Benson Monarch

[dorrisant]

Mike,
Very nice! So, do you like the way the circuit works with the p-to-p better?
That is a better use of the BJr... I love the form factor, hate the circuit. A speaker change to just about anything helps. Also, do change your reverb tank. The original BJr tank is for a solid state circuit. Matching up a correct tank will make for much better impedance matching and therefore much better reverb.

Read here for the breakdown on selecting your new tank:

https://www.cedist.com/tech-articles/sp ... d-compared

Todd,
Feed that switched bypass cap with a 100k resistor from the cathode pin. That will allow the cap to voltage-load somewhat while it is disengaged. Then when you engage the switch your not suddenly switching to a different DC level... thus no popping.

[T Wilcox]

Todd,
Feed that switched bypass cap with a 100k resistor from the cathode pin. That will allow the cap to voltage-load somewhat while it is disengaged. Then when you engage the switch your not suddenly switching to a different DC level... thus no popping.

Thanks I will give that a shot! It's really not a problem since I wouldn't be switching it on the fly but if that works out I might add a relay and make it a foot switchable boost.

[alexrussomusic]

Hey all - got one of these recently and it seems to have a little more hum than I'd like, so I cracked open to look for obvious suspects...and there were some surprising circuit differences. v1b had a 1k cathode instead of 2k2, the split plate was a 5k8 instead of 10k w/ a .1uf feeding the PI. Also completely different power supply string with an 88k supplying the preamp node??? its 32 uf->1k-> 32 w/a 47uf (? its hot glued value side down) --> 88k --> 47uf and that supplies both the preamp & phase inverter.

I'm also very interested in seeing the power supply . This would make sense if...

30uf -> 1K -> 20uf (47 + 32 in series) -> 8.8k -> 47uf

For a head (no reverb) you would get similar voltages and filtering to the 20,47,20,20 power supply we are running on the reverb builds. That should supply enough filtering to run the PI and pre-amp tube together. A few people stumbled onto a very similar formula when trying to sort things out initially. That also looks very similar to what Aaron initially traced out and Chris Benson commented on... If you recall, we were thinking 30,30,30 with 1k and 10k dropping resistors.

There's no way it's an 88k resistor in the power supply. That would drop way too much voltage to be viable.

Mike

Hey Mike - I know its totally nuts but that's what it looks like re: 88k. I agree that an 8k8 would make way more sense. Screen capacitors appear to be in parallel. I assume it's a 20 uf & 32 uf tied together for ~50 uf. Hard to tell without unsoldering. Cap can is a 32 / 32. Part of me wonders if the reason for the upped capacitance is to deal with the larger caps (1ufs in the PI & a 1uf feeding the PI). I'm attaching pictures.

FWIW this is a Non reverb head. But the chassis is drilled in such a way that a reverb circuit could be added.

IMG_4043 2.JPG

[T Wilcox]

Thanks for the pic Alex, curious to know if that black ground wire coming off the can cap runs all the way back to the input ground or bus bar at the pots
Im doing a similar build next with a little off the reservation but ordered a 40-20-20-20 can cap but may regret it since the power section and preamp section grounds cant be separated

Todd

[pullshocks]

Hey Mike - I know its totally nuts but that's what it looks like re: 88k. I agree that an 8k8 would make way more sense. Screen capacitors appear to be in parallel. I assume it's a 20 uf & 32 uf tied together for ~50 uf. Hard to tell without unsoldering. Cap can is a 32 / 32. Part of me wonders if the reason for the upped capacitance is to deal with the larger caps (1ufs in the PI & a 1uf feeding the PI). I'm attaching pictures.

FWIW this is a Non reverb head. But the chassis is drilled in such a way that a reverb circuit could be added. IMG_4043 2.JPG

That does look like an orange stripe. WTF?

[brewdude]

Any voltage readings around the “88k” resistor? Current calculated across this resistor may indicate the value of resistance.

[mikeywoll]

Hey Mike - I know its totally nuts but that's what it looks like re: 88k. I agree that an 8k8 would make way more sense. Screen capacitors appear to be in parallel. I assume it's a 20 uf & 32 uf tied together for ~50 uf. Hard to tell without unsoldering. Cap can is a 32 / 32. Part of me wonders if the reason for the upped capacitance is to deal with the larger caps (1ufs in the PI & a 1uf feeding the PI). I'm attaching pictures.

FWIW this is a Non reverb head. But the chassis is drilled in such a way that a reverb circuit could be added. IMG_4043 2.JPG

That's very helpful, thanks Alex.

I see a 1K and 11K resistor there, am I missing something? Maybe this is like that blue/gold dress that went viral?

I see Brown,Black,Red and Brown,Brown,Orange for the 2 resistors.

Chris Benson said he "pins the screens where he likes them" using a 47uf cap, so it all makes sense.

32uf -> 1k -> 50uf -> 11k -> 32uf for the non-reverb circuit fits pretty well with what he told us for the reverb circuit, 22uf -> 1k -> 47uf -> 4.7k -> 22uf -> 4.7k -> 22uf. Very similar voltage drop for both circuits with the added capacitance/filtering with fewer nodes in the non-reverb circuit.

I bet that the other changes make up for the small gain difference between the reverb and non-reverb circuits. In older builds Benson said all coupling caps were 0.1uf vs using 0.022 in the PI of newer builds. Guessing you have an older one, or he ran out of 0.022uf caps.

Mike

[mikeywoll]

Mike,
Very nice! So, do you like the way the circuit works with the p-to-p better?
That is a better use of the BJr... I love the form factor, hate the circuit. A speaker change to just about anything helps. Also, do change your reverb tank. The original BJr tank is for a solid state circuit. Matching up a correct tank will make for much better impedance matching and therefore much better reverb.

Read here for the breakdown on selecting your new tank:

https://www.cedist.com/tech-articles/sp ... d-compared

Todd,
Feed that switched bypass cap with a 100k resistor from the cathode pin. That will allow the cap to voltage-load somewhat while it is disengaged. Then when you engage the switch your not suddenly switching to a different DC level... thus no popping.

I was really surprised how bad the Blues Jr circuit is. I could never dial in a satisfying tone on the BJr. It was always either ear piercing or the bottom end fell apart. It's the same cab, speaker, reverb tank, and transformers, but there is a world of difference in the sound between the original amp and the monarch circuit. The boxy sound BJr is known for is probably 95% circuit. This thing is not at all boxy. The tweed BJr comes with a Jensen reissue that sounds great at low to moderate gain with this amp (especially in the "American" setting). The speaker is a little too wide range for great overdrive, but still sounds pretty good turned all the way up. I ordered a Benson branded speaker, just have to find time to drive across town and pick it up .

The reverb tank is an 8eb2c1b, which should be perfect for this reverb circuit. It's a moderate decay Ruby branded tank, which is just not high quality. The tank was flat out terrible in the BJr. It sounds great at low to moderate (aka values I'll actually use) settings in this circuit. Turned up beyond 3/4, it's noisy and the reverb sounds like silverware falling down stairs . I may swap it in the future, but it's at least as good as my mxr pedal as is.

I was mostly motivated to switch to PTP because I was worried that my volume knob issue was evidence of some parasitic coupling, and worried that the subtle artifact I was hearing with the amp cranked was another symptom. After playing through a few cab/speaker combos and cranking the amp, I can say for certain that the PTP rebuild sounds better cranked up. That artifact is gone and the overdrive sounds excellent. The amp even seems a little more punchy than before when turned up. It really sounds exactly like the demos online. At lower volumes/gain both builds were pretty similar. They were both equally quiet. Maybe the PTP is a little more "high fi" sounding. I settled on setting the trim-pot by playing an A note on loop through the amp, cranked, into a dummy load, and balancing the ac voltages on the power tube grids. In both layouts, peak voltage was stable at around 43 volts. I did tons of soldering and resoldering on the eyelet build, so it's possible that the small differences I noticed were caused by some minor problem I missed. But overall, I'm a believer in the PTP now. I started out skeptical.

Mike

[mikeywoll]

Todd,
Feed that switched bypass cap with a 100k resistor from the cathode pin. That will allow the cap to voltage-load somewhat while it is disengaged. Then when you engage the switch your not suddenly switching to a different DC level... thus no popping.

Thanks I will give that a shot! It's really not a problem since I wouldn't be switching it on the fly but if that works out I might add a relay and make it a foot switchable boost.

Foot switchable boost sounds like a great idea.

Is it really this simple?

Screenshot_20210504-000000.png

[dorrisant]

Foot switchable boost sounds like a great idea.

Is it really this simple?

Screenshot_20210504-000000.png

Yes, it is. Try different values of course.

[roberto]

It is that simple, but I'd prefer to add a very small complication like a simple fet to locally switch the bypass, and avoid to have that shielded cable onstage.
Just my preference.

[pullshocks]

Hey Mike - I know its totally nuts but that's what it looks like re: 88k. I agree that an 8k8 would make way more sense. Screen capacitors appear to be in parallel. I assume it's a 20 uf & 32 uf tied together for ~50 uf. Hard to tell without unsoldering. Cap can is a 32 / 32. Part of me wonders if the reason for the upped capacitance is to deal with the larger caps (1ufs in the PI & a 1uf feeding the PI). I'm attaching pictures.

FWIW this is a Non reverb head. But the chassis is drilled in such a way that a reverb circuit could be added. IMG_4043 2.JPG

That does look like an orange stripe. WTF?

OK, on my tablet it looked like grey grey orange. On my phone and desk top it looks like brown brown orange.

[T Wilcox]

Todd,
Feed that switched bypass cap with a 100k resistor from the cathode pin. That will allow the cap to voltage-load somewhat while it is disengaged. Then when you engage the switch your not suddenly switching to a different DC level... thus no popping.

Thanks I will give that a shot! It's really not a problem since I wouldn't be switching it on the fly but if that works out I might add a relay and make it a foot switchable boost.

Foot switchable boost sounds like a great idea.

Is it really this simple?

Screenshot_20210504-000000.png

Nice and easy, might throw this together tonight

[brewdude]

I’ve lost track...
Is there an updated schematic for these current successful builds?

[T Wilcox]

I’ve lost track...
Is there an updated schematic for these current successful builds?

I created this in Kicad and I think its accurate but maybe someone else can put a second set of eyes on it and verify?

There are only 3 significant changes from the original schematic
1. the power supply nodes
2 the US/UK switch wiring
3 the plate resistor on reverb return triode to 470K

Edit: updated reverb return plate R to 470K