Hey guys,
As I was thinking of a design for a simple tube amplifier, I thought it might be a good idea to have two inputs on it, one line input (eg. CD-player) and one guitar input (so there has to be a guitar preamp in it). So basically:
(guitar preamp) > preamp > phase shifter > push-pull stage > output transformer
The guitar preamp can be switched on or off accordingly.
I've got a couple of tubes, some E88CC's and RCA6350's, which are both twin triodes. My goal is not to produce some kind of high-end amplifier with components over $200, it's just for fun.
Schematic
What I'd like to know is: is my design going to work? Are there any special points I should take into consideration or other suggestions?
Because no attention is spent on tube design in school, I don't know how to calculate them so my resistor values might be a bit off. And what about the input impedances? Are they in the right direction?
On another note: maybe it would be cool to include a switchable tube overdrive. Any ideas how I can achieve this?
By the way i've grounded the low side of the filaments supply to reduce humming.
Thnx
Schematic
As I was thinking of a design for a simple tube amplifier, I thought it might be a good idea to have two inputs on it, one line input (eg. CD-player) and one guitar input (so there has to be a guitar preamp in it). So basically:
(guitar preamp) > preamp > phase shifter > push-pull stage > output transformer
The guitar preamp can be switched on or off accordingly.
I've got a couple of tubes, some E88CC's and RCA6350's, which are both twin triodes. My goal is not to produce some kind of high-end amplifier with components over $200, it's just for fun.
Schematic
What I'd like to know is: is my design going to work? Are there any special points I should take into consideration or other suggestions?
Because no attention is spent on tube design in school, I don't know how to calculate them so my resistor values might be a bit off. And what about the input impedances? Are they in the right direction?
On another note: maybe it would be cool to include a switchable tube overdrive. Any ideas how I can achieve this?
By the way i've grounded the low side of the filaments supply to reduce humming.
Thnx
Schematic
Don't do it just yet. It will not work as is and is therefore dangerous. 
In my opinion, you should read http://www.tubecad.com/articles_2003/Grounded_Cathode_Amplifier/Grounded_Cathode_Amplifier.pdf
and read it until you understand it.
If I sound like a parent, I don't mean to. You seem to have the talent. Push on and learn
BTW, welcome to the forum!
In my opinion, you should read http://www.tubecad.com/articles_2003/Grounded_Cathode_Amplifier/Grounded_Cathode_Amplifier.pdf
and read it until you understand it.
If I sound like a parent, I don't mean to. You seem to have the talent. Push on and learn
BTW, welcome to the forum!
Your design looks ok-ish in principle, but some points below:
1)Your B+ is way too low, you probably need about 300V or so. Try using a 230V 1:1 transformer or put the 2 115V secondaries in series.
2)Also use at least 100uF or so for the first power supply cap for less ripple.
3)Have another dropping resistor and cap for the preamp stage power supply to isolate it.
4)Also remember plate voltage is V anode - V cathode. Don't be afraid to run the 6350 valves at limits- just be careful of max plate dissipation (Va-Vc) x Ic.
5)You need coupling capacitors and grid leak resistors on the first two valves.
6) Ground one side of the speaker, and run a negative feedback loop from the + speaker terminal to the cathode of V1A, you can switch it off if you want more distortion, on when you want hi-fi.
7) Put a volume control in front of V1B, so you can choose to overdrive the preamp, power amp (or both )
The 6350 appears to be like a 12BH7, so you might easily get a couple of usable watts. Run it in class AB (with low idle current) for the most power. As for tube overdrive, just switch in that extra stage. Put cathode bypass caps on the preamp valves for more gain (say 100uF on V1A and 1uF on V4A ) Check the resistor values, I don't know what they should be sorry but have a look around, you'll get an idea.
Have a look at some Fender schematics for more ideas... You could switch to a 'long-tailed pair' phase inverter for more drive.
Looks like it would work in principle though Good luck
1)Your B+ is way too low, you probably need about 300V or so. Try using a 230V 1:1 transformer or put the 2 115V secondaries in series.
2)Also use at least 100uF or so for the first power supply cap for less ripple.
3)Have another dropping resistor and cap for the preamp stage power supply to isolate it.
4)Also remember plate voltage is V anode - V cathode. Don't be afraid to run the 6350 valves at limits- just be careful of max plate dissipation (Va-Vc) x Ic.
5)You need coupling capacitors and grid leak resistors on the first two valves.
6) Ground one side of the speaker, and run a negative feedback loop from the + speaker terminal to the cathode of V1A, you can switch it off if you want more distortion, on when you want hi-fi.
7) Put a volume control in front of V1B, so you can choose to overdrive the preamp, power amp (or both
)The 6350 appears to be like a 12BH7, so you might easily get a couple of usable watts. Run it in class AB (with low idle current) for the most power. As for tube overdrive, just switch in that extra stage. Put cathode bypass caps on the preamp valves for more gain (say 100uF on V1A and 1uF on V4A ) Check the resistor values, I don't know what they should be sorry but have a look around, you'll get an idea.
Have a look at some Fender schematics for more ideas... You could switch to a 'long-tailed pair' phase inverter for more drive.
Looks like it would work in principle though
Good luck
Allright, those are some good hints
I've adapted my design accordingly:
>>Check it out here<<
Plate voltage: 330V
Cathode current: 27.5mA (peak 333mA)
Plate dissipation: 3.85W / triode
I think I might damage it if I increase B+.. See 4)
got half an E88CC left, can I use it for something or should i just run it on one triode?
@lndm: Why is it dangerous? is there something wrong with the power supply? By the way that link was a big help, thanks
6350 Datasheet
E88CC Datasheet
I've adapted my design accordingly:
>>Check it out here<<
Maximum ratings for the 6350:
Plate voltage: 330V
Cathode current: 27.5mA (peak 333mA)
Plate dissipation: 3.85W / triode
I think I might damage it if I increase B+.. See 4)
Right, forgot to change that.. I've switched to a 220uF 250V cap.
Added a 100uF cap and a resistor..
Max dissipation is 3.85W, so that makes 154V @ 25mA.. Does that sound about right? The datasheet mentions some "Characteristics range values for equipment design", which state that at Va=150V and Vg=-5, Ip is(should be?) between 6 and 16mA.
Done
I don't really know how.. I'm sorry, I'm rather new to this tube designing.. By the way i've
got half an E88CC left, can I use it for something or should i just run it on one triode?
I think I've got it figured out now.. But what makes the difference between overdrive and the "normal situation"? Could I use the second half of V4 to create some kind of switchable clean/overdrive feature?
I still don't quite understand what determines the class. Okay, of course the idle current (which is when the grid is connected to ground via an 1M resistor and nothing else right?), but how do you determine the idle current? How much is low?
@lndm: Why is it dangerous? is there something wrong with the power supply? By the way that link was a big help, thanks
6350 Datasheet
E88CC Datasheet
Actually you want to maximise the voltage swing on the output transformer, which means higher voltage at low current. A wild *** guess a B+ of 300 volts should be about right, adjust the cathode resistors to give maybe 5-10mA per section, giving less than 2W disspiation at idle per section. You won't damage them a bit!
Basically these things are like a more powerful 12BH7 which is kind of like 12AU7/ECC82. They can take a beating. The whole class AB thing is harder to explain, but basically if you run them at high voltage and low idle current, you will get more power than if you run them with high idle current in class A. Valves are voltage amplifiers rather than current amplifiers.
As for your input section, you should limit the B+ to about 120V or so, these ECC88s like low voltage apparently. Adjust that dropper resistor R19 to lower the voltage. To use up that extra section, I suggest using a fender type driver design like the 12AT7 at http://www.freeinfosociety.com/electronics/schematics/audio/pictures/fenderab673.gif
I don't know much about the ECC88 sorry, so no ideas on what values you would need... But it would have better drive to the output stage with a driver liek that. Here is an interesting schematic http://www.tubes.mynetcologne.de/roehren/el86pp/el86pp_amp_full.gif
from http://www.tubes.mynetcologne.de/roehren/el86pp/schematics_e.html
For negative feedback, add a 100 ohm resistor between the 560 ohm of V1a and ground. Make sure the cap is also raised above ground. Then connect an appropriate resistor from the speaker + terminal to the junction of the 560 and 100ohm resistors. you could start with 10K or so, but for best results calculate how much gain you want. See above schematic.
Its getting there!
Basically these things are like a more powerful 12BH7 which is kind of like 12AU7/ECC82. They can take a beating. The whole class AB thing is harder to explain, but basically if you run them at high voltage and low idle current, you will get more power than if you run them with high idle current in class A. Valves are voltage amplifiers rather than current amplifiers.
As for your input section, you should limit the B+ to about 120V or so, these ECC88s like low voltage apparently. Adjust that dropper resistor R19 to lower the voltage. To use up that extra section, I suggest using a fender type driver design like the 12AT7 at http://www.freeinfosociety.com/electronics/schematics/audio/pictures/fenderab673.gif
I don't know much about the ECC88 sorry, so no ideas on what values you would need... But it would have better drive to the output stage with a driver liek that. Here is an interesting schematic http://www.tubes.mynetcologne.de/roehren/el86pp/el86pp_amp_full.gif
from http://www.tubes.mynetcologne.de/roehren/el86pp/schematics_e.html
For negative feedback, add a 100 ohm resistor between the 560 ohm of V1a and ground. Make sure the cap is also raised above ground. Then connect an appropriate resistor from the speaker + terminal to the junction of the 560 and 100ohm resistors. you could start with 10K or so, but for best results calculate how much gain you want. See above schematic.
Its getting there!
smsmonster, I was taught in college to have a staunch attitude toward safety. I'd like to move on now (and maybe be helpful?).
The plate resistances for the first three stages get smaller, I see the intention, (and with valves being voltage controlled devices) there is not usually so much variation. I don't use the e88cc, but I assume 15k sounds like a good start. It would be similar for all three stages. You can adjust the cathode resistor until the plate sits at the desired voltage. The link I gave you would set you up IMO, helping you find the ideal Ra and Rk.
You have three stages feeding off the pre supply rail. This can cause regenerative feedback (google motorboating). Try not to run more than two stages off one rail, just repeat R19 and C1.
R19 should be The desired voltage drop divided by the current drawn by the stages it is supplying. Usually between 1k-50k
P1 could probably 250k or 100k. The 1M resistor following may load down the 500k pot giving inaccurate performance. The previous stage will handle it.
Grid stopper resistors. Putting small (typically 100-1000 ohm) resistors at (right at) the grids can eliminate oscillation that occurs due to the input capacitance.
Try not to run the early stages at max diss or max current (for longevity and for sound), it is not necessary.
Also, I would leave the feedback loop until I had the rest working (if you want it at all).
Class A means the amplifying device conducts current throughout the entire wave cycle.
You can measure current by measuring the voltage across a resistor and dividing by the resistance.
Good luck with this.
PS, SHiFTY, just realised I overposted you, got distracted.
The plate resistances for the first three stages get smaller, I see the intention, (and with valves being voltage controlled devices) there is not usually so much variation. I don't use the e88cc, but I assume 15k sounds like a good start. It would be similar for all three stages. You can adjust the cathode resistor until the plate sits at the desired voltage. The link I gave you would set you up IMO, helping you find the ideal Ra and Rk.
You have three stages feeding off the pre supply rail. This can cause regenerative feedback (google motorboating). Try not to run more than two stages off one rail, just repeat R19 and C1.
R19 should be The desired voltage drop divided by the current drawn by the stages it is supplying. Usually between 1k-50k
P1 could probably 250k or 100k. The 1M resistor following may load down the 500k pot giving inaccurate performance. The previous stage will handle it.
Grid stopper resistors. Putting small (typically 100-1000 ohm) resistors at (right at) the grids can eliminate oscillation that occurs due to the input capacitance.
Try not to run the early stages at max diss or max current (for longevity and for sound), it is not necessary.
Also, I would leave the feedback loop until I had the rest working (if you want it at all).
Class A means the amplifying device conducts current throughout the entire wave cycle.
You can measure current by measuring the voltage across a resistor and dividing by the resistance.
Good luck with this.
PS, SHiFTY, just realised I overposted you, got distracted.
Hey thanks you guys
I've been calculating and reading all night, I came up with this:
Drawing.
I think I can get it.. But I'd have to know what the gain of V1A has to be, so I have to know what kind of signal I have to provide to the resistor network consisting of R6,7,8,9.
As you can see, I've calculated that the gain of V4A should be 5x to go from 200mV to 1V and that the gain of the phaseshifter (cathodyne) is 0.9X. So the overall gain is 4.5 or 0.9, multiplied with the gain of V1A. This number should, according to my thinking, be equal to the required grid swing on the input of my power stage. (200mV input * 5 * gain of V1A * 0.9 = Vg) or (1 * gain of V1A * 0.9 = Vg) right?
What I experienced to be confusing: when calculating the voltage drop across the plate resistor, should you use the idle current as a starting point or the current swing (magnitude of the amplified current)? I just took the idle current of 16.5mA as you can see, 1.1V bias, 90V plate voltage, so that leaves 91.1V on the low side of R10 and B+ on the high side. But what I don't understand: The low side of R10 is connected -more or less directly- to the grid of V1A. Does this imply that the grid voltage of V1A will be 91.1V? Or am I wrong referring it to ground?
If figured the push-pull stage always has to be working in Class B. Am I correct? Or can a push-pull stage run in Class A as well? Anyhow, if it has to be Class B, I'd have to set the idle current up a bit right? Or is my configuration of the power stage already good? I haven't touched that part
Furthermore, is it just me or are my resistor values very low? Everywhere I look, I see values between 1k-100K and i've got 68 ohms.
As for the powersupply and R19: If all the tubes are run on 90V, how can i decouple the powersupply rail? Should i just put some caps between each stage and no resistors or do I have to do it in another way? Again, the story about the voltage swing across the plate resistor comes to mind, which I don't really get yet..
I'm really excited now, can't stop drawing and calculating and I have to work tomorrow in about 6 hours Thnx for the help so far
By the way I'm aiming for about 2-5W output power.. So if it can work using a 160VDC powersupply it'd be greatly appreciated
Drawing
I've been calculating and reading all night, I came up with this:
Drawing.
I think I can get it.. But I'd have to know what the gain of V1A has to be, so I have to know what kind of signal I have to provide to the resistor network consisting of R6,7,8,9.
As you can see, I've calculated that the gain of V4A should be 5x to go from 200mV to 1V and that the gain of the phaseshifter (cathodyne) is 0.9X. So the overall gain is 4.5 or 0.9, multiplied with the gain of V1A. This number should, according to my thinking, be equal to the required grid swing on the input of my power stage. (200mV input * 5 * gain of V1A * 0.9 = Vg) or (1 * gain of V1A * 0.9 = Vg) right?
What I experienced to be confusing: when calculating the voltage drop across the plate resistor, should you use the idle current as a starting point or the current swing (magnitude of the amplified current)? I just took the idle current of 16.5mA as you can see, 1.1V bias, 90V plate voltage, so that leaves 91.1V on the low side of R10 and B+ on the high side. But what I don't understand: The low side of R10 is connected -more or less directly- to the grid of V1A. Does this imply that the grid voltage of V1A will be 91.1V? Or am I wrong referring it to ground?
If figured the push-pull stage always has to be working in Class B. Am I correct? Or can a push-pull stage run in Class A as well? Anyhow, if it has to be Class B, I'd have to set the idle current up a bit right? Or is my configuration of the power stage already good? I haven't touched that part
Furthermore, is it just me or are my resistor values very low? Everywhere I look, I see values between 1k-100K and i've got 68 ohms.
As for the powersupply and R19: If all the tubes are run on 90V, how can i decouple the powersupply rail? Should i just put some caps between each stage and no resistors or do I have to do it in another way? Again, the story about the voltage swing across the plate resistor comes to mind, which I don't really get yet..
I'm really excited now, can't stop drawing and calculating and I have to work tomorrow in about 6 hours
Thnx for the help so far By the way I'm aiming for about 2-5W output power.. So if it can work using a 160VDC powersupply it'd be greatly appreciated
Drawing
smsmonster, R6,7,8,9 are not a divider network, R6,7 are grid stopper resistors and can be ignored (not omitted). To know how much swing to provide for the output, bias the output stage, and look at the voltage across R's 11,12,13,14. The voltages should all be the same. You should be able to swing at least 4-8 times this IMO. ie, up to zero, and back down to twice the voltage, plus 6-12dB for good measure (more is good).
As I have said, I am not familiar with e88cc but I know that 16mA is way more than necessary here (but it might be perfect for this device), thing is, a large plate resistor can help you get higher gain if desired.
You need a coupling capacitor. Without one, you are absolutely correct about your assumption. The stage 'idles' at a certain quiescent point. This current is DC. The signal is AC, it 'rides' on the DC so the _average_ current in the stage is always the DC. Since capacitors block DC not AC, you can set the DC for each stage separately and still feed the signal through.
Best to use the manufacturers recommended application as a starting point for biasing your output stage. You should get a hold of http://www.duncanamps.com/tdslpe/tdslpe_install.exe if you don't already have it. Plenty of info plus links to the data sheets and manufacturers recommended applications.
Class B is where one 'side' of the PP amplifies one half of the signal. It is standard to overlap slightly (idle both sides up slightly) as this gives the best results. This is called class AB. Class B is uncommon in audio but we just call it class B for simplicity. Once you get going, you can bias the stage up to full class A if you want to.
The 68 ohm resistors do look unusual. but according to TDSL, 15mA current, Va=90V, Vg=1.2V so it looks like you've done your homework. Thing is, as I mentioned, 16mA is higher than is common for a pre stage.
As far as decoupling is concerned, The earliest stages need less swing anyway, and you can use less than 90V on the plate if you wish. Do use resistors with the caps.
Good to hear you are doing well.
As I have said, I am not familiar with e88cc but I know that 16mA is way more than necessary here (but it might be perfect for this device), thing is, a large plate resistor can help you get higher gain if desired.
You need a coupling capacitor. Without one, you are absolutely correct about your assumption. The stage 'idles' at a certain quiescent point. This current is DC. The signal is AC, it 'rides' on the DC so the _average_ current in the stage is always the DC. Since capacitors block DC not AC, you can set the DC for each stage separately and still feed the signal through.
Best to use the manufacturers recommended application as a starting point for biasing your output stage. You should get a hold of http://www.duncanamps.com/tdslpe/tdslpe_install.exe if you don't already have it. Plenty of info plus links to the data sheets and manufacturers recommended applications.
Class B is where one 'side' of the PP amplifies one half of the signal. It is standard to overlap slightly (idle both sides up slightly) as this gives the best results. This is called class AB. Class B is uncommon in audio but we just call it class B for simplicity. Once you get going, you can bias the stage up to full class A if you want to.
The 68 ohm resistors do look unusual. but according to TDSL, 15mA current, Va=90V, Vg=1.2V so it looks like you've done your homework. Thing is, as I mentioned, 16mA is higher than is common for a pre stage.
As far as decoupling is concerned, The earliest stages need less swing anyway, and you can use less than 90V on the plate if you wish. Do use resistors with the caps.
Good to hear you are doing well.
Hey, I'm at work now so I can't upload a schematic but i have been on it during my lunch break
So far, i've concluded that the gain of V1A should be 5.5x. This because of the bias current on V2A will be around 1mA (class B). To achieve this, Vg has to be -9V. Therefore, the cathode resistor should be 9k if I'm correct.
In order to have 16mA of current through V2A, the voltage on the grid has to go to -4V. This means there has to be a swing of 5V on the grid of V2A? (of course this same story applies to V2B, V3A and V3B)
Then: if the input signal's magnitude is 1V, and the gain should be 5.5*0.9, which is almost 5x or 5*1 voltage swing.
If the gain is to be 5.5x then Ra must be 520ohm because
Gain = (mu*Ra)/(Ra+rp) = (33*520)/(520+2600) = 5.5x
As for the power supply: as 3*16.5mA flows through V1 and V4, and the voltage swing has to be (4*9 = 36)V, B+ must be 90+36 = 126V. Therefore, R19 becomes (156-126)/(3*0.0165)=606 ohms (in which 2 Watt is dissipated).
Are my calculations correct? And is there something dangerous about my design? Should I put another resistor between V4 and V1 in the B+ line? Thanks a lot
So far, i've concluded that the gain of V1A should be 5.5x. This because of the bias current on V2A will be around 1mA (class B). To achieve this, Vg has to be -9V. Therefore, the cathode resistor should be 9k if I'm correct.
In order to have 16mA of current through V2A, the voltage on the grid has to go to -4V. This means there has to be a swing of 5V on the grid of V2A? (of course this same story applies to V2B, V3A and V3B)
Then: if the input signal's magnitude is 1V, and the gain should be 5.5*0.9, which is almost 5x or 5*1 voltage swing.
If the gain is to be 5.5x then Ra must be 520ohm because
Gain = (mu*Ra)/(Ra+rp) = (33*520)/(520+2600) = 5.5x
As for the power supply: as 3*16.5mA flows through V1 and V4, and the voltage swing has to be (4*9 = 36)V, B+ must be 90+36 = 126V. Therefore, R19 becomes (156-126)/(3*0.0165)=606 ohms (in which 2 Watt is dissipated).
Are my calculations correct? And is there something dangerous about my design? Should I put another resistor between V4 and V1 in the B+ line? Thanks a lot
I think I'll hold off till you can supply a schematic of where you are up to, I may be able to help more then.
A few points. The plate resistor on V1A. For starters, gain is lower than expected. You need to use a plate resistor at least twice the valve's plate resistance (rp) which with your operating points is around 2,650 ohms. You should really go for three times rp or higher. I think Your plate resistor should be at least 10k, and I think others use 25k. The higher values will provide less distortion. If this is a problem with supply voltage, reduce the current from 15mA. Try to set the plate at around half the supply voltage. This suggests 180V supply, say for eg, 15k plate resistor and 6mA current. Feel free to up the supply voltage if desired.
All this is good as you will want more than 5v swing to drive your output stage. Don't underestimate a person's tendency to push a low powered amp. It is essential that it sounds ok when you do. First thing is to make sure the driver stage doesn't stress.
And yes, I feel you should put an RC before V4 (decouple the supply rail). Not only is it necessary, but it will lower the hum (as v4 is amplifying low level signals).
A few points. The plate resistor on V1A. For starters, gain is lower than expected. You need to use a plate resistor at least twice the valve's plate resistance (rp) which with your operating points is around 2,650 ohms. You should really go for three times rp or higher. I think Your plate resistor should be at least 10k, and I think others use 25k. The higher values will provide less distortion. If this is a problem with supply voltage, reduce the current from 15mA. Try to set the plate at around half the supply voltage. This suggests 180V supply, say for eg, 15k plate resistor and 6mA current. Feel free to up the supply voltage if desired.
All this is good as you will want more than 5v swing to drive your output stage. Don't underestimate a person's tendency to push a low powered amp. It is essential that it sounds ok when you do. First thing is to make sure the driver stage doesn't stress.
And yes, I feel you should put an RC before V4 (decouple the supply rail). Not only is it necessary, but it will lower the hum (as v4 is amplifying low level signals).
It's me again..
Schematic
I've used TubeCAD to calculate some values, really handy program. Entered my parameters manually and got the following:
V4A: (guitar)
V1A: (line)
Also I've decoupled V4A with 33K and 100uF. I've moved the decoupling capacitor from the wiper of P1 to the high side of P1.. I thought it would be better that because of the DC current that will flow thru it..
Schematic
I've used TubeCAD to calculate some values, really handy program. Entered my parameters manually and got the following:
V4A: (guitar)
An externally hosted image should be here but it was not working when we last tested it.
V1A: (line)
An externally hosted image should be here but it was not working when we last tested it.
Also I've decoupled V4A with 33K and 100uF. I've moved the decoupling capacitor from the wiper of P1 to the high side of P1.. I thought it would be better that because of the DC current that will flow thru it..
Very well, i've upped the plate resistances and fiddled around a bit with load resistors to get the gain i wanted..
See here.
Just one thing: The gain of V1A is way too high (25.66x). But because of P1 I can't change that.. What can I do about it? It needs to be 5.5x
Nearly there.. I can feel it
See here.
Just one thing: The gain of V1A is way too high (25.66x). But because of P1 I can't change that.. What can I do about it? It needs to be 5.5x
Nearly there.. I can feel it
Ok.
1. I would reduce the values of R19 and R23 to give higher supply rails.
2. R4 and R5 should be between 3k and 10k each and identical.
3. Remove R24, it is a problem.
4. Place a grid resistor from V1a to ground (250k-1M).
5. Recalculate R2 and R3 (in future, it is helpful to include the voltages at the plates).
6. You say the input to V1a is 1v. I assume you mean peak to peak. In this case with approx 25 gain you have 25Vp-p. With an output stage grid to cathode voltage of -5V, you need 10Vp-p as an _absolute minimum_. IMO, 25Vp-p is only just enough in practice.
I agree that having V4a to make up the difference between 200mV and 1V is indeed overkill, but do not try to lower gain by using a tiny plate resistor. It only results in high distortion.
1. I would reduce the values of R19 and R23 to give higher supply rails.
2. R4 and R5 should be between 3k and 10k each and identical.
3. Remove R24, it is a problem.
4. Place a grid resistor from V1a to ground (250k-1M).
5. Recalculate R2 and R3 (in future, it is helpful to include the voltages at the plates).
6. You say the input to V1a is 1v. I assume you mean peak to peak. In this case with approx 25 gain you have 25Vp-p. With an output stage grid to cathode voltage of -5V, you need 10Vp-p as an _absolute minimum_. IMO, 25Vp-p is only just enough in practice.
I agree that having V4a to make up the difference between 200mV and 1V is indeed overkill, but do not try to lower gain by using a tiny plate resistor. It only results in high distortion.
So I've been thinking today..
New schematic!
There is something I keep bumping into. As for V4A, the idle current, cathode resistor and gain are numbers which just don't fit together.
But that's of later concern: first off, I've adjusted V1A so it has a swing of +/-50V, which should be enough for the power stage (which is biased at -9V, so 36V-72V should do it right? It's idle current is set at 5mA as you can see.
How do I calculate the gain of V1B (cathodyne phase splitter)? And I presume it has to be correctly biased as well.. Is it particularly important that it is biased in class A? What determines the idle current and what is a realistic value? From then on I can calculate R19.
R23 is high, but OK. It has to drop 30 volts, but just at 1mA so that makes 30K right? About power supplies: Is C4 large enough? I could make it 2200uF or something, I have some really large capacitors with high voltage ratings.. (Yes those are expensive, but I can take them out written-off equipment at work (which by the way are electronic chopper installations for large DC-motors) pretty handy eh?)
V4A: ah our little problem child
What concerns me: the -Ug/Ia curve is not at all linear at 1mA (idle current). Does this introduce serious distortion? And the gain is way too high (about 6 times too high).. Can I resolve this by using a 1:6 resistor divider or am I thinking too simple?
New schematic!
New schematic!
There is something I keep bumping into. As for V4A, the idle current, cathode resistor and gain are numbers which just don't fit together.
But that's of later concern: first off, I've adjusted V1A so it has a swing of +/-50V, which should be enough for the power stage (which is biased at -9V, so 36V-72V should do it right? It's idle current is set at 5mA as you can see.
How do I calculate the gain of V1B (cathodyne phase splitter)? And I presume it has to be correctly biased as well.. Is it particularly important that it is biased in class A? What determines the idle current and what is a realistic value? From then on I can calculate R19.
R23 is high, but OK. It has to drop 30 volts, but just at 1mA so that makes 30K right? About power supplies: Is C4 large enough? I could make it 2200uF or something, I have some really large capacitors with high voltage ratings.. (Yes those are expensive, but I can take them out written-off equipment at work (which by the way are electronic chopper installations for large DC-motors)
pretty handy eh?)V4A: ah our little problem child
What concerns me: the -Ug/Ia curve is not at all linear at 1mA (idle current). Does this introduce serious distortion? And the gain is way too high (about 6 times too high).. Can I resolve this by using a 1:6 resistor divider or am I thinking too simple?
New schematic!
Now that you mention your phase splitter, lets set that up.
The first thing to do is to move P1 to in between C4 and R20. This is because V1a is needed to bias V1b. Next, remove C15 and R21 and replace with a direct connection. Then you adjust the value of R2 until R4 and R5 each have about 35V across them. If you can increase your supply rail, this might be good, or leave it for now.
Hope you appreciate leaving this step till later as it may have just confused matters earlier. In this case V1b needs to sit comfortably in the middle of the two resistors with some swing available on each end. To change the characteristics of the V1b stage, just change R4,5 together.
As far as your ps caps, they look like they will work. I like to avoid large values due to the current surges that can be associated with them and because it is hard to find quality an a larger value cap IMO. If you have an issue with ripple/hum, upping C4 would be the place to start, otherwise IMO not necessary.
As far as V4a, you are correct about not running the current too low. IMO you could try a different device. Until it is built, you may not be entirely sure you will even need this stage. Even so, your idea about attenuating the signal is doable, if not the most elegant method. I guess that perfectionism is the enemy of a finished product (safety aside).
The first thing to do is to move P1 to in between C4 and R20. This is because V1a is needed to bias V1b. Next, remove C15 and R21 and replace with a direct connection. Then you adjust the value of R2 until R4 and R5 each have about 35V across them. If you can increase your supply rail, this might be good, or leave it for now.
Hope you appreciate leaving this step till later as it may have just confused matters earlier. In this case V1b needs to sit comfortably in the middle of the two resistors with some swing available on each end. To change the characteristics of the V1b stage, just change R4,5 together.
As far as your ps caps, they look like they will work. I like to avoid large values due to the current surges that can be associated with them and because it is hard to find quality an a larger value cap IMO. If you have an issue with ripple/hum, upping C4 would be the place to start, otherwise IMO not necessary.
As far as V4a, you are correct about not running the current too low. IMO you could try a different device. Until it is built, you may not be entirely sure you will even need this stage. Even so, your idea about attenuating the signal is doable, if not the most elegant method. I guess that perfectionism is the enemy of a finished product (safety aside).
Very nice
I've adjusted things according to your last post and am hoping to visit the shop on Tuesday (shop's closed on Monday )
For now, i've cut out the guitar pre-amp bit. If it doesn't amplify enough yet, I can use it to crank up the voltage (or perhaps as a distortion stage?).
As for the Output Transformer: What is the output resistance of my power stage? In other words: what kind of input impedance do I need on my OPT?
I was thinking of this transformer.
Input: 2x2000 ohm
output: 4 or 8 ohm
Power rating: 40W
It will cost me about 52 euro's (or 115 new-zealand dollars). Is this overdone? Or is any (non high-end) transformers usable as well? (obviously not a power supply transformer) I want to use an 8ohm speaker.
I was told I should use it in 4000 : 4 mode for 8000 : 8.. Is this realistic?
I'm going to the shop on Tuesday, and start building
I've adjusted things according to your last post and am hoping to visit the shop on Tuesday (shop's closed on Monday
)For now, i've cut out the guitar pre-amp bit. If it doesn't amplify enough yet, I can use it to crank up the voltage (or perhaps as a distortion stage?).
As for the Output Transformer: What is the output resistance of my power stage? In other words: what kind of input impedance do I need on my OPT?
An externally hosted image should be here but it was not working when we last tested it.
I was thinking of this transformer.
Input: 2x2000 ohm
output: 4 or 8 ohm
Power rating: 40W
It will cost me about 52 euro's (or 115 new-zealand dollars). Is this overdone? Or is any (non high-end) transformers usable as well? (obviously not a power supply transformer) I want to use an 8ohm speaker.
I was told I should use it in 4000 : 4 mode for 8000 : 8.. Is this realistic?
I'm going to the shop on Tuesday, and start building
The spec sheets don't advise on PP operating points. I don't have experience with these output devices and my guess is as good as any . Get a second opinion. Enough secondary taps gives you a good range of experimentation though.
The OPT ratio can be used 4000:4 and 8000:8 no problem, it is the ratio not the absolute numbers that count. The limit typically is that the primary reflected impedance from the secondary needs to be lower than the reactance of the primary inductance. The manufacturer can advise here.
Best of luck
. Get a second opinion. Enough secondary taps gives you a good range of experimentation though.The OPT ratio can be used 4000:4 and 8000:8 no problem, it is the ratio not the absolute numbers that count. The limit typically is that the primary reflected impedance from the secondary needs to be lower than the reactance of the primary inductance. The manufacturer can advise here.
Best of luck
This is it. That's how I'm going to build it and from then on I'll see what I can improve I can calculate on and on for hours, but things will just work out differently in practice.. It's a bit trial-and-error i think
Anyway, I'll just go for the 8000 ohms guess. Or is there any way of measuring what the output impedance is (by means of a DVM or oscilloscope)?
By the way here's the datasheet for the OPT.. Sounds good so far.
http://www.amplimo.nl/download/3A524.pdf
I'd like to thank you for your time and effort.. It's been great I'll start building this week and post some pictures of the process and when I'm done.. Think it'd be great, we have some really nice measuring equipment (portable digital scopes, digital four-channel scopes with color displays nice)
Reckon I'll start with my 230V-115V isolation transformer.. Might be tricky to get my hands on one in our local store.
I can calculate on and on for hours, but things will just work out differently in practice.. It's a bit trial-and-error i think Anyway, I'll just go for the 8000 ohms guess. Or is there any way of measuring what the output impedance is (by means of a DVM or oscilloscope)?
By the way here's the datasheet for the OPT.. Sounds good so far.
http://www.amplimo.nl/download/3A524.pdf
I'd like to thank you for your time and effort.. It's been great
I'll start building this week and post some pictures of the process and when I'm done.. Think it'd be great, we have some really nice measuring equipment (portable digital scopes, digital four-channel scopes with color displays nice) Reckon I'll start with my 230V-115V isolation transformer.. Might be tricky to get my hands on one in our local store.
Ta 
BTW did you know that you can save on parts by replacing R11,12,13,14 with a single 2250 ohm resistance from ground to all four cathodes, and in parallel a single 100uF instead of C5,6,7,8 as in the williamson amp. This is a classic amp and many have been modeled on it. Your amp bears some similarity to the Williamson. I have built one and it was worth the effort.
BTW did you know that you can save on parts by replacing R11,12,13,14 with a single 2250 ohm resistance from ground to all four cathodes, and in parallel a single 100uF instead of C5,6,7,8 as in the williamson amp. This is a classic amp and many have been modeled on it. Your amp bears some similarity to the Williamson. I have built one and it was worth the effort.
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