
I'll take my chances as well - I build just for me (some may find it embarrassing how many amps I have

Niki
Moderators: pompeiisneaks, Colossal
Yeah I mean, for one I have a chokeMasco wrote: ↑Sat Dec 05, 2020 2:55 pm Cool. Looks like the schematic and layout are conflicting. Maybe the layout has the B+ node for 1st stage on the wrong side of the 10K.
Not much filtering in this, 6 triodes sharing same point. Is that part of the magic?
I think using 6L6s would be my choice. And decoupling more stages.
Let us know how she turns out!
I built it basically as drawn, without the reverb. I think @chasbenson meant the DC heater circuit. I don't see why the power supply for the B+ nodes would cause hum as drawn.lespaulnmarshall wrote: ↑Fri Dec 18, 2020 4:17 pm Just Curious, has anyone figured out the power supply error in the monarch schematic?
I'd like to try and build one of these over the holidays. I'm thinking maybe the error is the number of stages connected to each node, or maybe something specific to the reverb version?
Thanks, you're probably right. Maybe Chris adds some 100 ohm artificial center tap resistors to gnd before the recifier? Thinking of just going for it, pretty sure I have most of the parts laying around already. Especially since yours is working well! Very neat little build btw!turbofeedus wrote: ↑Fri Dec 18, 2020 6:30 pmI built it basically as drawn, without the reverb. I think @chasbenson meant the DC heater circuit. I don't see why the power supply for the B+ nodes would cause hum as drawn.lespaulnmarshall wrote: ↑Fri Dec 18, 2020 4:17 pm Just Curious, has anyone figured out the power supply error in the monarch schematic?
I'd like to try and build one of these over the holidays. I'm thinking maybe the error is the number of stages connected to each node, or maybe something specific to the reverb version?
This is one of those things best answered with trial and error. I generally separate preamp and power amp grounds, but not more categorization than that. In fairness, it hasn't resulted in any real difference.
Very low is a matter of perspective. There are many components out there that would be totally destroyed if they came into contact with a 20V potential.
No, just a mistake.
Unless you're specifically going for a original look, you never HAVE to ground the case of the pot by soldering to the case directly (I believe that's what you're talking about?). It's already grounded through the chassis where it's attached.BobL wrote: ↑Sun Feb 07, 2021 6:22 am 4. I do want to ground the right lugs on the reverb and volume pots to the pots themselves, correct? I guess it might be better to do the same w/ the middle lug on the tone pot? I'm never entirely sure what I can/can't do in terms of sharing/not sharing ground lugs.
The high voltage for the tube plates is tube rectified with a 5AR4(GZ34). The heaters are being rectified with a full wave bridge rectifier as you correctly identified, but the 4700uF is there purposefully to filter the "dirty" DC voltage coming from the rectifier. Those are solid state silicon diodes, so that would count as solid state rectification, yes. So in other words, the high voltage (B+, HT, etc.) is tube rectified, and the heaters are solid state rectified. You would still refer to this as a tube rectified amp, except with DC heaters (tube rectification of heaters in basically not a thing, not in guitar amps at least).BobL wrote: ↑Sun Feb 07, 2021 6:22 am 5. I have a tube rectifier in this chassis, and so the setup looks much like dorrisant's layout, but I'm wondering about the notation on the schematic, which appears to have a bridge rectifier and... I'm just not familiar with what this would be - solid state rectification of some sort, yeah? I guess you don't need the 4700uF cap if you aren't using the bridge rectifier (I only know what that is from looking it up). Regardless of tube or solid state rectification, it would seem the the PI balance pot is used to adjust bias, is that right? If not... I'm unclear on how I would adjust the bias (and then would that just be adjusting tone?).
Yep - I just found this and thought the Monarch would be a good match for the parts I already had in this chassis, and they sound awfully nice as well.turbofeedus wrote: ↑Mon Feb 15, 2021 1:14 am Welcome, just a quick note that the benson preamp pedal is supposedly based off the chimera, not the monarch. The preamp schematic is out there, the chimera not AFAIK.
And is the higher power here resulting from the differences of cathode bias vs a fixed bias? The amps I have built before are fixed bias, and I've got something like 30mV on pin 8 V5/V6, but obviously wired differently. I've surpassed my understanding here, I guess, and I suspect part of that is wrapping my head around the circuit as a whole vs. looking at small pats of it. The above makes sense, just wasn't sure about the starting point of 'let's call the cathode voltage an even 20V'. This is the 19.66V on the cathode in Aaron's schematic? Apologies, as I know I'm missing something important here, but I can't quite wrap my head around which way is 'up' when it comes to the ground - here is my understanding: the cathode is the negative, so flow would be from the cathode, across the grid, to the plate, so current is flowing up through the cathode resistor into the tube, as I understand it, and the cathode cap is boosting gain - so is the 20V we are protecting things from on the other end of that ground? This is where I get pretty lost...turbofeedus wrote: ↑Mon Feb 15, 2021 1:14 amVery low is a matter of respective. There are many components out there that would be totally destroyed if the came into contact with a 20V potential.
Recall that voltage, resistance, and current are intimately related by Ohm's Law (V=IR). In this case, the power law (P=IV) is also relevant.
Let's call the cathode voltage an even 20V. One side of the cathode resistor is connected to ground, so the entire 20V at the cathode is dropping across the resistor.
The resistance as stated is 250Ω. We can now calculate the current through the resistor.
20=I(250)
I=20/250=0.080A or 80mA (remember cathode current is plate current + screen current, and that 80mA is two tubes!)
We can then use the power law to calculate how much power in wattage the resistor will need to handle.
P=0.080(15)=1.6W
So with 20V at the cathode, and a 250Ω resistor, the resistor will need to dissipate ~1.6W.
We like to overbuild things, so a 5W resistor as shown is common. I suppose that's the quick answer to your question.
As I understand it, I can keep my current heater wiring all tube based without causing actual problems - in this chassis, I just don't think I have the physical space for the bridge rectifier. I'm curious what the advantages might be to DC filaments on the preamp tubes?turbofeedus wrote: ↑Mon Feb 15, 2021 1:14 amThe high voltage for the tube plates is tube rectified with a 5AR4(GZ34). The heaters are being rectified with a full wave bridge rectifier as you correctly identified, but the 4700uF is there purposefully to filter the "dirty" DC voltage coming from the rectifier. Those are solid state silicon diodes, so that would count as solid state rectification, yes. So in other words, the high voltage (B+, HT, etc.) is tube rectified, and the heaters are solid state rectified. You would still refer to this as a tube rectified amp, except with DC heaters (tube rectification of heaters in basically not a thing, not in guitar amps at least).BobL wrote: ↑Sun Feb 07, 2021 6:22 am 5. I have a tube rectifier in this chassis, and so the setup looks much like dorrisant's layout, but I'm wondering about the notation on the schematic, which appears to have a bridge rectifier and... I'm just not familiar with what this would be - solid state rectification of some sort, yeah? I guess you don't need the 4700uF cap if you aren't using the bridge rectifier (I only know what that is from looking it up). Regardless of tube or solid state rectification, it would seem the the PI balance pot is used to adjust bias, is that right? If not... I'm unclear on how I would adjust the bias (and then would that just be adjusting tone?).
Ok, now that I understand that this is cathode biased and have read up some more on that, this makes more sense. It seems like that pot exists to allow you to adjust the amp to the point where it sounds 'right', based on " The divider needs to attenuate the signal by the same proportion that the paraphase amplifies it, thereby giving overall unity gain." That sounds like 'use your ears'...turbofeedus wrote: ↑Mon Feb 15, 2021 1:14 am The PI balance is simply a voltage divider controlling the amount of signal going to the grid of the second 6V6. You can read more about paraphase PIs here: http://www.valvewizard.co.uk/paraphase.html
This is a cathode biased amp, also called automatic bias. Bias is set as a function of the plate voltage, screen voltage, and cathode resistance. In practice, you would change the operating point (or bias point) by altering the cathode resistance (for a given plate and screen voltage). You can read more about that here: http://aikenamps.com/index.php/what-is-biasing
I would also recommend Robrob's tube bias calculator, see if you can calculate how much power the plates in the monarch are dissipating (the answer may surprise you!): https://robrobinette.com/Tube_Bias_Calculator.htm
PRR wrote: Plotting loadlines is only for the truly desperate, or terminally bored.
Not exactly. Cathode bias amps are usually set up to idle at higher dissipation than fixed bias, but this is only a correlation. The 30mV you've been reading at pin 8 on your previous projects is likely a special cathode resistor used for sensing the current through the tube. This resistor is NOT for establishing bias. Generally a fixed bias amp is thought of as having the cathode directly connected to ground, but often we will install a small 1Ω resistor from cathode to ground to "sense" the current flowing in the tube. Note then that since the resistance is 1, for Ohm's Law (V=IR), you can directly say V=I. So if you measure 30mV across the current sense resistor, that's equal to 30mA current in the tube.
Correct, when referring to cathode voltage, that is specifically the voltage difference between the cathode pin of the tube and ground.
Technically that is true, electrons are literally flowing from the cathode to the plate, through the control grid. But be careful; what you're describing is called "electron flow", and is generally backwards from how we think of the directionality of electricity. We usually work in "conventional flow" which is that electricity flows from positive to negative. In a more physical sense, with think of the "holes" left by the movement of electrons, rather than the electrons themselves. I would also caution you to thinking of things as negative and positive in an absolute sense. Negative and positive only exist in relation to something else. This is why you'll often see the phrase "with respect to", meaning when compared to, or in relation to. Nothing is negative or positive by itself. So yes, the cathode is negative with respect to the plate, but concurrently positive with respect to the control grid (generally). Why that is necessary for the tube to not self-destruct is a good question for which to seek an answerBobL wrote: ↑Mon Feb 15, 2021 6:15 pm here is my understanding: the cathode is the negative, so flow would be from the cathode, across the grid, to the plate, so current is flowing up through the cathode resistor into the tube, as I understand it, and the cathode cap is boosting gain - so is the 20V we are protecting things from on the other end of that ground?
Don't think of heaters as being "tube based" or "solid state based", the real distinction here is whether the heaters are being rectified into direct current(DC) or staying alternating current(AC), as is would be straight from the secondary of the power transformer. Theoretically rectifying the heaters to DC can reduce noise and heater hum in the amp. This can be very useful for high gain amps (Soldano is a famous user of DC heaters) where that hum induced on an early preamp stage would be amplified several times over cascading gain stages. But, AC heaters have been used for tube preamps for nearly a century without much fuss. For an amp like the monarch, with only a couple preamp stages, DC heaters seems like overkill.BobL wrote: ↑Mon Feb 15, 2021 6:15 pm As I understand it, I can keep my current heater wiring all tube based without causing actual problems - in this chassis, I just don't think I have the physical space for the bridge rectifier. I'm curious what the advantages might be to DC filaments on the preamp tubes?
You could use an oscilloscope, maybe even a DMM. For my build, I just set the pot at noon, and haven't seen much reason to move it.
Correct, 12.4W dissipation for a 14W tube. The interesting bit here is that setting the bias for 100% dissipation is the boutique standard for cathode bias, and in fact even higher in some cases. People really like to run 6V6 hot.