same as allmost every single tube pre-amp for guitar ever made only some of the Values are different ....... the same pretty much as any marshal , fender . orange ect ......
pretty much the same as this :
DIY 12AU7 Tube Preamplifier Project
But with a 12ax7
pretty much the same as this :
DIY 12AU7 Tube Preamplifier Project
But with a 12ax7
Hmm... I admit to not being a guitar amp specialist (I have only built one) but I don't remember seeing too many that have a LTP at the input. Does yours have an input pot like that? If so I hope it is larger that 100K as I don't think your guitar will do its best running into such a small load. It is traditional to have a 68K grid stopper at the input at least on common cathode stages. I would presume that it would be advisable on a LTP also.
Do you have that switchable feedback tone control on yours or do you just use the signal as is from one or both of the outputs? Single ended output or balanced?
For comparison this is what I built. In retrospect it probably would have been better had I decoupled the power supply for U6 and U7 as they can be switched to cascaded mode but it worked pretty well. Note that I use straight common cathode until the final phase inverter stage which is LTP.
http://www.diyaudio.com/forums/atta...h-pull-guitar-amp-schematic-sought-wesamp.pdf
Do you have that switchable feedback tone control on yours or do you just use the signal as is from one or both of the outputs? Single ended output or balanced?
For comparison this is what I built. In retrospect it probably would have been better had I decoupled the power supply for U6 and U7 as they can be switched to cascaded mode but it worked pretty well. Note that I use straight common cathode until the final phase inverter stage which is LTP.
http://www.diyaudio.com/forums/atta...h-pull-guitar-amp-schematic-sought-wesamp.pdf
What is this preamp stage feeding?
A high impedance input on a Power amp ....
Are you using a grid stopper?
Yes , 27K .....
Are heater wires twisted ??
Yes ...
Are you using shielded or twisted pair wiring from the input to the first grid?
I"m useing shielded wire (connected at one end) for all audio inputs and outputs and twisted pair for all PSU leads .....
Is the power supply properly decoupled?
What makes a PSU properly decoupled and not properly decoupled ?
Did you take care to route input and output or power leads away from each other and not parallel to each other?
Not sure cuz I don"t really understand the question .....
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I don"t really have a schematic cuz Like I said it is like hundreds of others ....
I guess I"ll probably be going back to the drawing board .....
A high impedance input on a Power amp ....
Are you using a grid stopper?
Yes , 27K .....
Are heater wires twisted ??
Yes ...
Are you using shielded or twisted pair wiring from the input to the first grid?
I"m useing shielded wire (connected at one end) for all audio inputs and outputs and twisted pair for all PSU leads .....
Is the power supply properly decoupled?
What makes a PSU properly decoupled and not properly decoupled ?
Did you take care to route input and output or power leads away from each other and not parallel to each other?
Not sure cuz I don"t really understand the question .....
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I don"t really have a schematic cuz Like I said it is like hundreds of others ....
I guess I"ll probably be going back to the drawing board .....
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PS decoupling is the RC filter on the B+ between stages. I like to have one between each inverting stage if possible. For layouts you want to be sure to keep low level signals (inputs) away from high level signals (outputs, power section etc) and AC power lines. Where they do come close you don't want them running parallel to each other.
For the hum it may help to elevate the heaters using a voltage divider on the B+ so that it is at a DC level above that of the cathodes of the preamp tubes. I presume that you have the heater center tap connected to ground right now correct? You didn't leave them floating did you?
Is that a solid state power amp? If so it might not be all that high of impedance (10 or 20k). I don't think that will cause the problems your describing but it might result in high frequency roll off. Putting a cathode follower between the gain stage and the power amp would fix that.
One thing you can try is to take a non-conductive probe (wood or bamboo chopstick is traditional
) and gently move some of the wires around while the unit is powered on and see if you can find a wire or component that is causing the problem. Sometimes you find the noise and oscillation changes when certain parts are moved or touched and that can give you a clue to the problem. These high gain circuits can be tricky.
For the hum it may help to elevate the heaters using a voltage divider on the B+ so that it is at a DC level above that of the cathodes of the preamp tubes. I presume that you have the heater center tap connected to ground right now correct? You didn't leave them floating did you?
Is that a solid state power amp? If so it might not be all that high of impedance (10 or 20k). I don't think that will cause the problems your describing but it might result in high frequency roll off. Putting a cathode follower between the gain stage and the power amp would fix that.
One thing you can try is to take a non-conductive probe (wood or bamboo chopstick is traditional
) and gently move some of the wires around while the unit is powered on and see if you can find a wire or component that is causing the problem. Sometimes you find the noise and oscillation changes when certain parts are moved or touched and that can give you a clue to the problem. These high gain circuits can be tricky.Well the transformer I have for the heaters has a center tapped 6.3v (3.15v 0v 3.15v) so I didn"t use the center tap , I just used the other wires to get 6.3v AC , then rectified it and used a 4r 5w resistor to get the voltage to 6.3dc .....
Is this the wrong way to do it ?? also am I supposed to connect the heaters DC ground to the Plates DC ground ??
Thanx
Is this the wrong way to do it ?? also am I supposed to connect the heaters DC ground to the Plates DC ground ??
Thanx
If you are headed to the drawing board....
One thing to consider would be upping your B+ voltage - you don't need much current to run preamp tubes, so you could easily convert your PS to voltage doubler with a couple more caps.
With a more common B+ voltage you could just lift the preamp section component values from a known (eg Fender) design...
Hmm.. I have never used DC heaters. The guitar amp I made had no hum with AC heaters so I didn't bother. Maybe the DC guys can chime in but I think that DC heaters are usually done with some filtering to get rid of the 120Hz buzz. Once the filtering is added I don't think you will get adequate heater voltage.
I would recommend going to AC heaters. Take a voltage divider say 220K top resistor ( connected to B+) and 100K bottom (connected to ground). Connect the center tap of the heater winding to the junction of the two resistors which will be at roughly 1/3 B+. Then connect the string of heaters between the two ends of the heater winding. Twist the wires from the transformer to the first tube's heater tightly and do the same from each tube to the next if you have more than one. This should give you a nice quiet heater. Route the heater wires away from signal lines and if possible along the edge of the chassis. Again be sure that if you have to cross signal lines that you do so at a 90 degree angle. If you still have hum it is probably a ground issue, layout problem or bad PS filter cap.
Hope this helps. Don't give up. You will find the solution eventually.
P.S. if you up the B+ as suggested by others just adjust the voltage divider accordingly to give you 30 or 40 volts.
I would recommend going to AC heaters. Take a voltage divider say 220K top resistor ( connected to B+) and 100K bottom (connected to ground). Connect the center tap of the heater winding to the junction of the two resistors which will be at roughly 1/3 B+. Then connect the string of heaters between the two ends of the heater winding. Twist the wires from the transformer to the first tube's heater tightly and do the same from each tube to the next if you have more than one. This should give you a nice quiet heater. Route the heater wires away from signal lines and if possible along the edge of the chassis. Again be sure that if you have to cross signal lines that you do so at a 90 degree angle. If you still have hum it is probably a ground issue, layout problem or bad PS filter cap.
Hope this helps. Don't give up. You will find the solution eventually.
P.S. if you up the B+ as suggested by others just adjust the voltage divider accordingly to give you 30 or 40 volts.
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The transformer i am using is a center tapped 220v , but I was only useing one 110v side because the only caps that i have that are at least 350v are 47uF and I only had 3 of them but I have a bunch of 250 v 100uF so I used those .... Are three 47uF caps good enough for a single tube and a 10 henry choke ??
Thanx
Thanx
the hum can also be poor caps in the grid power supply.
Did you look for ground loops, maybe a bad or broken connection to ground or a blown grounding resistor. I had an amp with a hum that was there and then not and it turned out to be a resistor with intermittent opens.
To many guitar amps don't have the best ventliation and that damages both the resistors and caps which are usually rated at 85c. Peavey amps have as many as five to seven 12ax7 in a sealed compartment with zip air flow and caps rated at 85c and carbon film too.
Did you look for ground loops, maybe a bad or broken connection to ground or a blown grounding resistor. I had an amp with a hum that was there and then not and it turned out to be a resistor with intermittent opens.
To many guitar amps don't have the best ventliation and that damages both the resistors and caps which are usually rated at 85c. Peavey amps have as many as five to seven 12ax7 in a sealed compartment with zip air flow and caps rated at 85c and carbon film too.
I believe the tubes are Mesa Boogie branded Sovtec"s , one might be a fender branded sovtec .......
Actually the hum goes away when you play a note or short the input , I think it is probably oscilating and my layout is probably all wrong .....
One more question , are you supposed to ground the heater supply to the plate supply ??
Thanx
Actually the hum goes away when you play a note or short the input , I think it is probably oscilating and my layout is probably all wrong .....
One more question , are you supposed to ground the heater supply to the plate supply ??
Thanx
If you have access to a tube tester, you should have them checked anyway. If the shinny silver flashing is at all brown or the edges are that could be your hum.
D.C. heaters on a guitar does not make good sense to me. DC will burn the tube heaters up faster than AC. It might be one thing on the first amp stage, but after that I see no point. I only know of high end phono amps that use DC heaters. This DC thing may be a wrong turn for your project.
Never seen ( maybe some one else has) tube heaters tied to the plate circuit. The gird, in some cases...again for the first stage of amplification for weak signals.
I'd still check out a bad ground angle.
D.C. heaters on a guitar does not make good sense to me. DC will burn the tube heaters up faster than AC. It might be one thing on the first amp stage, but after that I see no point. I only know of high end phono amps that use DC heaters. This DC thing may be a wrong turn for your project.
Never seen ( maybe some one else has) tube heaters tied to the plate circuit. The gird, in some cases...again for the first stage of amplification for weak signals.
I'd still check out a bad ground angle.
Hmmm... it occurs to me that if the hum disappears when the input is grounded then it is probably not a problem with the B+ as such. Most likely induction or heater problem. It is possible that the ripple on the heater voltage is being capacitively coupled to the cathode. I am not sure if shorting the input would damp this out or not. When you short the input how are you doing it (shorting plug, wire to the chassis etc.)?
If it is induction in the grid circuit you might be able to confirm it by using your chopstick to gently move the input wire around and see if the level changes. If there is a pot at the input it might be getting picked up there also. If you have an input pot is the case of the pot grounded?
Can you determine for sure whether it is 60(50) or 120(100) Hz hum. That will help locate the possible problems. BTW, is Canada 50Hz or 60Hz power? Of course since you are using unfiltered DC on the heaters both the heater and the B+ could be sources of the higher frequency hum as only the pre-rectified side of things runs at the lower frequency. If the heaters were AC you could possibly eliminate that possibility.
How are you rectifying the heater? You said you have a separate transformer for the heater but what circuit are you using to rectify? I assume that you are using a bridge. If so you are right not to connect the center tap to anything but I really think that you want to connect the negative pole of the DC side of the bridge to ground (at the same point that the B+ is grounded). Try doing that and see if things improve or better yet tie it to an elevated voltage via. a voltage divider. But in reality I don't think the unfiltered DC heaters is a good idea anyway. If you are going to do the DC with a dropping resistor why not at least put a filter cap in front of the dropping resistor to smooth out some of the nasty ripple (or again just simplify and go AC)?
Just a few more options to look at. If you think of it as a detective story it can actually be kind of fun if you don't have a deadline.
If it is induction in the grid circuit you might be able to confirm it by using your chopstick to gently move the input wire around and see if the level changes. If there is a pot at the input it might be getting picked up there also. If you have an input pot is the case of the pot grounded?
Can you determine for sure whether it is 60(50) or 120(100) Hz hum. That will help locate the possible problems. BTW, is Canada 50Hz or 60Hz power? Of course since you are using unfiltered DC on the heaters both the heater and the B+ could be sources of the higher frequency hum as only the pre-rectified side of things runs at the lower frequency. If the heaters were AC you could possibly eliminate that possibility.
How are you rectifying the heater? You said you have a separate transformer for the heater but what circuit are you using to rectify? I assume that you are using a bridge. If so you are right not to connect the center tap to anything but I really think that you want to connect the negative pole of the DC side of the bridge to ground (at the same point that the B+ is grounded). Try doing that and see if things improve or better yet tie it to an elevated voltage via. a voltage divider. But in reality I don't think the unfiltered DC heaters is a good idea anyway. If you are going to do the DC with a dropping resistor why not at least put a filter cap in front of the dropping resistor to smooth out some of the nasty ripple (or again just simplify and go AC)?
Just a few more options to look at. If you think of it as a detective story it can actually be kind of fun if you don't have a deadline.
60Hz, like the USA.
That's why we can be on the same grid.....so a problem up here can lead to a blackout in NYC and a spike in births 9mos later..but this may be ancient history for you!
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