Benson Monarch

[pullshocks]

Looks like the amp in the picture is being wired with AC heaters. Heater wiring is twisted all to all tubes. The heater CT appears taped off. Heaters appear referenced to ground via 2 100 ohm resistors on a short terminal strip mid chassis.

[mikeywoll]

Looks like the amp in the picture is being wired with AC heaters. Heater wiring is twisted all to all tubes. The heater CT appears taped off. Heaters appear referenced to ground via 2 100 ohm resistors on a short terminal strip mid chassis.

I've noticed on recent ads for the Monarch Reverb, and the Benson website, the D/C filaments are not mentioned any longer. Curious.

[dorrisant]

I didn't use DC filaments and get zero hum. I have no desire to change it. I can see that change happening.

[pullshocks]

Maybe I'm overthinking this, but I wonder why they chose that spot on the chassis to do the ground reference, as opposed to the pilot light or other spot in the power supply area. Simplest explanation would be there is plenty of room there in mid chassis, and leaves room to add the DC heater components later if they want to. But is it also better from a grounding standpoint?

[T Wilcox]

I built mine with DC heaters and so far I love everything about this amp but keep in mind the pics and schem Aaron posted on page 2 were done because he had the amp in his shop to repair an issue with the heater circuit. He did not specify what the issue was unfortunately.

[mikeywoll]

I think the take-home for me is that there have been some changes and variation in the circuit over the years. The recent photo also has 0.02uf caps in the PI (as Chris Benson mentioned), for example.

The reverb driver seems to really appreciate the added current from the additional PS node. In my case, with an under-spec power transformer, having that last node take off in parallel to the pre-amp woke the amp up a bit. With a better power transformer maybe setting that up in series after the pre-amp node (like in the photo I think) would work better. Its also probably not necessary to add the extra node if the 4 node PS is working well in a particular build.

I would wager that Benson tweaks each amp at build time to get everything sounding just right.

Mike

[Synchu]

Slightly less involved demo of my build. Just a clean guitar and a small lead in the second part. Reverb from the amp.
Haar telecaster only. Low output pickups.
So versatile and great sounding amp.
I’m enjoying it a lot. Waiting for the Celestion G12H30 cab and will post on it as well.


Niki

https://soundcloud.app.goo.gl/k7rPZMP4HTgQeAnG8

[dorrisant]

Very nice demo, as expected!

[CraigGa]

I wasn't able to find any off the shelf transformers in the UK so I've asked for a quote to get some made.

I've specced 150mA of HT current, I just wanted confirmation that's plenty before I go any further.
I've seen people in this thread say 120mA so there should be a bit spare.

Craig

[Synchu]

150mA is more than enough.
I’d also look to spec the 5V for 3A, so it can handle most popular rectifier tubes. Gives room for experimenting.

Niki

[CraigGa]

Thanks Niki,
I looked up a chart that said some rectifiers use 3A so I went with 4A for both heater windings.

They should get back to me with a price today.

Craig

[Unipotential]

Just curious, has anyone tried Merlin's floating paraphase PI with this instead of the adjustable 10k pot? Shared Rk, second triode fed from voltage divider of 910k and 1M...

[mikeywoll]

After a bit more experimentation, forum searches, and lots of zooming into the pictures in this thread, I think I've figured out the circuit based on this picture:

benson-wiring.jpeg

I believe there is a 2K2 dropping resistor between the PI and Pre-amp filter cap (pink blur across the leftmost can cap leads, looks like a carbon film resistor). That feeds another 2K2 dropping resistor to the reverb filter cap, which is located at the reverb tube. In addition, the split plate load resistor in the photo is 16K (brown, blue, orange) actually probably 5k6 see below . Built to this schematic the amp is loaded with those amazing harmonics, is crazy touch sensitive, has plenty of gain on tap (my strat overdrives at around 1 O'Clock), has the right pick attack/compression, is essentially noiseless with A/C filaments, and the reverb is lush. I would imagine it would only sound better with better iron than my repurposed Blues Jr. transformers.

I've attached a quick demo (and the component tracks). I recorded with a bit of amp reverb (9 O'Clock-ish), no pedals. I added a touch of reverb and delay in post. I didn't mess with the EQ on the recording. The rhythm tracks are a les paul, neck pickup, American mode, tone rolled off a bit on the amp. The lead part is the bridge pickup, British mode, tone up around 2 O'clock-ish.

Schematic is attached and the zip file contains the Front Panel Designer file I created for the control panel. This panel will work with the existing holes in a Blues Jr. chassis. Just need to drill out the chassis a bit to fit full sized pots (as well as drill out and mount chassis mounted tube sockets, terminal strips, etc...). There are a couple layouts in this thread that will work for this schematic, with minor changes.

Enjoy!

Mike

Edit: Updated schematic to fix cap value on tone switch, moved pre-amp voltages to tube socket
Edit: Updated schematic R8 value; VVR resistor to ground increased to 220K; Improves VVR performance; Corrected mosfet pins
Edit: Updated schematic R16 value; Split plate load resistor decreased to 10K; Improves gain/feel. Amp photos likely 10K on original chassis photo, 5.6K on updated photo; Would consider lower values 5.6K-10K for humbuckers, 10K-15K for single coils
Edit: Corrected artificial center tap for filaments
Edit: Updated to match recent circuit photos - 11/2021

Mellow LP Jam - 2.mp3

Mellow LP Jam - 2 R2.mp3

Mellow LP Jam - 2 R1.mp3

Mellow LP Jam - 2 L.mp3

[CraigGa]

Thanks Mikey that sounds great,
Previously we'd determined that the preamp and reverb were fed in parallel via 4K7s from the PI node so yours is a subtle change to that.

What difference does the split load resistor make? would a 10k/15k switch be useful?

Thanks
Craig

[CraigGa]

I just noticed that you have 0.022uF across the tone switch when all the previous drawings have 250pF, that's a big difference.

Craig