Benson Monarch

[mikeywoll]

Moving the ground for the balance pot waaaay down to the end w/ the preamp ground does seem to have gotten rid of the bad hum... but also introduced a far more irritating oscillating click and static...

I have the power amp grounded to the same lug as the CT for the PT primary - is this a bad idea?

Using a chopstick to move the input wire around has no impact on this noise - changing the balance pot value impacts overall noise, but doesn't remove it.

Here is the noise: http://www.boblefevremusic.com/temp/mon ... lation.m4a

Hey Bob, I just watched "Raven Lee" on your web site. Great stuff!!!

That new noise sounds awful. I've pretty much lost track of your layout and ground system, maybe post a current gut shot? Did you bring the PI balance pot ground to the negative terminal of the Node C filter cap? If not try that.

Oh, hey, thanks! We got a whole record of that stuff out there. Bass player in that video died about a month ago, so it's been rough going lately.

I moved the 68K resistor on the input to the tube lug... seems to have changed some things but not improved things...

The balance pot ground I took all the way down to the preamp lug, but I can try it on node C as well.

What is very interesting is that the buzz (sounds like 120 to me now) is *very* dependent on if the amp is upright or not. I leaned it forward to chopstick some stuff and it reduces greatly (and yeah, I followed with my ear to make sure it wasn't just that I couldn't hear it as well ).

It also seems to be very influenced by the reverb circuit - if the tank is plugged in, the oscillation is mostly gone, but the buzz is *bad* now.

If I move the wire going from the reverb pot to the volume pot, it significantly impacts the buzz. If I turn up the reverb knob, it significantly impacts the buzz. If I remove the reverb tube, the oscillation starts to come back, especially with the amp tilted at about 30 degrees...

I had a similar issue in the first amp I built, a JTM45, and it turned out to be a bad capacitor that only was bad when the amp was upright (it was a pain to chase down!)

I had a similar oscillation pop up a while back when I was trying different caps in the reverb circuit, so now I'm thinking there is a very strong chance that the issue is with the .1uF cap I put in there. I'm going to replace that cap and see what happens.

The reverb signal grounding can be really fussy and easily creates ground loops depending on the grounding scheme of the tank. Ya know, If the cap isn't the issue.

Mike

[dorrisant]

I don't follow... If his transformer reflects 6k6 when connected with an 8ohm speaker on the 8 ohm tap, then all is good. Putting an 8 ohm speaker on a 16 ohm jack will reflect 3k3 ohms to the tubes, right? That doesn't sound like the correct option?

Mike

You are correct. I got to thinking about this and came to the same conclusion. Came back to edit. Man this thread is flying by...

[T Wilcox]

I don't follow... If his transformer reflects 6k6 when connected with an 8ohm speaker on the 8 ohm tap, then all is good. Putting an 8 ohm speaker on a 16 ohm jack will reflect 3k3 ohms to the tubes, right? That doesn't sound like the correct option?

Mike

You are correct. I got to thinking about this and came to the same conclusion. Came back to edit. Man this thread is flying by...

Either way I never want to pass up on chances to learn something new and this did get me thinking and could very well come in handy in the future if I come across an OT without specs. I have also always thought it was "somewhat okay" to to halve or double the impedance although I do not do this in practice. I have read that it is okay (nothings gonna smoke though not necessarily wise) to say..use an 8 ohm cab with the ohm selector on either 4 or 16 but to never use a 4 ohm cab with it set to 16 or vice versa!? I know I have done this by accident with no major problems, at least that I know of. maybe I've just been lucky

I still would like to play this amp some more and do more testing but as of last night since correcting the miswired primaries (a result of being colorblind ) I hooked it to the scope and adjusted the Pi pot to get both sine waves perfectly balanced and played it at full volume (neighbors probably loved this ) and I have no splatty overdrive or discernible noise other than the hiss you would get with the volume past noon on any amp. I will probably try to slightly imbalance the Pi a little to see if there is any tonal effect that my ears approve of but if not I am pretty happy with the amp as it is at this point

Todd

[BobL]

The reverb signal grounding can be really fussy and easily creates ground loops depending on the grounding scheme of the tank. Ya know, If the cap isn't the issue.

Mike

This may be where I need to look next.

Replaced that cap w/ a .047uF cap, and things are much improved, if not fixed. The hum/buzz I'm getting now is a bit quieter, and it is not affected by the physical attitude of the amp, so I think I did have a bad cap.

Volume and reverb pot settings have a big impact - turning up the reverb pot all the way really buzzes, which is what is making me think maybe the grounds there are what to look at next.

Attached is the layout that reflects what is wired in the amp. I guess the thing to try would be to run the preamp ground down to the lug, and then just ground both reverb grounds directly to the lug instead of having them share w/the preamp prior to the lug?

Might also try shortening that ground for the PI pot as suggested and run it to the ground side of node C.

Benson Layout- corrected.png

[pullshocks]

I had the house to myself so was able to crank it up to 5. This time I played a guitar with humbuckers, and yes it has some drive.
Take my comments with a grain of salt as I have a lot of high frequency hearing loss. So I don't hear the full harmonic content.

Having said that, I keep coming back to a PI balance setting of 4k ohms, but it sounds good at settings anywhere from 0 to 8 k ohms. At 0 ohms, I believe it is effectively a single ended amp, and I'm actually thinking of building a single ended version. 15 watts is really overkill for my needs

I've got to say, this thing has a LOT of bass. It is really awesome how solid it is, not blaaaaty. But with humbuckers it's almost too much. Chris Benson mentioned changing the power amp coupling caps from .1 to .02 uF. I split the difference a .047. I'm going to try the .022s and also try reducing the coupling cap into the PI. And maybe make the V1 cathode caps switchable.

[mikeywoll]

The reverb signal grounding can be really fussy and easily creates ground loops depending on the grounding scheme of the tank. Ya know, If the cap isn't the issue.

Mike

This may be where I need to look next.

Replaced that cap w/ a .047uF cap, and things are much improved, if not fixed. The hum/buzz I'm getting now is a bit quieter, and it is not affected by the physical attitude of the amp, so I think I did have a bad cap.

Volume and reverb pot settings have a big impact - turning up the reverb pot all the way really buzzes, which is what is making me think maybe the grounds there are what to look at next.

Attached is the layout that reflects what is wired in the amp. I guess the thing to try would be to run the preamp ground down to the lug, and then just ground both reverb grounds directly to the lug instead of having them share w/the preamp prior to the lug?

Might also try shortening that ground for the PI pot as suggested and run it to the ground side of node C.

Benson Layout- corrected.png

Most advice I've read suggests grounding the reverb leads directly to the chassis away from everything else. This is how it's done on the real deal. I found mine quiet enough when grounded directly to my pre-amp chassis ground lug.

Mike

[BobL]

I think I have it figured... put the PI balance ground to the negative side of the node C/D caps, grounded the reverb to the chassis on the same lug as the preamp, but on a separate lead.

It's not *silent* but it's dang quiet. If I roll that PI balance all the way counter clockwise it is quietest (and the amp volume is lower), but it sounds good... I can still turn the PI pot up a fair bit without it being bad news.

I still don't love the reverb... seems splashy instead of rich, but it's probably an improvement over the original settings. I will give it a run with the band tonight and see how it does there.

Thanks for all the help getting through this and figuring it out, fellers...

Oh, and the switch doesn't pop when I switch from A to B now... also, B has a lot more low end and overall seems richer - I'll be curious to see how it does with folks, and which setting I prefer.

I'm running it into 2 Eminence GA SC64s, btw.

EDIT: With the volume on 0, you can still play audibly. That's strange... anyone else experiencing that? Feature, or bug?

Wired like this (will update original as well):

[mikeywoll]

I think I have it figured... put the PI balance ground to the negative side of the node C/D caps, grounded the reverb to the chassis on the same lug as the preamp, but on a separate lead.

It's not *silent* but it's dang quiet. If I roll that PI balance all the way counter clockwise it is quietest (and the amp volume is lower), but it sounds good... I can still turn the PI pot up a fair bit without it being bad news.

I still don't love the reverb... seems splashy instead of rich, but it's probably an improvement over the original settings. I will give it a run with the band tonight and see how it does there.

Thanks for all the help getting through this and figuring it out, fellers...

Oh, and the switch doesn't pop when I switch from A to B now... also, B has a lot more low end and overall seems richer - I'll be curious to see how it does with folks, and which setting I prefer.

I'm running it into 2 Eminence GA SC64s, btw.

EDIT: With the volume on 0, you can still play audibly. That's strange... anyone else experiencing that? Feature, or bug?

Wired like this (will update original as well):

Congrats!

I didn't try playing mine with the volume off

Could it be coupling through the pre-amp filter cap grounds? Maybe try and ground the pre-amp filter caps directly to your pre-amp ground lug vs through the pre-amp bus?

Just guessing here?

Mike

[chopstuck]

I had reverb Buzz too. Solved by grounding the tank to the chassis with an alligator clip. I also returned all the reverb grounds to the input jack ground.
Don't use those handy grounding lugs on the RCA jacks !

[mojotom]

I had the house to myself so was able to crank it up to 5. This time I played a guitar with humbuckers, and yes it has some drive.
Take my comments with a grain of salt as I have a lot of high frequency hearing loss. So I don't hear the full harmonic content.

Having said that, I keep coming back to a PI balance setting of 4k ohms, but it sounds good at settings anywhere from 0 to 8 k ohms. At 0 ohms, I believe it is effectively a single ended amp, and I'm actually thinking of building a single ended version. 15 watts is really overkill for my needs

I've got to say, this thing has a LOT of bass. It is really awesome how solid it is, not blaaaaty. But with humbuckers it's almost too much. Chris Benson mentioned changing the power amp coupling caps from .1 to .02 uF. I split the difference a .047. I'm going to try the .022s and also try reducing the coupling cap into the PI. And maybe make the V1 cathode caps switchable.

I read somewhere Benson uses .02 on the whole power amp on his production Monarch amps but his personal amps still uses .1.

[dorrisant]

I read somewhere Benson uses .02 on the whole power amp on his production Monarch amps but his personal amps still uses .1.

From his 1st post here...

I'm bored on a plane and came across this. I must say I'm flattered to join the ranks of traced stuff on here...I've used this forum as a resource many times over the years, especially when I was doing repair work. Hopefully it is helpful of me to point out at first glance of the shared schematics that the power supply for the Monarch is not correct and will probably hum as drawn, the Tall Bird output buffer will not work as drawn, and the Vinny power tube grid stopper should be 27k, not 2.7k. bonus note: we use all .02 for the PI caps in the Monarch these days since gets rid of a bit of tubbiness but I haven't switched my personal amps over because it really doesn't change much.
Regards,
Chris

[mojotom]

Ah ah too much smoking amp-crack

[mikeywoll]

EDIT: With the volume on 0, you can still play audibly. That's strange... anyone else experiencing that? Feature, or bug?

Wired like this (will update original as well):

I also get bleed through of the signal with the volume off. The good news is that it isn't something "wrong" with your build. The bad news is that it is likely a layout issue.

Mike

[BobL]

The good news is that I don't plan on playing the amp with the volume off.

Took this to band practice last night, and it did really well. Does particularly well with certain drive pedals - super rich, harmonic overdrive.

My main kinda 'base' drives are a Bondi Sick As (modified Klone), and a Seymour Duncan Forza - the Sick As didn't sound that great, but the Forza sounded outstanding (total sleeper pedal people overlook).

I have an RYRA Klone that I use for a lead boost, and that did pretty well too. The amp has a pretty different and kind of pronounced midrange vs the Fender style amps I usually play, so having a drive with a mid control (like the Forza) was really useful. Cool shimmer on the cleans, and seems like way more volume on tap as built now vs. as built previously. I really didn't have it turned up much at all. Tight low end vs. my Fenders.

Overall, feeling pretty happy with it, though I still don't care for the voicing of the reverb. Too splashy.

[CraigGa]

I've not checked your layout Bob but the 12DW7 tridodes are not the same and the schematic on here doesn't specify which is which.
I presume the 12AU7 side (pins 1,2,3) is used as the driver and the 12AX7 side (pins 6,7,8) is used for the recovery?

Craig