Welcome, Guest. Please login or register.

July 09, 2020, 06:34:29 AM

Login with username, password and session length

Recent Posts

 

Author Topic: My Attempt at a Hybrid Design  (Read 9469 times)

Bakeacake08

  • Regular
  • **
  • Posts: 42
  • Chip Points: 1
    • View Profile
My Attempt at a Hybrid Design
« on: December 20, 2013, 06:41:39 PM »
I started out wanting to build an all-tube amp from the ax84.com, but I was still learning about electricity and circuitry, so the high voltages made me a little nervous, which led me to research solid-state amp design. Then I found out that you could make a pedal using tubes, so I thought it might be cool to have a solid state clean channel and a tube crunch channel. That seemed rather complex for my first design attempt (at this point I had decided that I wanted to try to design something myself), so I settled on building a hybrid amp, which leads me to this post. I have come up with the attached schematic and I was hoping to inquire of people with actual knowledge and experience in the hopes of not killing myself of those around me with my first amp project.


I have a few specific questions, but I would also love to hear any overall comments anyone might have as well.


1) C17 is used as a reservoir cap. I estimated there will be a quiescent current somewhere around 3-400mA. Is this enough capacitance? Too much?


2) For the heater circuit, I saw somewhere online about using a zener diode to drop the voltage inside the operating range of a 12V voltage regulator. Then I added an 80R resistor to supply the required 150mA. Is this the same current that will be going through the zener as well? Also, does this idea even make any sense? I was hoping to be able to use the same (relatively cheap) transformer instead of having to get one just for the heater supply. (*crosses his fingers*)


3) I'm not super familiar with how to choose/implement fuses, so any comments on my current scheme would be great.


4) C2 and C4 are cathode bypass caps. Does 10uF seem like a reasonable value? I know there is such a thing as partially bypassed and fully bypassed, but I don't know how different capacitor values will affect the sound.


5) The tone section is a passive James/Baxandall type circuit. I know that there will be some insertion loss; will scaling down the resistor values by, say, a factor of 10 (and scaling up the caps accordingly) affect this loss at all? From what I understand, this could at least cut down on the Johnson noise, but I'm not very familiar with the concept of insertion loss.


General comments on the design: I chose the TDA2050 because I looked up the schematic for the Marshall 15DFX I used to have and that's what it used, so I figured it would work. That chip has a max voltage rating of 50 volts, so I picked at 36V RMS transformer, which will put out 35 volts after losing 1V in the bridge rectifier. All resistors will be .5W unless otherwise noted. I've read thousands of pages of electrical theory in the past few months, but this is the first non-9V-battery project I'm attempting, so I hope I'm at least on the right track with this.


I greatly appreciate any feedback I you have for me.


(You know, to reduce the amount of distorted ideas I might be throwing out. Waka waka.)


Matt
« Last Edit: December 20, 2013, 10:57:52 PM by Bakeacake08 »

Roly

  • Legendary
  • ******
  • Posts: 2184
  • Chip Points: 288
    • View Profile
    • Australian Valve Amps
Re: My Attempt at a Hybrid Design
« Reply #1 on: December 21, 2013, 07:56:38 AM »
Welcome @Bakeacake08.


Until you have some experience building other people's designs you are really biting off quite a mouthful to start out with a design of your own.

1) It's a bit hard to have too much filter capacitance, but you can certainly have too little, and 68uF is well short.

The object is to store enough charge between charging impulses from the transformer and rectifier so that the capacitor can return the charge slowly for the amplifier, without the voltage dropping too much in between pulses.

The amount of charge in Coulomb is Q and relates as;

Q = I t = C V

That is charge equals Amps by Seconds equals Farads by Volts.

A bridge is a full wave rectifier so the period of interest is between half cycles, 1/100th sec for 50Hz mains or 1/120th for 60Hz.

Given the identity above, the current draw, and the capacitance value, and peak or starting voltage, you should now be able to do a transformation that will tell you how much the voltage drops between recharge pulses.

As we generally look for a hum voltage (drop from peak) of less than 5% my guess is that you will find this cap value is about one-tenth of what you will require.  But I'm not going to do the calculation for you because this is something any aspiring designer has to get used to doing all the time.

2)  Nothing wrong with the idea of a zener pre-dropper, but keep in mind that it will dissipate power equal to;

P = E I

i.e. the zener voltage times the heater current.  Perhaps a better idea would be a well chosen power resistor.

Compared to driving the heater directly from a 6.3 or 12.6 volt tranny either is a very wasteful way of getting your heater power (the 12A?7 series twin triodes can be wired for either six or twelve volt operation).

If you are going to use the 12AU7 in 12 volt mode, why the 80 ohm resistor at all?

3) The value of a fuse is determined by the current draw of the circuit being protected.  You estimate the current draw on the supply to be 300-400mA, so the fuse value must be higher than this, but low enough so the supply (transformer) is capable of actually blowing it under fault conditions.  As a general rule this will be around 150% of the expected current.  You also have to allow for the charging surge into the filter cap, so you can either use a much higher rated fuse, say 200%, or use a "slow-blow" type.

On the mains side the rating will be similar except that it will be reduced by the ratio of the transformer (i.e. depending on 110 or 240V supply).

4) At this point I have to say I'm not thrilled by running a valve such as a 12AU7 on a supply voltage as low as 35 volts - these are much happier with 150-200 volts, and where you can use common and time proven values.

10uF bypass is around half of the more normal 25uF, but then this is when you have 1k5-2k2 in the cathode.  Because of your reduced supply you have a much lower cathode resistor value, which in turn will need a much higher bypass capacitance.

The cathode bypass caps determine the low frequency rolloff point.  This is normally set to allow a fullish bandwidth, but is sometimes reduced to provide a rising characteristic giving a treble boost effect suitable for lead playing.  The identity of interest here is;

Xc = 1/2 Pi f C

where:
Xc is the capacitive reactance in ohms
f is the frequency in Hertz
C is the capacitance in Farads

5) Scaling the tonestack values changes the network impedance.  As a rule of thumb these networks like to be driven from an impedance that is one tenth of their characteristic, and feed into a load that is ten times (we'll return to this).

The insertion loss is the loss in dB between the circuit with and without the network, typically about 20dB.  Duncan's Tone Stack Calculator is a fun toy that will help you play around with this.

Have you calculated the Johnson noise of this tonestack?  When you do you will realise that it is almost nothing, microvolts, compared to the signal level, volts, and therefore should be of no concern at all.

A 35Vrms transformer will give you a peak rectified voltage of root(2) times, 1.414 * 35 = 49.49 or almost 50V on idle.

The 12AU7 is the lowest gain of the 12A?7 family, and apart from not being made any more, I'm dubious that you will get much useful gain on your low supply voltage.  The 12AX7 would be the more logical choice since it is still made.

Your 50k gain pot is effectively in parallel with the anode load of the first stage for AC, so its effective AC load will be 33k//50k or around 20k.  There is no reason not to make this 500k.

Since there is no cap between the wiper of the Gain pot and the grid of V1b there is no need for R4.

Between the tonestack and the power amp you have an interesting situation with the various impedances.  The tonestack needs to work into a much higher impedance, again say a 500k Master pot, but this in turn must work into a much higher impedance than the 22k of R13.


This is not exhaustive, and overall this is an interesting paper exercise, but I would again say that you would be better off following a well worn path with a design such as the AX86 before striking out on your own.

There are a few simple rules about not killing yourself with high voltage, the most basic being - don't touch it.

I deliberately haven't spoon fed you because if you have aspirations to be a designer you need to get into the habit of following up on a few clues and hints.
If you say theory and practice don't agree you haven't applied enough theory.

phatt

  • Legendary
  • ******
  • Posts: 2118
  • Chip Points: 256
    • View Profile
Re: My Attempt at a Hybrid Design
« Reply #2 on: December 21, 2013, 08:12:02 AM »
I'm not a big fan of low voltage valve stomp box circuits but if you must?
My first thought is the tone stack Definitely needs a hi Z buffer stage before sending it to the power amp as they do not have a high enough Z input and the tone circuit will not work well,, as well as massive loss of energy.

Others here are better qualified to comment on tecky details.

Meantime google *Mesa Vtwin Schemo* it will give you a good idea of how to go about mixing Valves with SS opamp circuitry.

Feel free to try mine which is a spinoff design of Vtwin;
http://www.ssguitar.com/index.php?topic=2895.0
It gives the classic rock sounds and more.

If you want modern metal style try this one;
http://www.freestompboxes.org/viewtopic.php?f=28&t=15954

Meantime Don't be put off by High voltages as long as you have a healthy respect for ALL electrical circuitry and ask questions if in any doubt you will be fine.

Both circuits above use a trick little 555 circuit to generate the HT voltage and mine has worked flawlessly for many moons now.

My advice is build the poweramp separate or purchase a basic SS Guitar amp and build the rest as a pedal.

Oh and Welcome to the show,
Cheers, Phil.

Seems Roly has covered all the detail well while I went for coffee,, No matter. :tu:

Bakeacake08

  • Regular
  • **
  • Posts: 42
  • Chip Points: 1
    • View Profile
Re: My Attempt at a Hybrid Design
« Reply #3 on: December 23, 2013, 10:30:14 PM »
Well first of all, I have to say that I've been studying electrics for a while now, and I am thoroughly impressed with how helpful everyone is. I've read tons of message board replies that say something to the effect of, "You shouldn't do it that way because of this, but if you want to, here's how you would do it." It's very fun to learn with so many willing teachers out there.

@Roly:

I realize that doing my own design is a huge mouthful, but I'm really enjoying the challenge, and right now I'm more interested in learning the theory of how all the different pieces come together. I still might actually build a proven project for my first one, but right now I have a newborn and lack a shop, so it's easier to get the learnin' in.

Anyway, I've done some more homework, and here's what I've come up with. First, I'm not sure where I got 68uF from. I think I did the calculations before I figured out everything that was going to be in my circuit, and missed some decimal places maybe? Anyway, I t = C V, so Creservoir = I t / V.

(.4A) x (1/120) / 1.75V [5% of 35] ~1900uF, so I'd round to 2000uF. I don't know if I should have used peak or RMS voltage, but this one comes out higher, so I'd go with that.


For the bypass capacitors, I found an old magazine article (http://www.rfcafe.com/references/popular-electronics/bypass-capacitor-jan-1962-popular-electronics.htm) that said Xc should be about 1/10th of the cathode resistor, so in my case Xc = 56. Since the lowest guitar frequency is 80Hz (or maybe 70Hz if I play in drop D, which I do sometimes), I'll pass everything over f=60Hz.

Cbypass = 1 / (2 x pi x 56 x 60) ~ 47uF.


I still might be confused about the mains fuse value, but I might have it: My transformer will drop the voltage by a factor of .3 (36V / 120V = .3, so Vsecondary = Vprimary x .3), which means it will raise the current by the same ratio (Iprimary / .3 = Isecondary. So to figure the primary current draw, Iprimary = Isecondary x .3 (I did some equation rearranging on paper that I didn't type out).

So if the secondary current is 400mA, multiplying by .3 will give me a primary current draw of 120mA, and I should choose a fuse based on that number.


I didn't realize the 12au7 was not in production anymore. I found an article comparing different tubes at low voltages (>12 V) where he did a bunch of measurements and determined that it had the best performance. I'm not married to it though, so I will change it to the 12ax7.


I put the 80ohm resistor in there because I figured that would limit the current to the required 150mA. But on reflecting on other schematics I've seen in which they connect directly to the 12.6V (or 6.3V), I suppose that the filament itself might limit the current, so the resistor wouldn't be needed.

I did not calculate the Johnson noise of the circuit. I read about it in one of the many articles/books I have read, but I haven't studied it very deeply at all; I mostly just knew it existed and that I might have to take it into account. But I do appreciate your point on the impedance mismatches. I had not considered that (obviously), but now it jumps out at me while I'm perusing various schematics, so thanks for putting it in my head.

The second grid-bias resistor can be removed because the grid is already referenced to ground via the gain pot.

Speaking of the gain pot, I'm not quite clear about how it loads the anode. I get how the two resistors are in parallel and how that makes 20k. I don't know what that means as far as how the anode is being loaded. Does this "count against" the output impedance of the first triode, or the input impedance of the second?

@phatt

I haven't had a chance to look at your schematics yet (my computer has been weird about not loading webpages lately), but I'm really interested in your 555 design. That's the chip that kinda made everything click for me when I was first learning about transistors. And I appreciate your encouragement about working with high voltages. I actually feel a lot better about it since I started with this project, but I'm really into it now, so I figure I might as well finish it. I'll see if I can get it recorded when I'm done and maybe it'll surprise you and sound sorta good--and by that I mean maybe it will make a sound.  :)

Roly

  • Legendary
  • ******
  • Posts: 2184
  • Chip Points: 288
    • View Profile
    • Australian Valve Amps
Re: My Attempt at a Hybrid Design
« Reply #4 on: December 24, 2013, 06:55:55 AM »
Congrats on the new member of the family.  If it's your first you will find that this little person will change your life pretty radically, but try not to miss a minute because the next thing you know they will be all grown up and having kids of their own.


Filter cap at 2000uF o.n.o sounds much more like it - but I'd still get the heater off the back of this supply.

The cathode bypass Xc at 1/10th of Rk also sounds reasonable because the output impedance of the valve is effectively in parallel and is a low value (roughly the cathode resistor divided by the gain).  This time constant defines a lower hinge frequency or -3dB point.

Your fuse calculation is correct, the primary current will be the secondary current by the transformer turns ratio.

You can always arrange to get less gain from a 12AX7 but you can't get more out of a 12AU7, which may explain why nobody bothers to make them any more.

Indeed, the heater resistance is what limits the heater current (and makes it get red hot).

Johnson noise is typically at a very low level, typically only microvolts, and we normally only bother about it in high sensitivity circuits.  About the only time you will see it considered in audio will be in microphone preamp stages where the initial level is only a few millivolts and we want the residual noise to be many dB below that.  In a valve circuit the dominant noise source is normally shot noise from the valve itself, and is generally one or two orders of magnitude higher.

Quote from: Bakeacake08
Speaking of the gain pot, I'm not quite clear about how it loads the anode.

The HT line is ground for AC signals thanks to the bypass cap, so the anode resistor and following stage load appear in parallel to the anode.  Again a typical rule of thumb is that the following load should be ten times the anode resistor.  The source impedance of this stage is the anode resistor in parallel with the effective resistance of the valve.  Naturally this changes as the valve turns on and off but a typical idle value is around 36k, so we want the load of the next stage to be no less than ten times this, so we want our tonestack to present 360k or more.

A way to vary the first stage gain directly is to place a pot in series with its cathode bypass cap.

As @phatt suggested, the simplest way to solve the impedance mismatch problem between the tonestack and the s.s. chip amp is a buffer, and in this case you could simply insert a FET Source follower stage, say an MPF102 or similar, which will give you very high impedance to the tonestack and a low impedance drive into the chip amp.
If you say theory and practice don't agree you haven't applied enough theory.

Bakeacake08

  • Regular
  • **
  • Posts: 42
  • Chip Points: 1
    • View Profile
Re: My Attempt at a Hybrid Design
« Reply #5 on: January 04, 2014, 07:28:17 PM »
I've finally found some time to tweak my schematic, so here is the new version with some revisions:- Changed reservoir cap to 2000uF- Removed regulator circuit- Replaced it with its own transformer- Removed R4 (V1b grid resistor)- Changed gain and volume pots to 500k- Added JFET buffer between tone stage and power ampI don't quite get JFETs yet. I understand the basic principle of them, but for some reason understanding all the details is giving me trouble. I found this buffer schematic online, and it seems to be basically the same as a BJT emitter-follower circuit. What should I be looking for as far as actually calculating correct values? Is there maybe a good tutorial somewhere I haven't found yet? I put 10Meg for the bias resistors so the input impedance will be 5Meg. I don't know why I put 3k3 for the source resistor other than that's what I saw on the schematic.


How does the input impedance for a tone stack work? In this case, I guess that it's the sum of the resistors on the bass side, as those are the first ones the signal sees--though I'm positive it's more complicated than that. I also guess that it will be quite low compared to what I need (as you mentioned it will need at least 360k), so it might be reasonable to say that a buffer before the tone stage would do some good as well.


And speaking of these buffers, is a simple version like this good enough, or would it be worth the trouble of adding complexity via a complimentary feedback pair and/or using a constant current source, or something of that nature?


That is certainly not the end of my questions, but it is for right now. Thanks again for all your help, and I hope you had a fantastic holiday season!


Matt

Roly

  • Legendary
  • ******
  • Posts: 2184
  • Chip Points: 288
    • View Profile
    • Australian Valve Amps
Re: My Attempt at a Hybrid Design
« Reply #6 on: January 05, 2014, 04:50:56 AM »
If you're using LTSpice for drafting your circuits, there are a couple of pot symbols available that allow simulation.  I can post the files if you need 'em.

My off the cuff first response would be that I would scale your tonestack up by a factor of ten in resistance, down in capacitance.

There is now no reason why the Master can't be increased to 1Meg Log.  You could also experiment with doing the same with the Gain control.

You should have a cap between the wiper of the Master pot and the Gate of the FET, otherwise the pot resistance will upset the biasing of the FET mid rail by the two 10M's, and greatly reduce signal headroom.  You can work it out using Xc = 1/2 Pi f C, but my guess would be 0.1uF.

J1 is a Source Follower with is related to the Emitter Follower and Cathode Follower, and is similar in operation to a normal triode, the Source/Cathode sits a couple of volts below the voltage on the Grid/Gate.  And like the other Followers its output impedance is a lot lower than the Source/Cathode resistor implies, roughly that resistor divided by the device gain.  So 3k3 may approximate to only 30ohms source impedance.

If you don't need such a low source impedance (for the chipamp, which we don't) then the idle current through the FET buffer can be reduced by increasing the Source resistor.  The two 10M resistors set the gate to 1/2Vcc, 35/2 = 17.5V, so the Source will be somewhere around 16V.

16V across 3k3, 16/3.3 = 5mA Id.  A resistor of e.g. 15k would reduce this considerably and shouldn't have any impact on audio performance.  That's how you bias a Cathode/Emitter/Source-Follower.  :-)

Next in the signal chain we have two high-pass networks that set the low frequency response of the chipamp stage, C10 and R13, and C14 and R15.

e.g.
RoT; The input corner frequency is when the resistance of R(13) is the same as the reactance of C, R = Xc = 1/2 Pi f C giving the hinge frequency fL and -3dB point.

Transpose Xc for f
Xc = 1/2 Pi f C

f = 1/2 Pi Xc C

set Xc = R, C = C, evaluate for fHz.

Repeat for C14 and R15, and for good measure, R17 and C15, the Zobel stability network, then C16 and RL.  :-)

Physically the Zobel network, R17 and C15, need to be as close as possible to the power output chip, with the shortest possible low resistance connections.

Fuses - work out how much current, worst case, the chip amp is going to draw as this is the dominant load.  Fuse for 25 to 50% above that.  On the primary side, add the transformed heater power to the main amp power, again allow 25 to 50%.

Not shown of course is the all-important mains safety earth.

If you say theory and practice don't agree you haven't applied enough theory.

Bakeacake08

  • Regular
  • **
  • Posts: 42
  • Chip Points: 1
    • View Profile
Re: My Attempt at a Hybrid Design
« Reply #7 on: January 10, 2014, 08:44:46 PM »
That would be great if you could post those files, and maybe some instructions on how to use them. (I tried setting up a file for the 12ax7, but I couldn't figure out what I was supposed to do with the code I had.)


According to my calculations, a .1uF with a10Meg resistor will pass everything over about 1.6Hz. So I'd say you were right about that one.  :)


I've done a lot of research and found some good videos on JFETs and all the calculations, but I'm still puzzled where you got the 16V figure for Vs. In other words, why does one assume that Vs is a couple volts below Vg? I understand everything about what you said except for how you arrived at the Vs figure. My head is swimming with math right now (Shockley's equation, remembering how to algebra, etc.), so maybe it's simple and I'm just thinking too much. We'll see, I guess.


The power amp is set up directly from the datasheet, so I wasn't too worried about any of those values being off, but I calculated them just for fun and confirmed that they are acceptable.


As far as the safety ground goes, my understanding is that the best place to connect it is from the chassis to the ground of the reservoir capacitor (and all that to earth ground, or course). I was aware of this, and I actually drew it on to one of my hand-written schematics I did at work on a previous draft. Do I need to add a safety ground to the heater supply as well? If so, how would this affect the signal circuit and what sort of things should I be aware of for that?

J M Fahey

  • SSGuitar Global Mod
  • Legendary
  • ****
  • Posts: 4142
  • Chip Points: 429
    • View Profile
Re: My Attempt at a Hybrid Design
« Reply #8 on: January 11, 2014, 12:12:49 AM »
FWIW .1uF +  10M will pass everything above 0.16Hz    .

Whales love those frequencies.

Bakeacake08

  • Regular
  • **
  • Posts: 42
  • Chip Points: 1
    • View Profile
Re: My Attempt at a Hybrid Design
« Reply #9 on: January 11, 2014, 01:01:51 AM »
Oh right. I just realized I did 1M instead of 10M when I did the calculation. Whoops.

Sorry for almost cheating the whales. :)

J M Fahey

  • SSGuitar Global Mod
  • Legendary
  • ****
  • Posts: 4142
  • Chip Points: 429
    • View Profile
Re: My Attempt at a Hybrid Design
« Reply #10 on: January 11, 2014, 10:00:40 PM »
 :)

Roly

  • Legendary
  • ******
  • Posts: 2184
  • Chip Points: 288
    • View Profile
    • Australian Valve Amps
Re: My Attempt at a Hybrid Design
« Reply #11 on: January 12, 2014, 07:36:54 AM »
Quote from: Bakeacake08
According to my calculations, a .1uF with a 10Meg resistor will ...

True, but not exhaustive.  The two 10Meg bias resistors are effectively connected in parallel for AC because the DC+ supply rail is actually ground for AC, so the resistance value is 10M//10M = 5M.


Quote from: Bakeacake08
where you got the 16V figure for Vs. In other words, why does one assume that Vs is a couple volts below Vg?

The FET datasheet essentially.  Taking the datasheet for the MPF102 as typical; the pinch-off voltage (the Gate voltage that reduces the channel current to some specified level, in this case 200uA) is Vgs(off).  In the MPF102 this has a rather wide spread from -0.5V to -7.5V, but it's a fair bet with a modern production FET it will be somewhere around -2 volts.  This makes it quite similar to the 12AX7 triode valve.

If the supply is 35V and the bias divider is 1:1 then the voltage on the Gate must be half the supply or 35 / 2 = 17.5 volts.  If the Gate is 17.5V and the pinch-off is -2V then the Source voltage must be 17.5 - 2 = 15.5V.  To get rid of the decimal part for simplicity I assumed in this case a Vgs(off) of -1.5V.


Quote from: Bakeacake08
As far as the safety ground goes, my understanding is that the best place to connect it ...

...depends on where you are.

In Australia the requirement is that the safety ground has a bolt of its own with suitable lockwashers etc.  This practice is varied sometimes in that the bolt can be shared, but the safety earth has to be the first thing on, next to the chassis, and that it has its own retaining nut and lock washer before anything else is added (e.g. amp signal grounds) so loosening signal grounds won't loosen the mains safety earths.

Quote from: Bakeacake08
Do I need to add a safety ground to the heater supply as well?

No.  However the heater supply must be defined for the sake of the valves and performance.  This means that at least one side be grounded (or connected to a heater bias supply).  It is more normal in audio amps with AC heaters to ground the mid-point of the heater supply.  In older amps the heater winding was centre-tapped and this was grounded, however much the same effect can be had by connecting two 100ohm resistors across the supply and connecting the mid point to ground.  If you really want to gild the lily this is a pot so the mid-point can be adjusted to minimise hum, and was then called a "hum-dinger".

This only works with a 6.3VAC heater supply and the twin triode heaters connected in parallel; it doesn't work with 12.6VAC series heater connection.

Grounding in audio amplifiers is a bit of a tricky subject and you will find my discussion here.  The key point with signal grounds is to avoid stages having return paths/resistances in common.  This generally means that power and  ground returns should run radially from the supply to each section, not daisy-chain from one to the next.



LTSpice

The attached "*.sub" and "*.lib" files go in their respective folders, the "*.asy" in /sym/.

The circuit requires an "include" command such as;

.include tube1.lib

...and the symbol value has to be set to the desired valve name, i.e. 12AX7.

Similarly for potentiometer.sub, ".include potentiometer.sub".

You can also put them wherever you please and use a full explicit path like;
.inc "C:\Program Files\LTC\LTspiceIV\Circuits\Valve Amps\tube1.lib"
If you say theory and practice don't agree you haven't applied enough theory.

Bakeacake08

  • Regular
  • **
  • Posts: 42
  • Chip Points: 1
    • View Profile
Re: My Attempt at a Hybrid Design
« Reply #12 on: January 17, 2014, 09:23:23 PM »
Okay, I've done some learning about JFET biasing, and here's where I'm at so far:Shockley's equation gives the drain current for a given value of Vgs: Id = Idss * [1 - (Vgs/Vp)]2The datasheet for the mpf102 gives a Vp value of -8V and an Idss of 2mA to 20mA. I assumed 10mA for my calculations.Putting that all into the math machines, a Vgs of -2V should yield a quiescent current of about 5.5mA. As Vgs = Vg - Vs, Vs should be around 19.5V, as we know Vg is set at 17.5V.19.5V / 5.5mA gives us a source resistor value of 3,545ohms, or 3k3, which (I just found out) happens to be what I had when I first added the JFET. Am I on the right track so far? I hope so, cause I'm going to keep going. :) You said that I could lower the current with a 15k resistor. Thinking it through, changing Rs to 15k (assuming a 19.5V drop across it) would lower the current to about 1.3mA--but this would also affect Vgs to -5V, meaning that Rs would actually be dropping 22.5V (17.5 - [-5]). This appears to be within the specs of the FET, which I think is why you said it shouldn't affect the performance too much. Do I appear to be understanding the calculations correctly? I forgot how much fun math can be to learn.


Also one other note about the mpf102: the datasheet says that the drai, drain-gate voltage max rating is 25V. I can't seem to find a drain-source voltage rating, but I know that I will be running at around 35 VRMS, which will peak at about 50V. Will this FET work in my circuit? I assume I should use the bigger number when figuring maximums, but even the RMS value seems too high.


Thanks again for all your help. Learning is fun!
« Last Edit: January 17, 2014, 09:36:43 PM by Bakeacake08 »

Roly

  • Legendary
  • ******
  • Posts: 2184
  • Chip Points: 288
    • View Profile
    • Australian Valve Amps
Re: My Attempt at a Hybrid Design
« Reply #13 on: January 18, 2014, 09:54:22 AM »
Quote from: Bakeacake08
Shockley's equation

Who?   ;)

Quote from: Bakeacake08
The datasheet for the mpf102 gives a Vp value of -8V and an Idss of 2mA to 20mA.

Maybe, and no offense to Shockley but a bias point Vgs of around -3V and Id of 1mA is a bit more realistic with an actual device.

Quote from: Bakeacake08
cause I'm going to keep going. :)

Please, be my guest (I'll just get the fire extinguisher handy).   8|

Quote from: Bakeacake08
Do I appear to be understanding the calculations correctly? I forgot how much fun math can be to learn.

That's a very pleasant contrast to some other stuff that is going on.   :dbtu:

Yeah, you are right, my bad, MPF102 Vdg is 25V so it is effectively a 25V device, however...

Working backwards from the output the peak-to-peak at clipping is your 35V supply rail.  The gain of the chip amp is roughly R16 over R15, Av ~= 22/.68 = 32 times, so the input for clipping is around 1V peak-to-peak centered around half supply.  You can go for a higher voltage FET, but I'm inclined to think you can get away with it with an MPF102.

I may have mentioned this before, but looking at your circuit, move the 35V supply to the chip amp end and add some RC supply decoupling for the preamp.

Quote from: Bakeacake08
but I know that I will be running at around 35 VRMS, which will peak at about 50V.

No you won't, you only have a single 35VDC supply.  35Vp-p = 17.5Vpk --> 17.5 / root(2) = 12.4Vrms.

BTW your main filter cap only needs to be a 50V rating (on a 35VDC supply).

If you used high intensity LED's you'll only need a mA or two and can increase the series resistors to 15k -> 33k and reduce their power rating.

You will need to select C16 to have a ripple current rating higher than your maximum output current.


You are getting to the point where you need to throw your ideas up against reality and see what sticks.   :tu:

HTH
If you say theory and practice don't agree you haven't applied enough theory.

Bakeacake08

  • Regular
  • **
  • Posts: 42
  • Chip Points: 1
    • View Profile
Re: My Attempt at a Hybrid Design
« Reply #14 on: January 24, 2014, 02:26:07 PM »
Quote from: Roly
Maybe, and no offense to Shockley but a bias point Vgs of around -3V and Id of 1mA is a bit more realistic with an actual device.
But . . . but the textbook says . . .

I guess I'll go with your actual, real-life experience on this one. :)  However, I'm still not sure what makes you think I'd get away with using the MPF102. I understand how you worked backwards to get to the 1V signal, but I'm under the impression that exceeding the max drain-gate voltage would do something like blow it up (or whatever happens to JFETs when they fail).


And speaking of headroom, working backwards from the power amp section, the max input is 1V, meaning the max input of the JFET stage is about 1V as its gain is roughly 1. So the max output from the tubes should be around 1V as well, correct? I know that there is some loss in the tone stage (20dB or so, I believe), but how does that relate to the voltage signal? Also, what is the best way to test the output voltage of my pickups? It seems I should know this in order to figure out the output of each stage. My instinct would be to hook up my DMM for AC voltage to the guitar and play a loud chord and hopefully get a reading. Maybe there's a better way?


Quote from: Roly
You will need to select C16 to have a ripple current rating higher than your maximum output current.

That makes sense. How do I calculate the maximum output current? The maximum that my transformer can put out?

Quote from: Roly
You are getting to the point where you need to throw your ideas up against reality and see what sticks.



Thumbs up indeed! Now I just need to explain to my wife why we need all these electronic parts instead of, for instance, fruit. You can live off pop tarts and hot pockets, right? :)
« Last Edit: January 24, 2014, 02:29:15 PM by Bakeacake08 »