So I just designed a valve amp...

[DIXØN]

Is there anyway I can make a full computer simulation of it before I go out and build it all up, only to find it don't work? Or can you just tell by looking at the schematic that it will or won't work? If so, could someone do that for me? :) I can post the schematic if needed...

[JimiB]

post away!

[DIXØN]

http://img.photobucket.com/albums/v306/Ali3nat0r/homebrew.gif

Photobucket has shrunk it a bit but it's still just about readable.

[andyc]

Why the bridge rectifier for the heater circuit?

[WesKuhnley]

Why 4 stages in series? Why set them all up the same way? I would put the first two stages in parallel, bias one hot, the other cool, use the third as a gain boost, 200K plate resistor, 1k5-2k2 cathode resistor, and drive your tone stack with a cathode follower rather than off the plate of your 4th stage.

Your schematic looks nice and clean, for all other intents and purposes, the amp appears like it would function.

[banjoze]

Great question in the original post above: Is there software available that validates or simulates what a circuit will do prior to actually building it? Back in 1985 I worked at McDonnell Douglas in St. Louis and remember seeing something like that there.

I'm sure it will not tell you how it will sound, but it would be cool if could show you problems in your design at an electrical level at least.

[banjoze]

Btw DIXON, what did you use to draw your circuit? Visio, other?

[rooster]

You may want to reference the grids on the 2nd, 3rd, and 4th stages to ground. I'd also think about using a fairly low resistor value. You are going to have unbelievable levels of gain buildup between those stages, and I'd wager quite a bit of grid blocking distortion. There is also no decoupling between your power supply nodes, and I have a feeling this thing is going to take off like the phasers from the Enterprise when you turn it on. As well, if you're looking to bypass your tone stack, simply either lift it from ground, or run your switch across a very high value resistor.

Not trying to be hypercritical, but this ain't the way to go. I've built quite a few amps, and this isn't going to get you where you want to go. I'd suggest going to a couple of "builder" boards, and possibly order a kit from Doberman amps to get started. Start small, work your way up. You're looking at something less than Pearl Harbor in 1941, but something more than accidentaly stepping on your wah pedal during a smokey jazz number.

rooster.

[VacuumVoodoo]

Great question in the original post above: Is there software available that validates or simulates what a circuit will do prior to actually building it? Back in 1985 I worked at McDonnell Douglas in St. Louis and remember seeing something like that there.

I'm sure it will not tell you how it will sound, but it would be cool if could show you problems in your design at an electrical level at least.

http://www.beigebag.com/

Has tube and transformer models. Not totally intuitive so learning curve is steep in the beginning. Free 30 day full feature trial version, goes into a limited feature after trial period expires.

[DIXØN]

Why the bridge rectifier for the heater circuit?

I heard Epiphone had to upgrade the Valve Junior with DC heaters because AC ones were noisier, or something...

I used ExpressSCH to draw the circuit, it and ExpressPCB come together as a free software bundle.

You may want to reference the grids on the 2nd, 3rd, and 4th stages to ground. I'd also think about using a fairly low resistor value. You are going to have unbelievable levels of gain buildup between those stages, and I'd wager quite a bit of grid blocking distortion. There is also no decoupling between your power supply nodes, and I have a feeling this thing is going to take off like the phasers from the Enterprise when you turn it on. As well, if you're looking to bypass your tone stack, simply either lift it from ground, or run your switch across a very high value resistor.

OK cheers mate, will have a think 'bout that and see what happens, is it just like WAY too high gain at the moment then or would it not even produce any sound at all? Also strange no-one said anything about the output stage since I guessed most of the resistor values in that bit.

On a side note, if you hook up the supply voltage and screen voltage for the output stage to a transistor based voltage amplifier controlled by a pot (basically a pot that sets the supply/screen voltage) would that mean you can get output valve overdrive at low volumes, or would it send my EL34s to the big recycle bin in the sky?

[embot]

Glassware has a CAD program that can simulate tube amps based on your circuit. Although I've personally have not tried it. I've seen it being sold in Antique Electronics Supply and have been curious about its capabilities.

[embot]

A few more comments based on your circuit. Since you mentioned the output stage.

You connected the grid of the el34 to the B+ voltage via 1k ohm/7 watt resistors. Normally a choke or at least another 1K (5W to 10W) resistor would separate B+ from the tie in point of the 2-1k Ohm/7 watt resistors. I'd also reduce the power rating of the two resistors from 7 watts to 1 or 2 watts. Just in case of a tube failure the resistors will fail before the output transformer burns.
The screen resistors, at 20K ohm, seem overkill. Normally I see 1K - 5.6K ohm resistors in this position.
At the phase inverter, I also usually see a .1uf capacitor connecting the input of ECC83 3b to ground (where the 1M ohm resistor is connected to 3b). I don't really know what the ommitance of this capacitor would do to your circuit. It's just that all long tailed pair phase inverters I've come accross has that capacitor.

[Wakarusa]

www.linear.com, download swcad3 -- it's a free SPICE simulator. SPICE models for tubes, speakers, transformers, etc. are available all over the place or you can work up your own. I've been using it to proof designs for years now -- with the caveat that the simulation software will never quite exactly predict final performance, but certainly good enough for testing for big flaws, stability, odd frequency shaping, etc.

[RedMan]

It would work OK if you used some lower valued grid to ground resistors say 47-100K on the latter gain stages, maybe another 100K pot on the last one. It actually wouldn't be too much gain then with that and the unbypassed cathodes but it would probably still be a bit noisy. I like the true tonestack bypass. It may be cool to have a gain stage bypass function in conjunction with that. Of course you would need to make it noise free somehow. I think building and listening is much better than computer modeling, but the modeling can save you some time getting close.

[donnyjaguar]

Yikes, good first try but there are a few obvious things that need to be addressed. Someone mentioned referencing the grid to ground. That first gain control looks a little low at 100kΩ as this will load down the first stage a lot. You have too many gain stages too, IMO. The tone defeat has two problems. One it will click/pop when switched, and the other is the level will be much louder when defeated. I'd put an attenuator in here to more close match a typical tone setting's level. The 2nd 100kΩ is also too low in value and will load down the tone stack severely. 1MΩ would be better here. Also, you don't want a 91k resistor in series driving effects, you want 0Ω here, IMO. You're missing a capacitor in the phase invertor - see 2nd grid. The power supply for the small signal tubes has no decoupling, guessing this is just an error. The circuit for the bias will work, but if you add a second RC filter and wire the adjustment as a rheostat (like in a Marshall) the tubes won't self-destruct if the wiper of the control goes open (most common failure mode, BTW). Also, the amplifier has no negative feedback, although this may be intentional. I like the DC filament supply but you'll need much more capacitance if you want to smooth out the DC and reduce hum to an absolute minimum.

Anyway, good effort and this can easily be turned into a rock and roll machine!!

[Wakarusa]

Finally got around to looking at the schematic. In addition to other's comments, I'd suggest doing the math to look at the expected frequency response of the entire amplifier. All four preamp stages are unbypassed and there's no global NFB (with the opportunities for frequency shaping there) and the interstage attenuation (the 75K series resistors) shouldn't have a huge filtering effect in and of themselves -- all in all, I think this thing would be somewhere between "bright" and "ice pick" - particularly with the tone stack cut out.

[DIXØN]

OK I'll stick a 100k res from the last 3 stages' grids to earth then. Since I play a majority of music, I thought it probably best to have a lot of gain on tap for when I do need it. Should probably put a switch in to divert the input from ECC83 1a to ECC83 2a, and move the gain control. I'll also add that cap in the power amp and upgrade the gain/level pots to 1M. And sort out that power supply error ;) oops...

Also, you don't want a 91k resistor in series driving effects, you want 0Ω here, IMO.

Hrm, isn't the output from a valve (even after going through the tone stack) like 150+ volts? I'd rather not put that into my digital delay pedal...

[RedMan]

Finally got around to looking at the schematic. In addition to other's comments, I'd suggest doing the math to look at the expected frequency response of the entire amplifier. All four preamp stages are unbypassed and there's no global NFB (with the opportunities for frequency shaping there) and the interstage attenuation (the 75K series resistors) shouldn't have a huge filtering effect in and of themselves -- all in all, I think this thing would be somewhere between "bright" and "ice pick" - particularly with the tone stack cut out.

That's part of the reason I suggested the low value grid resistors, it would serve to roll off some high end as well as the interstage attenuation that's needed there. It would also keep the bass from getting too flubby. Another cool thing to consider would be using a tone control between each stage instead of them all at once. I think amps sound better with more gain stages and the right attenuation/frequency compensation between them, even for a clean sound. Most people prefer the second channel on a BF Fender amp w/reverb over the first channel even if the reverb is off and it's set clean. That's 3 gain stages. This is just one more and I'm guessing he wants more gain than a BF Fender.

[Wakarusa]

That's part of the reason I suggested the low value grid resistors, it would serve to roll off some high end as well as the interstage attenuation that's needed there. It would also keep the bass from getting too flubby. Another cool thing to consider would be using a tone control between each stage instead of them all at once. I think amps sound better with more gain stages and the right attenuation/frequency compensation between them, even for a clean sound. Most people prefer the second channel on a BF Fender amp w/reverb over the first channel even if the reverb is off and it's set clean. That's 3 gain stages. This is just one more and I'm guessing he wants more gain than a BF Fender.

While we're goin' nuts with the man's design... :)
Rather than series interstage attenuation my preference would be to split the cathode resistors and bypass (or partially bypass) just one side of the split. You get cathode bypassing for frequency shaping and local cathode fed NFB for frequency response, gain control, and output impedance control. Much more elegant solution to my way of thinking

[DIXØN]

lol this is my first full design, I'm used to tweaking around with a Valve Junior and seeing what each components does. I have managed to get it sounding better than stock by increasing the anode resistor of stage 2 from 100k to 150k, that's given me more gain, as well as bypassing the interstage attenuation resistor with a 68k for even more (it's 1M stock, so works out as being 63k instead). I'm also toying with the idea of inserting the tonestack from that design above into it to see how that sounds. In the VJ, the cathode resistors on both gain stages are bypassed with 22uF caps, although one is electrolytic whilst the other is not. Does this have any effect on the tonal properties of the stage?

[ontariomaximus]

I suggest the following:
1. take one of the 12AX7 sections, say perhaps the 4th one and put it in between the fx return and the LTPI
2. You don't need 1 ohm 7watt cathode resistors to monitor current, 1 watt is PLENTY
3. if you need 4 gain stages in the pre why don't you try copying a topology of a known then tweak from there. I am thinking of a SLO100 (or supercharger GTO), Bogner XTC, Laney AOR50 or 100. I can almost guarantee what you have will sound like caca, so I won't go address the pre section.
4. You need a dropping string in the PS for the different nodes.
5. Choke in the PS after the main B+ cap 3-10h 40ma-100ma
6. Connect your suppressor grids to the bias supply
7. What is your B+?
8. Do ground your secondary, and what about nfb and presence?
9. 20k grid stoppers on the EL34's is a pretty high value
10. Why screw around with DC on the heaters? 99% of the time it's more trouble than it's worth. 47uf is WAY too low a value for ripple reduction
11. You want a 20k slope resistor?

[DIXØN]

ontariomaximus, I've seen a load of your custom amps on youtube. I'm gonna take a lot of your advice. :) And about the 20k slope res, I designed the whole tonestack in Duncans Tone Stack Calculator, that one gives pretty much the frequency response and adjustment I'm looking for. I'll also be using a Hammond 373BX power transformer, so B+ is about 350V.

[ontariomaximus]

You should hear my amps with someone who can really play. I'll be getting some new youtube clips up in the next little while.
You can forego the choke with a B+ of 350, I doubt the screens will be drawing a ton of current. Use a 100 ohm resistor in place of the choke, then a 1000 ohm 10w SHARED screen resistor (25w would be a little cooler) , then individual 100 ohm screen resistors. The Trainwreck Express uses a 1000 ohm shared screen R fwiw.
Here is an example of one of my hi-gain preamps.
http://s199.photobucket.com/albums/aa171/ontariomaximus/?action=view&current=SHOMar08.jpg

[allynmey]

Dixon, all those gain stages....just to be knocked down at each stage by a resistor? I'd use the first to amplify the input then into the tone stack. Use the second as a tone stack recovery. If you still want the second tube in there, make a Cathode Follower to drive the effects loop with a recovery stage. put pots on the input and output of the EFX loop and then into your phase invertor. If you are going to use the tone stack bypass like your picture has, you may need to knock down the signal a bit. Maybe a grid resistor or perhaps use the EFX send pot to lower output.

My .02

[DIXØN]

Ok I've updated the schematic to the following:

http://img.photobucket.com/albums/v306/Ali3nat0r/homebrew-1.gif

Photobucket didn't shrink it this time either... weird...

[rooster]

Well, I won't address the design of the preamp anymore, since you're set on it. I believe, and this is conjecture, that it's not going to be giving a good, smooth, controllable tone. But, that's my opinion. The Power supply, however, is not going to cut it. You need to decouple the filter caps in the power string. You essentially have one big cap with a bunch of lines coming off of it, and every current draw will affect every stage. Also, that 150uF on the filament line isn't close to enough if you expect to filter out the ripple. Personally, I've found that floating the filaments at about 70VDC or so tends to get rid of any buzz, and that's even at ridiculous amounts of gain. You'll need at least 2000uF for proper DC filtering the way you're setting it up, and I'd be thinking more along the lines of 4700uF myself, if I were to be doing that.

Good luck, make sure you build it with extra turrets/eyelets so you can tweak it after you get it done.

I'd still recommend building something like a P1 first, and going from there.

rooster.

[DIXØN]

Can you breadboard this sort of HV circuit, or would it arc across the tracks?

[teleamp]

Can you breadboard this sort of HV circuit, or would it arc across the tracks?

I would think that you would get too much cross-capacitance with a breadboard.

[DIXØN]

I would think that you would get too much cross-capacitance with a breadboard.

Ah shyte, turret boards confuse the hell out of me. I'm used to breadboarding then producing a PCB...

[teleamp]

Ok I've updated the schematic to the following:

http://img.photobucket.com/albums/v306/Ali3nat0r/homebrew-1.gif

Photobucket didn't shrink it this time either... weird...

The best thing for you to do is to build it and see how it sounds, then you will understand a lot of the suggestions that have been offered.


Instead of a breadboard, use a piece of plywood with wood screws for the turrets.

BTW, you didn't design an amp, you morphed some circuits. The real designers are the engineers who developed the tubes and the associated circuits which amp builders use to morph into amps.

[mooreamps]

BTW, you didn't design an amp, you morphed some circuits. The real designers are the engineers who developed the tubes and the associated circuits which amp builders use to morph into amps.


I think it is a fine design effort, none the less.

-g

[teleamp]

I think it is a fine design effort, none the less.

-g

I wasn't trying to burst his bubble, but pointing out.

One does not need to be an engineer or designer to build a great amplifier, some of the best amps are ones that circuits have been morphed together and tweaked to perfection.

Some of the worst sounding amps are a result of an amp being bult per load line specs.

[donnyjaguar]

One good thing about tube guitar amps is they can be totally out of whack yet still produce music. :)
The designer gets two thumbs up from me for scratch building! :BEER
This the way to do it. Kits are for young players!

I agree on him putting it together at this point and seeing what it does before optimizing the circuit.

DJ

[mark norwine]

1.) I agree with the poster who said the power supply will need more separation....

2.) what's with the isolation of the 3 secondary windings on the OT? That confuses me.

[DIXØN]

1.) I agree with the poster who said the power supply will need more separation....

2.) what's with the isolation of the 3 secondary windings on the OT? That confuses me.

I've seperated the power supply now, but can't be arsed to re upload the sch as of yet. The OT thing confused me too, it's a stock component in the software. The OT I'm planning on using isn't even wound like that, it is 16 ohm with taps at 8 and 4 (I think).

[epluribus]

The designer gets two thumbs up from me for scratch building! :BEER

DJ

Big +1! :BEER

This thread raises a ton of terrific questions, none of which I can answer as competently as the pros you've heard from, :) so color me tuned in.

--Ray