Benson Monarch - Finding 60hz hum?

[BobL]

So... been playing the 6G3 I finished recently (thread: https://el34world.com/Forum/index.php?topic=27030)


Just... not a fan. It does a cool thing with chords, but in practice, it's just not a circuit I see myself being able to use.


Been wondering what might be fun and feasible to put in this chassis, re-using as much of this build as possible (it's in a tweed style chassis), and the circuit from the Benson Monarch caught my eye - ampgarage had a thread where someone traced the circuit, and Chris Benson himself showed up in the thread with some feedback and advice, which was cool.  I tried posting in that thread there, but not getting any action, so thought I'd try here as well.


The original has some custom transformers, and uses a solid state rectifier (that I don't 100% understand, being unfamiliar), but it looks like people have tried the circuit out with tube rectifiers, so I thought I might give it a go, but I'm unsure of some things and need a bit of advice in a few areas. The original is all point to point on turret strips, but I thought for my chassis, it'd be good to stick with a board.



I guess to start off... what do I have wrong here? I'm fairly confident about the connections, but I wasn't sure if I was going about the grounding scheme badly, or if perhaps I wasn't translating aspects of the schematic correctly.


Is the PI balance pot on this basically what you would be using to bias the amp?


Other thoughts? As always, thanks in advance!


Here is the schematic drawn by Aaron at ampgarage (maybe he's here too?)



And here is my attempt at a layout (this is really the first time I've done something at this scale, so I'm sure there are mistakes):

[BobL]

Okay... maybe a follow up question instead...


With a 5U4GB, I can get voltage on my PT down to 334, theoretically, which is higher than the 278 listed in the schematic by a fair bit.  Do I need to make changes downstream from that to get lower voltages at the power tubes and preamp tubes? Or is it a 'try and see what you get' sort of situation, and just make sure voltages are safe for my tubes, and if so, see if I like how things sound before making other changes?

[Latole]

Okay... maybe a follow up question instead...


With a 5U4GB, I can get voltage on my PT down to 334, theoretically, which is higher than the 278 listed in the schematic by a fair bit.  Do I need to make changes downstream from that to get lower voltages at the power tubes and preamp tubes? 

No, 6V6 can play very well with 334 volts an up to 425 volts

[BobL]

Okay... maybe a follow up question instead...


With a 5U4GB, I can get voltage on my PT down to 334, theoretically, which is higher than the 278 listed in the schematic by a fair bit.  Do I need to make changes downstream from that to get lower voltages at the power tubes and preamp tubes? 

No, 6V6 can play very well with 334 volts an up to 425 volts

So, this would also create higher voltage at the preamp tubes due to a higher overall B+, right? What sort of impacts would that have on tone, higher gain? Less sag?

[Latole]

More gain.

[sluckey]

With a 5U4GB, I can get voltage on my PT down to 334, theoretically, which is higher than the 278 listed in the schematic

Drop the B+ even mor with a real 5Y3.

[BobL]

With a 5U4GB, I can get voltage on my PT down to 334, theoretically, which is higher than the 278 listed in the schematic

Drop the B+ even mor with a real 5Y3.

As opposed to a 5Y3GT, you mean? I will keep that in mind if things seem too hot with the 5U4GB.


I guess one thing I don't totally understand is how, if at all, the OT interacts with the circuit itself - the Monarch looks to have a 15w OT, but mine is rated at 20w... does that really just mean that I'll have the same thing, but louder, or will that have other impacts? If it's louder, that would theoretically get me back some clean headroom vs. the 15w OT for the same 'in the room' volume, I would imagine...


With a CT on my 6.3V heater wires on my PT, I guess I don't need to bother with the 100 ohm resistors on the pilot light, correct?


Thanks, all.

[Latole]

With a CT on my 6.3V heater wires on my PT, I guess I don't need to bother with the 100 ohm resistors on the pilot light, correct?


Thanks, all.

Correct.

[sluckey]

You won't need the CT or the resistors. I take that back. I suggest you dump the DC filament supply and run all filaments on AC. And connect the CT to ground. There's nothing special about this amp that would benefit from using dc filaments on the little tubes.

[Latole]

You won't need the CT or the resistors.

You are right, it is DC supply

[BobL]

You won't need the CT or the resistors.

You are right, it is DC supply

I understood this as the schematic having a DC supply, but w/ my layout and the tube instead of the SS rectifier, do I not end up with AC on the heaters?


EDIT: Oh, I just saw sluckey's edit... ok.

[Latole]

Schematic use tube rectifier for circuit B+ voltage
And SS rectifier for heater. Both are two different thing , apple and orange

[BobL]

Schematic use tube rectifier for circuit B+ voltage
And SS rectifier for heater. Both are two different thing , apple and orange

Interesting - that was why that section was confusing me. Not something I was familiar with at all.

[Latole]

Are you familiar with schematic  ? If yes, just follow

[BobL]

Are you familiar with schematic  ? If yes, just follow

Working on understanding it all better, but I just wasn't familiar with the solid state/bridge rectifier, and I wasn't sure if the tube part of that section was actually something that was part of the rectifier and didn't actually represent a tube - it makes sense to me now that you guys have provided some more explanation... and it seems obvious now, but it kinda threw me.

[BobL]

You won't need the CT or the resistors. I take that back. I suggest you dump the DC filament supply and run all filaments on AC. And connect the CT to ground. There's nothing special about this amp that would benefit from using dc filaments on the little tubes.

I'm curious as to why someone would use DC for the filaments on the preamp tubes?

[BobL]

Am I understanding correctly that to determine the impedance required for the reverb tank, I will measure ohms from the send to ground, and then from the return to ground, and get the tank that best fits the values in this chart for the type I want? This is with the amp off, correct?

[walkman]

What’s happing with the phase inverter balance pot... I have not seen that before.

[BobL]

What’s happing with the phase inverter balance pot... I have not seen that before.

Yeah, I hadn't either... in practice, honestly, I'm not noticing much impact on the amp having that thing all the way one way or the other, but I might play around with it some more.

[BobL]

Amp was working well - then I installed a new set of speakers, Eminence GA SC64s, to get some more clean volume (had P12Qs which are super quiet), briefly installed a reverb tank (worked, but apparently wasn't a match - I've since found/ordered the proper one for this amp), temporarily detached the controls (not the connections, just the nuts) to install a new face plate, and now I've got nasty hum that has me a little stymied.  Pretty sure it is 60hz.


When I turn the amp on, prior to the tubes warming up, I get audible but quiet hum through the speakers.  As the tubes warm up, the hum gets pretty dang loud.  If a guitar is plugged in, it adds a harmonic to the hum. Volume/tone/reverb controls have no effect on the hum whatsoever.


Amp sounds great when played, but it'd be nice to get rid of the hum.


Things I have tried:
Swapping all preamp tubes.
Pulling all tubes one at a time - the only one that gets rid of the hum is V2.
Moving the preamp ground to a lug (it was grounding to the input, but this is probably a better idea anyway).
Retouching all of the solder joints on and from the controls, as it seemed most likely that I had injured a joint when I pulled them back to ream the holes in the new faceplate to match (new faceplate is plastic, so I don't think that did anything).
Chopsticked like crazy to move wires, check joints, etc.
Attached different speakers.
Tried a different rectifier tube.
Added shielding to the back panel and installed.


What should I be doing next? The only thing I haven't tried that seems obvious to me is to replace the power tubes, so I will grab another donor amp to try that, but since pulling one or the other didn't seem to impact, I don't have high hopes.


Here is a clip of the hum - starts w/ nothing plugged into the amp, then you can hear me plug in a guitar, then at about :13 I turn up the volume and you will hear the additional buzz from the guitar.  As noted, turning up the volume w/o a guitar makes no difference in volume or tone of the hum.


https://drive.google.com/file/d/18FLyTltBTTrtKGetWKMCmRT7cBkSzoKO/view?usp=sharing

[T Wilcox]

assuming you have a shorting input jack and its humming with nothing plugged in check that the sleeve is still grounded after removing and reinstalling the front controls.
check all your grounds for continuity. If there was no hum before adding the faceplate then check all the connections you may have disturbed.

I think I will be building this amp with iron from a trainwreck build I never loved.
Which reverb tank did you end up finding was the correct one?
in the pics Arron posted on TAG the output cap from the 12DW7 looks to be 500Pf but the schem he posted shows 250Pf, which did you use? i wonder if this is what Chris Benson was talking about being wrong?

[BobL]

assuming you have a shorting input jack and its humming with nothing plugged in check that the sleeve is still grounded after removing and reinstalling the front controls.
check all your grounds for continuity. If there was no hum before adding the faceplate then check all the connections you may have disturbed.

I think I will be building this amp with iron from a trainwreck build I never loved.
Which reverb tank did you end up finding was the correct one?
in the pics Arron posted on TAG the output cap from the 12DW7 looks to be 500Pf but the schem he posted shows 250Pf, which did you use? i wonder if this is what Chris Benson was talking about being wrong?

Yep, I did check grounds - they seem ok. I may end up retouching all of the joints at some point to see if I've got a bad one.


I was told that the correct tank was an 8EB2C1B.

My cap is a 250pF... if my reverb seems janky with the correct tank I'll look at swapping that - thanks.

[BobL]

assuming you have a shorting input jack and its humming with nothing plugged in check that the sleeve is still grounded after removing and reinstalling the front controls.
check all your grounds for continuity. If there was no hum before adding the faceplate then check all the connections you may have disturbed.

I think I will be building this amp with iron from a trainwreck build I never loved.
Which reverb tank did you end up finding was the correct one?
in the pics Arron posted on TAG the output cap from the 12DW7 looks to be 500Pf but the schem he posted shows 250Pf, which did you use? i wonder if this is what Chris Benson was talking about being wrong?

FYI - I did swap out the 250pf for the 500pf... the difference was very minimal.  The reverb is kinda weird still - very splashy/bright, but also very quiet.


Would swapping the .022uf I have on that tube to a .1uf let more low end through?


I wonder if there is something still wrong with it.  I'm looking at that picture of the preamp, and it's really hard to make out exactly what is going on with V3...  Am I not using the two halves of the tube properly? Hard to tell where things are going there - and I don't see the 1K paralleling the 25uF cap that is in the schematic, does that seem like a 2.2K to you? Would increasing that from 1K to 2.2K add more gain to the reverb circuit by giving more current to the 24uF cap and help with my low volume?  I'm a bit over my head here...


Still haven't been able to track down my hum... :/

[PRR]

> reverb is kinda weird still - very splashy/bright, but also very quiet.

[BobL]

> reverb is kinda weird still - very splashy/bright, but also very quiet.

Thanks - of note is that my current values for those two locations are .022uF and 500pF - can you help me understand what these changes would actually be doing?

[PRR]

More bass.

[BobL]

Ok, so letting more low end in, then letting more out, I guess?  My initial thought was that replacing that .022 (.1 in the schematic) with the .1 would help with that, so glad I wasn't totally off base (bass?)

[PRR]

You say quiet and bright. Like only treble? Don't be timid. Let's try TOO MUCH bass. 10uFd in the drive is surely excessive. But if it is still "bright" then something else is not letting full bass/body go through. When you have "full bass" it will probably be "mud and thunder". THEN reduce caps for best musical balance.

[BobL]

You say quiet and bright. Like only treble? Don't be timid. Let's try TOO MUCH bass. 10uFd in the drive is surely excessive. But if it is still "bright" then something else is not letting full bass/body go through. When you have "full bass" it will probably be "mud and thunder". THEN reduce caps for best musical balance.

No... not only treble, just brighter than I would want it to be.  The .1 might be the largest I have on hand, so I might start there. My preference would be more in the realm of 'dark and lush', and this is bright and splashy...

[pullshocks]

I've got the Ampgarage Benson Monarch Reverb circuit up and running on my mini breadboard.  A few minor differences from the schematic:

• I did not do the DC heaters.
• I added screen resistors
• The V1a grid resistor is 33K not 68K as shown on the  schematic.
• My Pt is 265-0-265 so my plate voltages are about 15 v lower.

Like you, I have a fairly prominent hum when turning the amp on, even with the volume turned all the way down.  But not that noticeable once I start playing, so not a deal breaker right now,


As you know, the schematic did not specify a reverb tank.   I have a 12FBxxx hooked in.


I found the reverb quite weak, and also very trebly and harsh sounding, similar to what you describe.  Picking hard, the verb takes on a harshness that is not pleasant.  Initially I assumed this was due to the xFBxxx tank being mismatched to the cathode follower (see more below).


Based on PRR's advice, I upped the reverb output coupling capacitor from 250pF to 0.005uF (input coupling capacitor stayed at 1.uF).  Drum roll, please....


Reverb is much more prominent now, louder and fuller.  Too much verb with the pot on 10.   As the pot is turned down to 5, the reverb level gets in the range I like, and the tone is MUCH better, the spiky treble is gone.


This is with the volume on 2, and DiMarzio Fast Track 1 neck and Bluesbucker bridge, not hot pickups.  The amp is pretty loud, 2 is about as high as I can go.


I'll probably try a few other  values for the output coupling cap, not sure where that will end up


As far as the tank, I think a n xEBxxx may be better matched.  I know just enough to make me dangerous, but here is my rationale:


So if I understand this right, the input impedance of the tank needs to match the output impedance of the driver stage, in this case a12AU7 cathode follower stage.

In Merlin Blencowes Designing Tube Preamps for Guitar and Bass Second Edition, on page 115, he states the output impedance of a cathode follower is 1 divided by the transconductance, and for a 12AX7 (transconductance 1.5 mA/V) this works out to 667 ohms.
For a 12AU7 the transconductance is 2.2 mA/V, so the output impedance would be lower, more like 500 ohms.   If that is correct, the xEBxxx, at 600 ohms input impedence would be a better match.


but the xFBxxx seems to OK with the cap change.  I'm curious if the xEBxxx would be even better, but not curious enough to spend the money.

[BobL]

As you know, the schematic did not specify a reverb tank.   I have a 12FBxxx hooked in.


I found the reverb quite weak, and also very trebly and harsh sounding, similar to what you describe.  Picking hard, the verb takes on a harshness that is not pleasant.  Initially I assumed this was due to the xFBxxx tank being mismatched to the cathode follower (see more below).


As far as the tank, I think a n xEBxxx may be better matched.  I know just enough to make me dangerous, but here is my rationale:

Yeah, someone at Benson told me the specific tank: 8EB2C1B.


I will see if I have any caps in the range of what you tried and give them a go - thanks for testing that out.


Let me know if you figure out the hum... what Chris Benson said in that thread was that the schematic was wrong and 'would hum' as shown... Aaron then looked at the amp again and found that the B+ wiring was different... but I wonder if there is something else that is getting missed there.

[pullshocks]

Good to know there was confirmation on the reverb tank being an EB type.  Looks like Doug Hoffman does not carry reverb tanks anymore.  Where did you get yours?

[T Wilcox]

He probably did DC heaters for a reason
could always try elevating the heaters and see if the hum goes away, only 3 more components

[pullshocks]

I've tried my heater center tap on the 6V6 cathode.  Did not solve the problem.

[T Wilcox]

I'm not sure what you mean by tried it on the cathode?

From my experience the heater hum is usually being injected/amplified by the first gain stage, just keeps multiplying from there through the following gain stages
I've had great results by elevating the heaters to around 40vdc
and also on one build did DC (same exact config as the Benson) rectify the 6.3 with 4700uF 10v but only on the V1 tube

I have parts on order to build the Monarch reverb in to a trainwreck express chassis I never loved. It will be my first true point to point build and plan to really follow Bensons full approach

[BobL]

I'm not sure what you mean by tried it on the cathode?

From my experience the heater hum is usually being injected/amplified by the first gain stage, just keeps multiplying from there through the following gain stages
I've had great results by elevating the heaters to around 40vdc
and also on one build did DC (same exact config as the Benson) rectify the 6.3 with 4700uF 10v but only on the V1 tube

I have parts on order to build the Monarch reverb in to a trainwreck express chassis I never loved. It will be my first true point to point build and plan to really follow Bensons full approach

I will be interested to see how your build turns out, and if you end up w/ the same hum we're getting.


My issue w/ the DC heaters is going to be space, I think... I'd have to figure out where to put the diode bridge in my tweed chassis.

[T Wilcox]

Ill post back here and the TAG thread once I get it together.
Should be able to knock a lot out this Sunday hopefully
I modified the faceplate from the express for this build to keep the holes in the same spot and added a dwell knob but I wont implement the dwell until I have heard the original circuit first
New faceplate should arrive tomorrow from Indcom
looking forward to this build

[T Wilcox]

Another possible cause of the hum could be coupling between the PT and OT
remove just the preamp tubes, power it back up and see if the hum is there still
I dealt with this 9 years ago on the same trainwreck express build I'll be dismantling

A reply from user SteveM on the following TAG thread reminded me of this

https://ampgarage.com/forum/viewtopic.php?p=425669#p425669

[BobL]

Another possible cause of the hum could be coupling between the PT and OT
remove just the preamp tubes, power it back up and see if the hum is there still
I dealt with this 9 years ago on the same trainwreck express build I'll be dismantling

A reply from user SteveM on the following TAG thread reminded me of this

https://ampgarage.com/forum/viewtopic.php?p=425669#p425669

My PT/and OT are offset about 45 degrees... since my original build was a 6G3, I laid out the transformers like they would be a brownface, even though I've got the tweed chassis.


I swapped the 500pf (250pf originally) output cap on the reverb for .0047uF, and the input cap to .1uf... the reverb sounds better, but I *think* the hum is now worse, and for sure worse is that I now have an oscillating click, like you might get with a tremolo.


I think at this point I want to try doing the DC heaters on the preamp, but I'm not entirely sure what I should get in terms of a bridge rectifier, and having a hard time finding that 4700uF 25v cap as well.  I would imagine voltage for the bridge rectifier, I don't need a lot of voltage since it's the heaters?  Recommendations on voltage/amperage I should look at?  Or what diodes would be appropriate to build my own?  I probably have 4 diodes sitting around here...

[T Wilcox]

https://www.tubedepot.com/products/3a-1000v-bridge-rectifier

This is the rectifier I have on order

The MIC W04M that Benson used appears obsolete but it was only 1.5amp as compared to the 3 amp I ordered

For DC heaters I've read to calculate 400ma per 12ax7 as compared to 300ma for AC heaters. I've also read 600ma as a cautionary so I just use that although probably not necessary, either way 3amp should be more than enough


For the cap I ordered the following but I also have a stash of radial 4700uF 10v from Mouser that would be fine
https://www.tubedepot.com/products/illinois-5000f-55v

[BobL]

https://www.tubedepot.com/products/3a-1000v-bridge-rectifier

This is the rectifier I have on order

The MIC W04M that Benson used appears obsolete but it was only 1.5amp as compared to the 3 amp I ordered

For DC heaters I've read to calculate 400ma per 12ax7 as compared to 300ma for AC heaters. I've also read 600ma as a cautionary so I just use that although probably not necessary, either way 3amp should be more than enough

1000v seems like overkill? Do you introduce unwanted restistance using that vs. the 400v one they have?

[T Wilcox]

I'm not concerned with it being capable of handling "more" voltage, afaik there is no minimum voltage
1N4007 are 1000v as well and I use them everywhere

[BobL]

I'm not concerned with it being capable of handling "more" voltage, afaik there is no minimum voltage
1N4007 are 1000v as well and I use them everywhere

Did you find that big 4700uF 25v cap somewhere?

[T Wilcox]

http://www.learningaboutelectronics.com/Articles/Diode-resistance.php#:~:text=Just%20like%20a%20resistor%20or,It%20changes%20parabolically.

Your question made me curious so I found this. It appears below 0.7v silicon diodes do offer some resistance but that is well below what we are dealing with.

I edited my first reply with the cap info as well you are to quick for me lol

[BobL]

http://www.learningaboutelectronics.com/Articles/Diode-resistance.php#:~:text=Just%20like%20a%20resistor%20or,It%20changes%20parabolically.

Your question made me curious so I found this. It appears below 0.7v silicon diodes do offer some resistance but that is well below what we are dealing with.

I edited my first reply with the cap info as well you are to quick for me lol

Cool... I'm ordering the 400v version... we can compare notes. :)

[T Wilcox]

you saved 10cents atleast
As far as the cap its gonna see less than 10v but they only had the axial at 5000uF 55v which is max voltage and once again will be fine as long as that is not exceeded
I avoided using mouser that order or else I would have just got a 4700uF 25v which they have plenty of

[pullshocks]

BobL, what voltage are you seeing on you PI plates?  Aarons readings were 164.8 on both.  I am getting only135 on v3a and 127 on v3b.


All my other plate voltages are closer to  those on the schematic.


I suspect the hum is coming from the PI.  Pulling v1 does not affect the hum, pulling v2 does not affect the hum.  Pull v3 and the hum is gone.


I have been over and over my PI wiring, tried different tubes, checked component values...

[BobL]

BobL, what voltage are you seeing on you PI plates?  Aarons readings were 164.8 on both.  I am getting only135 on v3a and 127 on v3b.


All my other plate voltages are closer to  those on the schematic.


I suspect the hum is coming from the PI.  Pulling v1 does not affect the hum, pulling v2 does not affect the hum.  Pull v3 and the hum is gone.


I have been over and over my PI wiring, tried different tubes, checked component values...

135/148


I have the exact same thing - pull the PI tube, no hum.

[sluckey]

What happens to the hum if you ground V3 pin 7 fight at the socket?

I would move that 27K resistor to the return jack. Make it's ground connection to the ground lug on that jack. Then make a short connection from the jack to V3-7.

[pullshocks]

Thanks Sluckey, I want to make sure we are all on the same page as to tube nomenclature. 


When I say removing V3 eliminates the hum, I was  talking about the PI tube. 


The schematic is not labeled as to tube number.  From here on out I will not use tube numbers, I will say input tube, reverb tube, and PI tube.


Removing the reverb tube has no effect on hum


Removing the PI tube-- no hum. 


As far as the reverb tube connections, would it make a difference that I am using isolated RCA jacks, with a wire from their (isolated) ground lugs to my ground bus? 


EDIT--I can easily move the 27K resistor to the return jack, no problem there, just wanted to let you know about the isolated jacks.


BTW, disconnecting the tank has no effect on hum