I think its pretty cool how may people chimed in on this thread. Lots of folks who have studied tube guitar amps and learned much indeed. The quality of the responses is quite good. It could almost be (dare I say it) a primer for those who say, hey "I want to build a guitar amp". Anyway lets see what happens next.
Hey again guys, In the Plexi 6v6 schematic (http://mhuss.com/Plexi6V6/Plexi6V6.gifhttp://mhuss.com/Plexi6V6/Plexi6V6c.gif), I have a few questions:
1. The first 2 common cathode stages appear to be in parallel, can anyone explain why it is designed that way? I noticed there are different valued caps around them...do they just emphasize different frequencies?
2. can anyone explain what the "FAT" switch in at the cathode of the 3rd stage does (I realize that it decides whether the cathode is sitting at AC ground or not, but what does this mean for the amplifier?)
RTF this is kind of the topology you were describing correct? (except I took your explanation to have the first 2 common cathode stages in series rather than parallel)
Also, How difficult would it be to design in a 9v supply into the amp? (it may be useful for adding built-in effects if it isnt too difficult)
Thank you guys for all your help! Im starting to piece things together, and I'm hoping to start work on this very soon...thanks again!
1. The first 2 common cathode stages appear to be in parallel, can anyone explain why it is designed that way? I noticed there are different valued caps around them...do they just emphasize different frequencies?
2. can anyone explain what the "FAT" switch in at the cathode of the 3rd stage does (I realize that it decides whether the cathode is sitting at AC ground or not, but what does this mean for the amplifier?)
RTF this is kind of the topology you were describing correct? (except I took your explanation to have the first 2 common cathode stages in series rather than parallel)
Also, How difficult would it be to design in a 9v supply into the amp? (it may be useful for adding built-in effects if it isnt too difficult)
Thank you guys for all your help! Im starting to piece things together, and I'm hoping to start work on this very soon...thanks again!
Also, do you guys have any good suggestions for a power trafo? and I was looking into greinacher doubling the voltage, and it seems like the output is bipolar, but i dont think i want a bipolar supply for the amp. If i attach the negative end to ground will it still work? (meaning that the negative end of the trafo is tied to the negative end of the doubler cicuit)
lynchy217:
1. I wouldn't say they are parallel per se, they are mixed together. The Plexi amp has two channels, one a bit brighter, one a bit darker. They also have a bit different distortion characteristics. The coupling caps and the cathode bypass caps make the difference in tone, the cathode resistors alter distortion characteristics.
2. The 4.7µF cap when in circuit will add some gain and boost upper bass and mids (and highs), making for a 'fatter' sound. Thicker in the mids. I prefer this stage either bypassed with 680nF cap or unbypassed.
1. I wouldn't say they are parallel per se, they are mixed together. The Plexi amp has two channels, one a bit brighter, one a bit darker. They also have a bit different distortion characteristics. The coupling caps and the cathode bypass caps make the difference in tone, the cathode resistors alter distortion characteristics.
2. The 4.7µF cap when in circuit will add some gain and boost upper bass and mids (and highs), making for a 'fatter' sound. Thicker in the mids. I prefer this stage either bypassed with 680nF cap or unbypassed.
In mark Huss's updated schematic, it appears that there is only one input jack, so could I put a "bright" switch in there to choose which channel the signal goes through? This would make it behave a little more like the original marshall plexi, only replacing the two inputs with one and a switch correct?
Power supplies are inherently differential. We make them single ended by arbitrarily grounding a point. The attached "El Cheapo" schematic shows a Greinacher doubler supply employing SS diodes. You would use the 2 sections of a 6BY5 and smaller caps. in the doubler stack. 47 μF./250 WVDC 'lytics of good quality should do well. For a single PP 6V6 channel, a Triad N-68X, instead of the N-77U shown, is fine.
Attachments
Unless you intend to play loud enough to over drive the output power tubes to distortion there is littel point in the above. Power tubes running in there linier range sound like SS, or close enough to it.
At more normal volume levels any tube type sound you hear is in the preamp. I can turn up a preamp gain control until a preamp triode stage clips and then turn master volume down and get that tube breakup sound a "bedroom" volume.
A guitar amp iis not at all like a HiFi amp. The guitar amp strongly colors the sound. Even a very "clean" guitar amp runs at about 10% to 15% THD but it si the exact make up of the harmonic distortion products that define the sound. It's a black art with no correct answer.
I strongly suggest you build a known and common design first using all tubes and point to point hand wiring. This kind of amp can later easily be tweaked or re-built.
Yu asked what kind of hibred is best. I'd have to say put the tube in the preamp. You will notice the sound in the stages that have the most voltage gain. If you must build a hibred just used a single 12AX7 and a "chip amp" for power. It will have more of a tube sound and be much cheaper to build.
That said, build a basic Fender "Champ" as the first build and plan building more.
This is a guitar amp? Make all decisions based on the tone you want to hear.
Look what Fender did. Those old power supplies look very minimal with smallish filter caps. He did it for a reason. He saved a buck but he was also after a compression effect. Compression is caused by voltage drop in the power supply. Has the counter intuitive result of making the music sound louder because you can turn the volume up with out raising the peaks so much you get more average sound level. They compress the heck out of most pop music so as to boost average power. The musician term for this is "sag". It's one thing that gives these amps their unique sound. Again we are not building a HiFi amp. Hi output impedance in the PS can be a good thing if you have exactly enough of it people actully place resistors in seris wth the transformer to get it now but Fender used vacuum tube diodes
DOH! i dont know why i was thinking that the secondary coil of the power trafo needed to be grounded! thanks eli. and Chris, I think I am going with an all tub amp for this build. Ill play around with other architectures once i understand the function of tubes a little better. my understanding of sag was that because vacuum tubes have more resistance than SS diodes, higher transient signals cause the B+ voltage to lower. if you use a smaller cap, it will also cause a similar effect, but also introduce more of the rectified 60 Hz signal whcih will lead to 120Hz hum (in america) into the supply rail. does this hum show up in the output signal, or does the PP output stage somehow reduce it further? if so, does the supply rail have to be filtered again in between the output stage and the preamp?
I think I'll draw up a prelim schematic this weekend to see what you guys think about it.at this point, I feel like the PS is my biggest issue as Im starting to understand the signal stages a little more clearly.
thank again for all your help!
I think I'll draw up a prelim schematic this weekend to see what you guys think about it.at this point, I feel like the PS is my biggest issue as Im starting to understand the signal stages a little more clearly.
thank again for all your help!
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Yes to all. But the main reason the PS is filtered again between stages is to bepent feedback. THe same power supply filter section can feed stages that are out of phase. but if you feed in-phase stages you have positive feedback which might oscillate.
One thing the helps with hum in a guitar amp is that unlike a HiFi amp you know the lowest note going into the input just is a "low E". The low E in a standard tune guiter as about 80Hz. You can design the amp to block everything below 80Hz Same for the high end. You can kill ultrasonic oscillations by blocking everything above 10KHz.
Purposefully limiting the amps bandwidth to the 80-10,000 rank would make for a poor HiFi amp but that is all the guitar amp needs to have.
And yes the push pull output stage is resistant to power supply noise, but only "resistant". The preamp is much more in-tolerent of hum but because it uses so much less current you can filter it with another pi section.
But you know that the guitar itself is the biggest source of hum.
Adding a choke can help a lot. Just replace one of the resistors in the PS with a choke that is rated for enough current.
Hey again, I have attached my prelim schematics for the preamp and power amp, at least the signal flow parts. what do you guys think?
I am still working on trying to get the right resistor values and power supply design. I am having trouble figuring out the power supply voltages needed for each stage. Is there any reason why different stages need different voltages? I could just copy someone else's design, but I would rather learn this subject. I am sorry if I'm asking common sense questions here.
also, I noticed a lot of amp designs feed the voltage straight from the rectifier to the center tap on the output transformer. Since this is the most unfiltered piece in the power supply, does this contribute 120 Hz hum as well?
Chris thank you very much, the positive feedback situation makes a lot more sense to me. also very good call on limiting the bandwidth to that of the guitar.
thanks everyone for your input!
I am still working on trying to get the right resistor values and power supply design. I am having trouble figuring out the power supply voltages needed for each stage. Is there any reason why different stages need different voltages? I could just copy someone else's design, but I would rather learn this subject. I am sorry if I'm asking common sense questions here.
also, I noticed a lot of amp designs feed the voltage straight from the rectifier to the center tap on the output transformer. Since this is the most unfiltered piece in the power supply, does this contribute 120 Hz hum as well?
Chris thank you very much, the positive feedback situation makes a lot more sense to me. also very good call on limiting the bandwidth to that of the guitar.
thanks everyone for your input!
I think the preamp is missing at least one cap. From the junction of R6/8/13 to ground. Typically it would be a 22 or 47uf 400V electrolytic. That's a comples preamp for a first try. but if you like the mixing bring and not bright try it.
I assume you mean R6/7/13 right? I agree...I will update. Also, how difficult would it be? I dont wanna jump into the deep end without knowing how to swim, but i definitely do like the idea of having that functionality.
PS help please!!
Hey again everyone... I haven't had much time to work on this project as of recent, but I was wondering if anyone could help me with my power supply design. I know the basic topologies used for most power supplies, but I'm having trouble figuring out what kind of supply voltages I should be looking for in which parts of the circuit. I've been reading up on power supply design for the past week or so, but can't seem to find anything about what kind of voltage I should be looking for. Does anyone know any where I can go to find this kind of information? I realize most of it is a matter of design, but since I'm new to tubes I am not sure what voltages they require.
Also, I was wondering how much gain I should be designing for in the preamp. I realize that this is mostly a matter of design, but if anyone could help me with a ballpark estimate of what i can expect, it would be greatly appreciated. also, if I want to be able to get preamp overdrive distortion at lower volumes, would it be better to put an attenuator before or after the poweramp?
Thanks guys!
Hey again everyone... I haven't had much time to work on this project as of recent, but I was wondering if anyone could help me with my power supply design. I know the basic topologies used for most power supplies, but I'm having trouble figuring out what kind of supply voltages I should be looking for in which parts of the circuit. I've been reading up on power supply design for the past week or so, but can't seem to find anything about what kind of voltage I should be looking for. Does anyone know any where I can go to find this kind of information? I realize most of it is a matter of design, but since I'm new to tubes I am not sure what voltages they require.
Also, I was wondering how much gain I should be designing for in the preamp. I realize that this is mostly a matter of design, but if anyone could help me with a ballpark estimate of what i can expect, it would be greatly appreciated. also, if I want to be able to get preamp overdrive distortion at lower volumes, would it be better to put an attenuator before or after the poweramp?
Thanks guys!
About voltages: There is no why to avoid drawing a load line unless you are just copying another amp. Design the PS for the power section. Then rule of thumb is each PS section uses a resistor 10X larger than the last section. Try that, then adjust it later.
how much gain? Are you playing blues or metal? Answer is some place between 20 as much as you can get. One triode grain stage is the minium and many clean amps have just that.
If the "attenuator" is before the power amp then we call it a "master volume control" and all you need is a simple and cheap 1M pot. This lets you run the preamp gain as high as you like while keeping the power section from being over driven.
An attenuator place between the speaker and power amp section needs to be kind of beefy and rated at least double the amp's power. Use this if you want the over drive the power amp and not blow up you ears or the speaker.
Not reason not to have both although master volume controls as easy and cheap and power attenuators are much less common
awesome thank you!...but dont i need to know the supply voltage to draw a load line? Im sorry if this is a noob question.
so really the amount of gain doesnt matter I just want "a lot"? and I play both blues and metal(ish) depending on who im playing with...I would like the ability to do both. I take it metal requires a little more gain out of the preamp right?
also, i noticed a lot of designs connect the rectified signal directly to the center tap on the OP trafo, how much does this voltage matter? do I just want it to be big, or is there a way to decide how big it should be? (I assume it has something to do with the PP output stage right?
so really the amount of gain doesnt matter I just want "a lot"? and I play both blues and metal(ish) depending on who im playing with...I would like the ability to do both. I take it metal requires a little more gain out of the preamp right?
also, i noticed a lot of designs connect the rectified signal directly to the center tap on the OP trafo, how much does this voltage matter? do I just want it to be big, or is there a way to decide how big it should be? (I assume it has something to do with the PP output stage right?
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