Benson Monarch

[dorrisant]

Between the tube cathodes and the cathode resistor and cap.
Merlin Blencow has on this page (scroll down to Heater Elevation). http://www.valvewizard.co.uk/heater.html Robrob also shows it on his 5e3 pages.

Doing this w/o any other changes did not solve hum, FYI.

How did you configure your B+ dropping/filter string? Maybe post a pic.

[BobL]

Between the tube cathodes and the cathode resistor and cap.
Merlin Blencow has on this page (scroll down to Heater Elevation). http://www.valvewizard.co.uk/heater.html Robrob also shows it on his 5e3 pages.

Doing this w/o any other changes did not solve hum, FYI.

How did you configure your B+ dropping/filter string? Maybe post a pic.

It's as per original schematic and layout, not updated.

[pullshocks]

Pullshocks as built 4 14 21 revision 1.jpg

Bob L--without changing the power supply, elevating the heaters did not have any benefit for me either. By doing both revised power supply and elevated heaters, hum was mitigated. I also think you need to check the phase inverter connections as described above.



Here is a schematic crudely edited to show how I wired it. The voltages shown came from the original schematic. My voltages are appx 15 v lower.

EDIT--schematic updated to show 4.7K resistor between node C and D

[BobL]

Bob L--without changing the power supply, elevating the heaters did not have any benefit for me either. By doing both revised power supply and elevated heaters, hum was mitigated. I also think you need to check the phase inverter connections as described above.



Here is a schematic crudely edited to show how I wired it. The voltages shown came from the original schematic. My voltages are appx 15 v lower.

This is helpful. What value resistor did you put between nodes C and D?

[pullshocks]

Sorry, I thought I had it on there, will update the drawing. 4.7k, as indicated by Chris Benson.

[mikeywoll]

I think R22, the plate resistor for the reverb recovery stage, is 470k not 100k. Otherwise, there will be way more than 74 volts on that plate, since the "D" node is delivering 266 volts. It's hard to see in the picture we have, but it looks yellow, purple, yellow to me. Made a huge difference to the reverb quality on my build. Also, landed me right around 74 volts on the plate. The rest of my values are pretty darn close to the schematic.

Mike

[CraigGa]

I'm taking early retirement soon, I can see one of these in my future

[pullshocks]

I think R22, the plate resistor for the reverb recovery stage, is 470k not 100k. Otherwise, there will be way more than 74 volts on that plate, since the "D" node is delivering 266 volts. It's hard to see in the picture we have, but it looks yellow, purple, yellow to me. Made a huge difference to the reverb quality on my build. Also, landed me right around 74 volts on the plate. The rest of my values are pretty darn close to the schematic.

Mike

Good eye and good thought process. I swapped in the 470k and it brought my plate voltage reading down to 78, pretty close to the value indicated on the schematic. Interesting that it reads high, because I am about 10 volts low everywhere else. (No surprise, my PT is 265-0-265).

I did not get much of an improvement in reverb quality, probably because I am using the wrong tank.

[mikeywoll]

Also noticed that the tone switch is incorrect on the posted layouts and schematic. The volume pot should join the tone switch at the middle lug, via the capacitor not at the 470k resistor. Makes a huge difference in effectiveness of the switch and the tone of the amplifier.

Mike

Screenshot_20210415-183914.png

[roberto]

Mike, looking at the internal photos of the amp it is correct like in the schematic, and your correction is wrong.
The cap bypasses the 470k of the voltage divider, lowering the impedance at high frequencies and so increasing their gain.

[mikeywoll]

Mike, looking at the internal photos of the amp it is correct like in the schematic, and your correction is wrong.
The cap bypasses the 470k of the voltage divider, lowering the impedance at high frequencies and so increasing their gain.

It took me a while to work through it, but the schematic is definitely wrong. What Mr. Benson built here is much more complex than a simple bypass cap.

Watch some reviews of the actual amp in action... the reviewers all gush about how the switch effects the highs, mids, and lows together, creating 2 different amp voicings.

It switches in/out the 470k resistor, creating 2 different RC filters using the 2 resistors, the .022uf cap, and the 250pf cap. Basically, it's switching between 2 different fixed tone stacks. Very clever.

Screenshot_20210416-030101.png

It's hard to trace in the image, but you can see the wire poking backwards through the switch lug to join the 250pf cap (red arrow). This is the lead heading to the volume pot. I drew a purple dot to point out the lack of connection where the original schematic shows one. It's difficult to see clearly here, but I don't see any connections, other than the resistor, to that lug. I can't find any wires hiding under the switch, nothing.

The proof is in the final effect though. It was not a subtle change. I had the amp wired up according to the original schematic. It worked as a simple treble peak and was subtle. After switching that wire, the amp now really switches between a scooped blackface sound and a chiming british sound with pushed mids and tight bass. It's very obvious.

Mike

[roberto]

You are right, it's like in attachment:

Benson-Monarch.jpg

I did the same thing years ago by mistake (I forgot to add the resistor in || to the bright cap in a SLO-based amp), then I've used it in some amps and then forgot.

[pullshocks]

I find the switch very subtle. Look forward to trying this.
Thanks for posting.

[BobL]

I find the switch very subtle. Look forward to trying this.
Thanks for posting.

Yep, I've found the switch to be *incredibly* subtle.

I finally grabbed a few minutes this morning to update the layout I made with all of the changes from this thread...

Attached are the new layout and a version where I marked the areas that changed.

Will update the original location of the layout as well - let me know if I'm missing something still...

I plan to try and implement these changes and see where I end up.

[mikeywoll]

I finally grabbed a few minutes this morning to update the layout I made with all of the changes from this thread...

Attached are the new layout and a version where I marked the areas that changed.

Will update the original location of the layout as well - let me know if I'm missing something still...

I plan to try and implement these changes and see where I end up.

Hi BobL,

Thanks for updating the layout. It takes a lot of work. I think we got this one pretty much dialed in.

I noticed a couple things that are off on the updated schematic...

1) The phase inverter is still messed up on this one. It's a confusing design. Start with your original layout, then swap the wires connected to pin 2 and pin 7 of V2. That will work.

2) The 100K resistor at the right most "D" node (the plate resistor for V3) connected to the 500pf capacitor should be 470K.

3) This one is a little more of a style thing, and I apologize if its one you know.... In general, grid resistors should go directly on the tube pins, with as short a lead between the resistor and pin, as possible. They act to suppress RF in that configuration. It also saves space on your board. Often times, "Buzz" is just stray RF that gets amplified, and relocating resistors to the grid pins can make a huge difference in quieting an amp. I live near a radio tower. If I put my grid resistors anywhere other than on the tube, I end up with a great AM/FM radio. I can tune channels with the tone and volume knobs!

I linked a great resource here: https://www.aikenamps.com/index.php/gri ... -they-used

Thanks again, this is awesome!

Mike