Benson Monarch

[Synchu]

Cheers from the other side of the pond - great hints, great Chas - kudos
I'll take my chances as well - I build just for me (some may find it embarrassing how many amps I have ). Will do the head format with reverb.

Niki

[turbofeedus]

Chiming in with my build, started with this schematic.
Donor amp is a Webster 81-15.
Subbing 6SL7 in the preamp, just waiting for tubes in the mail.

build1-1.JPG

[dorrisant]

Looking good!

[Masco]

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!

[turbofeedus]

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!

Yeah I mean, for one I have a choke I might be enough changes away from not really being a Monarch anymore.
I also skipped the reverb, so only 4 triodes on the last node.
Hopefully the tubes are coming today.

EDIT: Done! Mean little thing, I like it a lot.

build_final-2.JPG

build_final-3.JPG

[turbofeedus]

Am I reading the schematic correctly, that tone switch is just bringing a 250pF cap in parallel with the 470K resistor?

[pullshocks]

If I understand correctly that creates the "treble peaker" as found in many Marshal and similar amps.

I'm still puzzled by the value of the tone cap being the same as the coupling cap off V1A

[lespaulnmarshall]

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?

[turbofeedus]

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?

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]

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?

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.

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!

[BobL]

Hi all - first time poster.

Backstory: I have built five or six amps, enjoy it a lot, and don't entirely (or vaguely sometimes) know what I'm doing when it comes to 'why'... but I've had good luck building a few things that I use a lot (my last build, a single channel, no reverb Super is what I use on this recording: https://www.youtube.com/watch?v=oX20cNO-uG4). I like blackface sort of tones and tweed aesthetics, so that's what my last few builds have been - the Super and then a PR in a Blues Jr I gutted, and then one I just finished which is a 6G3.

Long story short, I just very much don't like the circuit - that whole disconnect between hearing something and thinking it is cool and then playing it and realizing it ain't for you. So I've been looking at something I could adapt to what I've already built and get into some sounds I'd find more useful - I have a Benson Preamp pedal that I run always on, and it sounds great, so... that lead me here with hopes maybe I can salvage the spendy bits of this project.

What I'd like to do is yank the board I've got in there and replace it with this Monarch circuit. To that end, I've drawn up a layout using Aaron's schematic and dorrisant's layout as references, and I was hoping that I could get some feedback on what I've done and where the inevitable issues are. I've tried to keep things aligned with the sockets/front panel holes already in this chassis.

I have some more specific questions regarding the transformers I have, but thought I'd start by seeing if folks think this layout checks out?

I have a couple specific questions:
1. Is grounding the PI Balance pot on the same ground as the filter caps a bad idea in terms of noise? I had also thought about using the same ground as the preamp, but I thought that also was maybe asking for trouble.
2. I'm unclear why dorrisant's layout shows the 250 ohm resistor on the cathode bypass cap as being so large? Voltage there would be very low, right?
3. I show a 470K resistor in series with the .022uf coming off of the plate on V1 in the schematic, but didn't see this in dorrisant's layout - was this a purposeful omission?
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.
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?).

I guess I'll see if folks think what I have here is worth pursuing before I get into it any further...

Thanks a bunch in advance! The thread has already been super interesting and informative!

EDIT 4/16/2021 - Incorporated changes from discussion into layout.

EDIT 4/21/2021: Final wiring based on testing and hum elimination.

[turbofeedus]

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.

1. Is grounding the PI Balance pot on the same ground as the filter caps a bad idea in terms of noise? I had also thought about using the same ground as the preamp, but I thought that also was maybe asking for trouble.

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.

2. I'm unclear why dorrisant's layout shows the 250 ohm resistor on the cathode bypass cap as being so large? Voltage there would be very low, right?

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.
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.

3. I show a 470K resistor in series with the .022uf coming off of the plate on V1 in the schematic, but didn't see this in dorrisant's layout - was this a purposeful omission?

No, just a mistake.

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.

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.
The way the layout shows is fine.

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?).

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).

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

[BobL]

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.

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.

I really appreciate the the informative responses here.

2. I'm unclear why dorrisant's layout shows the 250 ohm resistor on the cathode bypass cap as being so large? Voltage there would be very low, right?

Very 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.

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...

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?).

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).

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?

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

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'...

EDIT: Looks like the tubes would be dissipating 12.4W? If I used 6V6GTs and had higher B+ voltage, everything goes up, I maybe have more gain, but theoretically things would work, yeah?

Again, thank you very much... I feel like the tadpole on the left side of the evolution chart most of the time.

[thetragichero]

keep in mind that in a fixed bias amp, the power tube cathodes are connected to ground. many of us install 1r metal film resistors purely to simplify measuring the bias current (1ohm makes the calculation of cathode current simple: a measurement of mV dropped across a 1ohm resistor allows us to "read" (infer) mA using the voltage setting on a meter due to ohm's law V = IR) power dissipated is minimal... P = V² / R. using your numbers P = (.03)² / 1 = (.00009) / 1 = .00009 W. many use 1/2w or 1w resistors solely for their size. also take a look at a fixed bias vs cathode bias circuit: in fixed bias the two grid resistors are connected to the negative bias voltage while in cathode bias the grid resistors are connected to ground. apples to oranges

[turbofeedus]

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.

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.

When I was learning about this, I had to go all the way back to understanding how tubes actually, physically work for this to make sense. That may benefit you as well. Uncle Doug's videos are very good: https://www.youtube.com/watch?v=x5SSKX74DKg

This is the 19.66V on the cathode in Aaron's schematic?

Correct, when referring to cathode voltage, that is specifically the voltage difference between the cathode pin of the tube and ground.
Aaron measured 19.66V between pin 8 and 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?

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 answer

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?

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.

That sounds like 'use your ears'...

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.

EDIT: Looks like the tubes would be dissipating 12.4W? If I used 6V6GTs and had higher B+ voltage, everything goes up, I maybe have more gain, but theoretically things would work, yeah?

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.
I'd recommend having a look at other 6V6 designs and seeing what they're doing for screen and cathode resistors. There are ways to calculate (or at least ballpark) those things before the fact, but I think that's a journey for another project.