Hi, I'm hoping for some well rounded advice/opinions about a Valve Wizard style low-voltage hybrid guitar preamp I'm building. My end goal is to best replicate a Fender Champ AA764 with one output for use as preamp pedal, and another through a 12V poweramp (currently a tiny TDA2030a module) so it could be used as a mini-head.
The restrictions I've given myself are 12V supply and 6111 or 12AU7 tube (I have heard the spiel about starved plate not being the same). It seems to be working fairly well, both into a guitar amp and through the TDA2030 module, but there are some things I could use some opinions on. Here is what I have on my breadboard at the moment, minus a 7806 for the heater (The 1Meg at the output is the input pot of the TDA2030 module):
I've used Merlin Blencowe's opamp biasing for the input triode so that I can hopefully have high input impedance but also overdrive the first triode without overdriving the opamp, but I'm not really grasping how the output range of the opamp and the swing range of the triode interact, or how to best choose a grid stopper.
Usually in opamp circuits the filters go to Vref instead of ground, so I'm unsure if going to ground and isolating the DC with C9 off the volume pot is the best way to go here?
I'm trying to use the second opamp buffer play the role of the second preamp stage, leaving the second triode to take the place of the power tube and be overdriven by the "preamp" section. The volume pot here seems to be messing with the biasing though, and is quite crackly when adjusted.
Any pointers or advice?
The restrictions I've given myself are 12V supply and 6111 or 12AU7 tube (I have heard the spiel about starved plate not being the same). It seems to be working fairly well, both into a guitar amp and through the TDA2030 module, but there are some things I could use some opinions on. Here is what I have on my breadboard at the moment, minus a 7806 for the heater (The 1Meg at the output is the input pot of the TDA2030 module):
An externally hosted image should be here but it was not working when we last tested it.
I've used Merlin Blencowe's opamp biasing for the input triode so that I can hopefully have high input impedance but also overdrive the first triode without overdriving the opamp, but I'm not really grasping how the output range of the opamp and the swing range of the triode interact, or how to best choose a grid stopper.
Usually in opamp circuits the filters go to Vref instead of ground, so I'm unsure if going to ground and isolating the DC with C9 off the volume pot is the best way to go here?
I'm trying to use the second opamp buffer play the role of the second preamp stage, leaving the second triode to take the place of the power tube and be overdriven by the "preamp" section. The volume pot here seems to be messing with the biasing though, and is quite crackly when adjusted.
Any pointers or advice?
My understanding is that when the volume pot changes, U3+ will see a jump in voltage followed by a decay back to Vr. Is this the crackling problem mentioned?
Is there a reason a 1M volume pot can't replace R10, and get rid of the 200k one (or change it to a regular 200k resistor if it's loading in a relevant way)?
Is there a reason a 1M volume pot can't replace R10, and get rid of the 200k one (or change it to a regular 200k resistor if it's loading in a relevant way)?
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Yes, actually it's a 1Meg pot, I was just laying a 220k resistor across it for some log action. I've removed it, but I see now you're right, even with a 1Meg pot the bias shifts up and down which is causing the crackling. Is there a way I could maintain the volume pot and somehow isolate it so this doesn't happen?
Here's an updated schematic. I changed the bias circuiting a bit, but taking more measurements from the breadboard I notice that the grid-cathode voltage is +0.25V (grid=2.97V, cathode=2.76V) instead of -0.25V as I'd expect! I'm pretty sure this won't hurt the valve at this voltage but I'm puzzled as to why. Any pointers on that would be appreciated as well.
Sorry, I meant to move the 1M pot to where R10 is right now, connect the wiper to U3+, and remove R10, since the volume pot will be serving its function anyway. The reason I thought to make it 1M and not 200k is that a 200k would attenuate bass.
Why is the -.25V difference expected, and doesn't it depend on the setting of U4 / Vr? (Well, I'm trying to understand the circuit here, but I am a complete beginner with valves and may not have anything useful to contribute.)
Why is the -.25V difference expected, and doesn't it depend on the setting of U4 / Vr? (Well, I'm trying to understand the circuit here, but I am a complete beginner with valves and may not have anything useful to contribute.)
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Unfortunately if I put the volume pot in place of R10 it won't attenuate the full signal when turned down.
The idea with the valves is to use the opamp (U2/U3) to set the DC level at the grid of U1/U5 and use a cathode resistor R5/R13 to push the voltage at the cathode positive in relation to the grid. I suspect my failure might be relying on LTspice to properly simulate valves at low voltages, although this is how it works in Merlin Blencowe's paper. You can see his paper here, it's quite interesting and an easy read: Triodes at low voltages
The idea with the valves is to use the opamp (U2/U3) to set the DC level at the grid of U1/U5 and use a cathode resistor R5/R13 to push the voltage at the cathode positive in relation to the grid. I suspect my failure might be relying on LTspice to properly simulate valves at low voltages, although this is how it works in Merlin Blencowe's paper. You can see his paper here, it's quite interesting and an easy read: Triodes at low voltages
> isolating the DC with C9
C9 polarity is drawn wrong. If built this way, this sure would explain crackle and other ill effects. Flipping it is fast.
But since C9 only drives 1Meg, WHY is it 10uFd? For 0.02Hz response?? Me, I see 1Meg, I think 0.01uFd (17Hz), not 10uFd. Bonus: a 0.01uFd will not be a Polar cap so it can go either way.
If you have another 100nFd, like you used at the other 1Meg point, that will do too.
C9 polarity is drawn wrong. If built this way, this sure would explain crackle and other ill effects. Flipping it is fast.
But since C9 only drives 1Meg, WHY is it 10uFd? For 0.02Hz response?? Me, I see 1Meg, I think 0.01uFd (17Hz), not 10uFd. Bonus: a 0.01uFd will not be a Polar cap so it can go either way.
If you have another 100nFd, like you used at the other 1Meg point, that will do too.
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Yep, a 100nF non-polar fixed my crackle problem, thanks PRR. Originally I used a 10uF to avoid any high-pass filtering completely since that capacitor isn't there in the original AA764 topology, but 1.7Hz is fine.
My understanding is that they're like JFETs and positive grid voltage is a bad thing. Working with Merlin's measurements most tubes at 12V have a cut-off of about Vgk = -0.5V so a Vgk of about -0.25V seems best but that's not alot of headroom coming in. However he also says "What’s more, this [...] allows you to play with zero and positive grid bias (in such a low-voltage circuit no harm will come to the triode by doing this)." I notice that there's a large linear region from +0.2V Vgk, but don't know how one would tell if they're really damaging the tube, other than by doing it.
My understanding is that they're like JFETs and positive grid voltage is a bad thing. Working with Merlin's measurements most tubes at 12V have a cut-off of about Vgk = -0.5V so a Vgk of about -0.25V seems best but that's not alot of headroom coming in. However he also says "What’s more, this [...] allows you to play with zero and positive grid bias (in such a low-voltage circuit no harm will come to the triode by doing this)." I notice that there's a large linear region from +0.2V Vgk, but don't know how one would tell if they're really damaging the tube, other than by doing it.
Everything I wrote was with the assumption that the volume pot was seeing DC 7.42V, which doesn't really seem true after looking at the schematic again. So, I guess none of it made sense, sorry.
So the crackle is because the capacitor is failing and starting to conduct DC?
So the crackle is because the capacitor is failing and starting to conduct DC?
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Thanks PRR that worked, I went for 100nF. Originally, I used the 10uF to avoid any high-pass filtering since that cap isn't there in the AA764, but 1.7Hz is good enough. No worries chaerin, I appreciate you chiming in anyways.
Now I'm moving on the tackle the biasing. I'm new to tubes, but it's my understanding that positive grid bias is a bad thing, like with JFET gates, however Merlin's paper says "in such a low-voltage circuit no harm will come to the triode by doing this".
From Merlin's measurements common tubes at Va = 12V have a cut-off at about Vgk = -0.5V, so -0.25V or so seems like an obvious choice but also seems likely to be driven to a positive Vgk anyways by a guitar, unless I attenuate the input. So I notice there's a second linear region, starting about Vgk = +0.2V which seems like it might be a better choice, but is there a way to confirm I'm really not damaging the tube?
Now I'm moving on the tackle the biasing. I'm new to tubes, but it's my understanding that positive grid bias is a bad thing, like with JFET gates, however Merlin's paper says "in such a low-voltage circuit no harm will come to the triode by doing this".
An externally hosted image should be here but it was not working when we last tested it.
From Merlin's measurements common tubes at Va = 12V have a cut-off at about Vgk = -0.5V, so -0.25V or so seems like an obvious choice but also seems likely to be driven to a positive Vgk anyways by a guitar, unless I attenuate the input. So I notice there's a second linear region, starting about Vgk = +0.2V which seems like it might be a better choice, but is there a way to confirm I'm really not damaging the tube?
then you need to read at least a tenth of what Rob Robinette has to say on the topic. Starting with Tube Guitar Amp Overdrive and, I guess, ending with Voicing Amps. The analysis of the JCM-800 is one point along that road not to be missed.
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I would not worry about "burning grids" in a 12V system, especially with 4.7K in series with the grid.
> positive grid bias is a bad thing
Does not hurt tubes, or JFETs, as long as the current thus the power is not large.
The "problem" is that it distorts.
> positive grid bias is a bad thing
Does not hurt tubes, or JFETs, as long as the current thus the power is not large.
The "problem" is that it distorts.
Thanks thoglette, I've been reading through Aiken Amps, Rod Elliott/ESP and Rob Robinette but that article on Tube Overdrive has lot of answers I'm looking for right now.
PRR, thanks for making it a little clearer what "burns" the grid.
While reading through the Tube Overdrive article I've been testing with a fully positive grid bias of about +0.3V, unfortunately I only have little DSO138 hobby oscilloscope so it's hard to see soft clipping. I do have some more questions though.
I believe I can see "grid clipping" of the positive lobe, and that makes sense, but should I expect a "negative bias excursion" given that I'm biasing with an opamp? Will the grid current flow back through the opamp and interact with the input capacitor and opamp biasing resistor, or will the opamp hold the bias steady?
PRR, thanks for making it a little clearer what "burns" the grid.
While reading through the Tube Overdrive article I've been testing with a fully positive grid bias of about +0.3V, unfortunately I only have little DSO138 hobby oscilloscope so it's hard to see soft clipping. I do have some more questions though.
I believe I can see "grid clipping" of the positive lobe, and that makes sense, but should I expect a "negative bias excursion" given that I'm biasing with an opamp? Will the grid current flow back through the opamp and interact with the input capacitor and opamp biasing resistor, or will the opamp hold the bias steady?
A properly operating opamp is a very, very good buffer; what happens on the output end will have no effect at all on the input side of things.
What is the actual measured DC voltage between ground and the output pin, for each of your two opamps? TLO72 opamps can only get their outputs within a couple of volts of either supply rail (ground or +12V in your case). I'm wondering if your circuit is asking the opamps to do what they cannot actually do, i.e., swing the output lower than that.
-Gnobuddy
I've been playing with bias quite a bit actually. I have the bias for both opamps set to a single bias point which is 10k pot between a 1k resistor to ground and a 15k resistor to +11.5V.
Both opamps/triodes have readings within 10mV so I've only listed the first pair.
Pot @ 0% (towards ground):
Output pin 0.58V, Grid 0.54V, Cathode 0.84V, Anode 10.36V, Vgk -0.31V
Pot @ 50%:
Output pin 2.70V, Grid 2.50V, Cathode 2.52V, Anode 9.33V, Vgk +0.05V
Pot @ 100%:
Output pin 4.93V, Grid 4.55V, Cathode 4.40V, Anode 8.66V, Vgk +0.16V
If the opamp is buffering that well all I should have to worry about is "regular" valve cut-off (probably around -0.5V with Va=12V) and opamp swing. As you can see there's a voltage drop (over the grid stopper?) between the output pin and the grid, but the biggest problem is that in order to get a reasonable Vgk the opamp bias needs to be low which also raises the plate voltage rather high.
An externally hosted image should be here but it was not working when we last tested it.
Both opamps/triodes have readings within 10mV so I've only listed the first pair.
Pot @ 0% (towards ground):
Output pin 0.58V, Grid 0.54V, Cathode 0.84V, Anode 10.36V, Vgk -0.31V
Pot @ 50%:
Output pin 2.70V, Grid 2.50V, Cathode 2.52V, Anode 9.33V, Vgk +0.05V
Pot @ 100%:
Output pin 4.93V, Grid 4.55V, Cathode 4.40V, Anode 8.66V, Vgk +0.16V
If the opamp is buffering that well all I should have to worry about is "regular" valve cut-off (probably around -0.5V with Va=12V) and opamp swing. As you can see there's a voltage drop (over the grid stopper?) between the output pin and the grid, but the biggest problem is that in order to get a reasonable Vgk the opamp bias needs to be low which also raises the plate voltage rather high.
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> Output pin 4.93V, Grid 4.55V, Cathode 4.40V, Anode 8.66V, Vgk +0.16V
0.38V over 4700 is 81uA
0.38V at 81uA is 30 MICRO watts!
6111 is rated nearly 1 Watt in Plate. Grid dissipation is not directly rated, but I would expect at-least 1% of Pdiss, or 10mW. 300X what you have. Also the 6111 does specify max 5.5mA DC grid current, 68X what you are doing.
You could be a LOT more abusive!! Whether it sounds "better", I do not know. But you got the thing set up, try it!
Happy results will probably be limited by positive grid resistance eating all the signal. For higher plate voltage, 4Vg-k sucks 5mA so about 800 Ohms, so you get at grid about 1/7th of what comes from the opamp through the 4700. At low plate voltage, 2V may get rg down to 500r, about 1/10 loss. So you can really beat that signal without ANY risk to the tube.
0.38V over 4700 is 81uA
0.38V at 81uA is 30 MICRO watts!
6111 is rated nearly 1 Watt in Plate. Grid dissipation is not directly rated, but I would expect at-least 1% of Pdiss, or 10mW. 300X what you have. Also the 6111 does specify max 5.5mA DC grid current, 68X what you are doing.
You could be a LOT more abusive!! Whether it sounds "better", I do not know. But you got the thing set up, try it!
Happy results will probably be limited by positive grid resistance eating all the signal. For higher plate voltage, 4Vg-k sucks 5mA so about 800 Ohms, so you get at grid about 1/7th of what comes from the opamp through the 4700. At low plate voltage, 2V may get rg down to 500r, about 1/10 loss. So you can really beat that signal without ANY risk to the tube.
Ah! I'm beginning to understand how this all comes together. It actually sound super creamy with lower grid resistors like 220R-2.2k. Unfortunately I'm also discovering the headroom woes of starved plate amplifiers are even worse when coupled with opamp range. Valve distortion is definitely on the menu here, but I'd like to have a reasonable clean range as well; maybe source-followers instead of opamps?
I've been too busy at work to stop by here, sorry about that. Looking back at the voltages you posted, 0.58V on the output pin of a TL071 - with respect to the negative supply rail - is not enough for the opamp to function properly.
With these sorts of opamps, my rule of thumb is to never ask the output to get closer to a supply rail than roughly three times the voltage dropped across a single silicon diode (that's about 0.7 volts). So, don't ask the opamp output to get within less than two volts of either supply rail.
(There are some specialized opamps that can swing their output to within a few millivolts of either supply rail, but the TL071 is definitely not one of them.)
What this probably means is that U2 and U3 are distorting heavily at some pot settings. The distortion they produce is unlikely to be sweet and musical; to put it very mildly, opamps are not usually known for that.
The simplest fix I can think of is to use a separate 9V battery to supply a much more negative voltage to the appropriate pin of the opamp(s). If you're really using TL071s, that means pin 4 goes to the (-) end of the additional 9V battery, and the (+) end of that battery goes to your ground.
-Gnobuddy
Since you mentioned source-followers (to replace the opamps), I might as well add that a source follower is also not going to work with only 0.58V on the source, and the gate tied to ground. You'd need a negative supply voltage to make that work, too.
And, while JFETs are less likely to create unpleasant distortion than opamps, they are also less predictable, and less well suited to producing just the right DC voltage to bias your vacuum tube.
You could, of course, AC couple a properly biased JFET source follower to a vacuum tube stage - no need for DC coupling, really.
-Gnobuddy
And, while JFETs are less likely to create unpleasant distortion than opamps, they are also less predictable, and less well suited to producing just the right DC voltage to bias your vacuum tube.
You could, of course, AC couple a properly biased JFET source follower to a vacuum tube stage - no need for DC coupling, really.
-Gnobuddy
Yeah, 0.58V is too low in general, but that's more to do with me struggling to get a negative Vgk. I've been working with LTspice but the model I have for the 6111 is likely tuned for a standard 100V B+, which (I think) has a higher anode resistance than at 12V, which is about 20k (I think).
In any case, I was really using the first opamp half to set the grid voltage and increase input impedance but I've now replaced it with a JFET source follower.
The second opamp basically did the same, but also taking the place of the second triode (in the Fender Champ AA764 circuit I'm trying to emulate) to drive the second triode (which takes the place of the 6V6 tetrode). Today I replaced that with a BS250 MOSFET which I hope will let me swing lower than an opamp keeping the second valve's grid lower too.
Here's what's on my breadboard right now:
The tonestack will be standard fender, I just made it BMP when opamps were taking up too much room. The voltage readings are LTspice's and not to be trusted.
I'm familiarising myself with a number of new concepts so comments and suggestions are totally welcome if I seem like I'm doing crazy things. 😱
In any case, I was really using the first opamp half to set the grid voltage and increase input impedance but I've now replaced it with a JFET source follower.
The second opamp basically did the same, but also taking the place of the second triode (in the Fender Champ AA764 circuit I'm trying to emulate) to drive the second triode (which takes the place of the 6V6 tetrode). Today I replaced that with a BS250 MOSFET which I hope will let me swing lower than an opamp keeping the second valve's grid lower too.
Here's what's on my breadboard right now:
An externally hosted image should be here but it was not working when we last tested it.
The tonestack will be standard fender, I just made it BMP when opamps were taking up too much room. The voltage readings are LTspice's and not to be trusted.
I'm familiarising myself with a number of new concepts so comments and suggestions are totally welcome if I seem like I'm doing crazy things. 😱
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