Hey there!
I am working on a little project to build my own all-valve guitar amp, of about 40W. So far I have designed a pre-amp stage, a gain stage and a phase-splitter. The pre-amp and gain are working just fine, delivering 50 Vrms at an input of 350 mV (according to the datasheet from Mullard, the necessary input for two EL34 in push-pull configuration to reach about 40W). This is the schematic so far:
As I said, the first two stages are working just fine, the problem comes with the phase-splitter. An input of 50 Vrms seems to much for it to handle and clipps the upper half-wave at both outputs. I tried a different bias but it's still not enough, the only way for it to give a nice sine wave as an output is to reduce the input voltage to about 16 Vrms, which is not enough for the power stage.
I've seen several schematics following the same basic blocks (pre-amp, gain, phase-splitter) and I asume they ought to work well. The only solutions I've come up with are trying a different order of blocks, but they would require more tubes than the current one:
1) Pre-amp (1xECC82), Phase-Splitter (1xECC83), Gain (2xECC82)
2) Phase-splitter (1xECC83), Pre-amp and Gain (4xECC82)
Does anyone else come up with another solution?
Thanks in advance!
I am working on a little project to build my own all-valve guitar amp, of about 40W. So far I have designed a pre-amp stage, a gain stage and a phase-splitter. The pre-amp and gain are working just fine, delivering 50 Vrms at an input of 350 mV (according to the datasheet from Mullard, the necessary input for two EL34 in push-pull configuration to reach about 40W). This is the schematic so far:
An externally hosted image should be here but it was not working when we last tested it.
As I said, the first two stages are working just fine, the problem comes with the phase-splitter. An input of 50 Vrms seems to much for it to handle and clipps the upper half-wave at both outputs. I tried a different bias but it's still not enough, the only way for it to give a nice sine wave as an output is to reduce the input voltage to about 16 Vrms, which is not enough for the power stage.
I've seen several schematics following the same basic blocks (pre-amp, gain, phase-splitter) and I asume they ought to work well. The only solutions I've come up with are trying a different order of blocks, but they would require more tubes than the current one:
1) Pre-amp (1xECC82), Phase-Splitter (1xECC83), Gain (2xECC82)
2) Phase-splitter (1xECC83), Pre-amp and Gain (4xECC82)
Does anyone else come up with another solution?
Thanks in advance!
The Williansom amplifier circuit is the archetype for a PP amp like this: voltage amp, phase splitter, driver stage, output tubes. You are omitting the driver stage. You can also use the Mullard 5-20 circuit which is in the Mullard Circuits for Audio Amplifiers
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What are your objectives for the amp? A lot of clean signal, some overdrive? Most guitar amps don't have two 12AU7 stages for the preamp. I don't see any volume or tone controls which would give the amp insertion loss. This will affect the design because the overall gain will be less. If you go with a treb, mid, bass Fender type tone stack, you can lose up to 80% of your signal right there. 12AU7 stages won't get it done.
If you are going for clean and only use guitar controls, then that's different, but that is not too common.
So your cathodyne PI will not be so overdriven if at all if you put tone and volume controls in.
I usually start from the output section and work towards the preamp. Figure out how much signal swing I want coming out of the PI for the power tubes, then keep going towards the input. Look at some classic Fender schematics for ideas.
BTW, welcome to the forum.
If you are going for clean and only use guitar controls, then that's different, but that is not too common.
So your cathodyne PI will not be so overdriven if at all if you put tone and volume controls in.
I usually start from the output section and work towards the preamp. Figure out how much signal swing I want coming out of the PI for the power tubes, then keep going towards the input. Look at some classic Fender schematics for ideas.
BTW, welcome to the forum.
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Thank you all for the quick responses. As for your questions, the objective for the amp is, as said, build a simple all-tube amplifier, I intend to use it for my bass. I want it to sound as clean as possible and there won't be any tone controls, just one volume control.
So if I understand correctly, if I shift the order of the stages it should work properly? As of now I have one ECC82 (double triode) and one ECC83 (double triode). Should I use one half of the ECC82 as preamp and the other half as a phase-inverter and then use each half of the ECC83 as a drive stage?
Thanks!
So if I understand correctly, if I shift the order of the stages it should work properly? As of now I have one ECC82 (double triode) and one ECC83 (double triode). Should I use one half of the ECC82 as preamp and the other half as a phase-inverter and then use each half of the ECC83 as a drive stage?
Thanks!
The title of this thread is mis-leading... You want clean sound for bass not over-driving the LTP, if so, there are literally thousands of available designs you can follow, just sim/build them and pick one that works for you...
I was wondering about why the value of the coupling caps were so high. And with both preamp cathodes fully bypassed I thought "too much bass". For guitar, this amp wouldn't have any high end response, but now it makes sense.
As Jazbo said, there are literally thousands of combinations, it can't possobly be discussed here. I was just surprized that there were no controls and if there were, the problem might be solved through insertion loss from them.
Try lowering R7's value in your sim and add a grid stopper to the PI and see what it says. Add the volume first to see what that does. Even a simple volume will give some signal loss. Use a mid position setting and see. You probably won't be running the amp flat out anyway.
As Jazbo said, there are literally thousands of combinations, it can't possobly be discussed here. I was just surprized that there were no controls and if there were, the problem might be solved through insertion loss from them.
Try lowering R7's value in your sim and add a grid stopper to the PI and see what it says. Add the volume first to see what that does. Even a simple volume will give some signal loss. Use a mid position setting and see. You probably won't be running the amp flat out anyway.
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Please post DC voltages on U3 (cathode and plate).
*If* you don't have space to get a large voltage swing, you won't get it.
FWIW you are too "tight" there:50V RMS=141Vpp on plate and 141Vpp on cathode=282Vpp swing just considering both resistors, you are leaving tube drop and other losses outside the equation.
You can barely meet specs by rising available +B as suggested above, but I'd rather drop the cathodyne inverter which is very poor and use a LTP type instead.
That said, sorry for this, but I suggest you build a classic Fender Bassman (Black or Silver face, not Tweed) or Ampeg (B15 is a killer classic and what you are trying to do).
Tube technology achieved its peak 40/60 years ago and has been explored to death, the standard by which all Bass amps are judged is Ampeg SVT, launched in 1969 and slightly modified in 1972 ... do the Math.
*If* you don't have space to get a large voltage swing, you won't get it.
FWIW you are too "tight" there:50V RMS=141Vpp on plate and 141Vpp on cathode=282Vpp swing just considering both resistors, you are leaving tube drop and other losses outside the equation.
You can barely meet specs by rising available +B as suggested above, but I'd rather drop the cathodyne inverter which is very poor and use a LTP type instead.
That said, sorry for this, but I suggest you build a classic Fender Bassman (Black or Silver face, not Tweed) or Ampeg (B15 is a killer classic and what you are trying to do).
Tube technology achieved its peak 40/60 years ago and has been explored to death, the standard by which all Bass amps are judged is Ampeg SVT, launched in 1969 and slightly modified in 1972 ... do the Math.
Juan has a point, an AMpeg B15 is just a killer sounding amp. The Ampeg design philosophy was clean and strong.
Hey there! Thank you all for the comments. Tomorrow I will try some stuff in the lab and I'll get back to you with results!
So, I got some news. I went with the (Pre-Amp -> Phase-Splitter -> Driver) design and got some nice results. I achieve an output of 47 Vrms with an input of 180mV, more than enough to drive the EL34 pair. Here are some pics from the scope through different input frequencies:
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
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