Benson Monarch - Finding 60hz hum?

[BobL]

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.

No improvement, unfortunately.

[ac427v]

Seems like you and Sluckey are heading towards the phase inverter as the hum culprit. I notice that the inverter stage gets high voltage from node B, a poorly filtered source shared by the power tube screens. You could change that by adding another stage of filtering with a 4.7k/2watt resister and another 22uf/450v cap. If needed, you could decrease the resistor to the last filter to 4.7k to keep that supply voltage about the same as it is now. I would love to see you get this sorted out because the reverb circuit interests me for a future build Keep going, you're my beta tester!

[sluckey]

What happens to the hum if you ground V1 pin 2 directly at the socket?

[BobL]

What happens to the hum if you ground V1 pin 2 directly at the socket?

Maybe I didn't totally follow what you were saying re: the reverb tube - in addition to grounding the 27K resistor to the lug of the return jack, you were saying *also* add a ground from pin 7, and then now you are saying add a ground from V1 pin 2, but keep everything else intact?

[BobL]

Seems like you and Sluckey are heading towards the phase inverter as the hum culprit. I notice that the inverter stage gets high voltage from node B, a poorly filtered source shared by the power tube screens. You could change that by adding another stage of filtering with a 4.7k/2watt resister and another 22uf/450v cap. If needed, you could decrease the resistor to the last filter to 4.7k to keep that supply voltage about the same as it is now. I would love to see you get this sorted out because the reverb circuit interests me for a future build Keep going, you're my beta tester!

Maybe of note here is that my filtering is a little bit higher than the spec - I couldn't easily lay my hands on 25uF higher voltage caps for my filters, so I used 30uFs... but I suppose that is different than adding an additional filtering stage as you suggest?


I've reverted (in stages) the caps on either side of the reverb tank, and I still have the oscillating click, so something else is going on there than the values causing the issue, I guess... I'm back at 500pF and .022uF.  I tried replacing first the 500pF, no change, then the .022uF, no change... so that's strange.

[sluckey]

Maybe I didn't totally follow what you were saying re: the reverb tube - in addition to grounding the 27K resistor to the lug of the return jack, you were saying *also* add a ground from pin 7, and then now you are saying add a ground from V1 pin 2, but keep everything else intact?

I'm not suggesting to permanently add a ground to anything. Use a gator clip test lead. Connect one end to chassis. Now temporarily connect the other end to V3-7. Still got hum? Next move the clip to V1-2. Still got hum?

[T Wilcox]

does adjusting the 10k pot change the hum at all?
I've never done a floating paraphase before but looking at other schems from Magnatones I notice the balance resistors are not equal and then a pot or resistor to tweak balance. The side the grid connects to the pot is always less resitance unlike the monarch schem which is opposite

Comparing this http://www.dougcircuits.com/M192.html
To this  http://ampgarage.com/forum/download/file.php?id=59923&mode=view

The difference I see is even with the benson 10kpot set all the way maxed (shorted) its still 220k vs 220k
in the other schem above the wire coming from the grid to the balnce pot has the capability of being much less than the other side. 102k adjustable to 222k

Just a guess as Im still learning about these!?
 

[BobL]

does adjusting the 10k pot change the hum at all?
I've never done a floating paraphase before but looking at other schems from Magnatones I notice the balance resistors are not equal and then a pot or resistor to tweak balance. The side the grid connects to the pot is always less resitance unlike the monarch schem which is opposite

Comparing this http://www.dougcircuits.com/M192.html
To this  http://ampgarage.com/forum/download/file.php?id=59923&mode=view

The difference I see is even with the benson 10kpot set all the way maxed (shorted) its still 220k vs 220k
in the other schem above the wire coming from the grid to the balnce pot has the capability of being much less than the other side. 102k adjustable to 222k

Just a guess as Im still learning about these!?

The tone of the hum changes as you adjust that pot, but I don't actually think the *amount* is changing...

[BobL]

Maybe I didn't totally follow what you were saying re: the reverb tube - in addition to grounding the 27K resistor to the lug of the return jack, you were saying *also* add a ground from pin 7, and then now you are saying add a ground from V1 pin 2, but keep everything else intact?

I'm not suggesting to permanently add a ground to anything. Use a gator clip test lead. Connect one end to chassis. Now temporarily connect the other end to V3-7. Still got hum? Next move the clip to V1-2. Still got hum?

Gotcha - hum doesn't change, but with V3 - 7 grounded, the oscillating click disappears.


I'm kind of wondering about removing the reverb from the circuit entirely... what considerations would I need to make when doing that? I'm guessing I would need to replace components that split from the reverb circuit to accommodate the changes in voltage occurring from not having that circuit in there? Is it only the grid resistor on V3 that you have to account for, or is there more to that?

[T Wilcox]

Probably off base here as well since pulling the reverb tube does not stop the hum but as far as the reverb ckt goes on the input grid to V2-2 shouldnt it have a grid leak resistor?
I am also curious how much ripple is still on the B+ tap as ac427v mentioned there is not a whole lot of filtering happening even with a 30uF at only the 2nd node
One more question, if the tone of the noise changes with the 10K pot are we sure it is 60HZ hum and not buzz? I found other threads with buzz problems due to unbalanced PI

[PRR]

Is the driver really powered from the screen B+ node? That's not enuff filtering, even all push-pull like it is. Note all 25uFd caps and only 1k to the screen filter. When Fender did like this he took 10K even 25k filter resistor.

Quick test: move the driver feed over with the other small tubes. Does that hum less?

[BobL]

Is the driver really powered from the screen B+ node? That's not enuff filtering, even all push-pull like it is. Note all 25uFd caps and only 1k to the screen filter. When Fender did like this he took 10K even 25k filter resistor.

Quick test: move the driver feed over with the other small tubes. Does that hum less?

Maybe I'm misunderstanding what you are saying, but all three preamp tubes are connected to the C node...

[pullshocks]

After looking at the photo on page 6 of the TAG Monarch thread, the PI plate resistors do come from node c.


A couple other things I notice, 


The reverb return triode Rk looks like a 2.2k (red red red) not 1k as shown in the schematic.


At the rectifier for the DC heaters, there are 2 100 ohm resistors that do not appear in the schematic

[T Wilcox]

The (2) 100 ohm resistors would be for an artificial center tap. If you have a center tap on your PT for the 6.3v ckt and it's grounded then they would not be used.
If neither were used it will most definitely be humming.
I dont remember where I found the attached DC heater pdf but it is a similar setup with much more filtering but shows the 100R's being used on both the AC side and DC side on the bottom setup with "NO" PT center tap. The top one shows the AC side uses the PT center tap but also uses 100R's on the DC side

Good catch on the 2k2 for the reverb return cathode! I already mentioned the mica cap coming off the plate of the same triode appears to be 500pF in the pic as well. It clearly has 500 printed on it in the pic

[BobL]

The reverb return triode Rk looks like a 2.2k (red red red) not 1k as shown in the schematic.


At the rectifier for the DC heaters, there are 2 100 ohm resistors that do not appear in the schematic

I actually noticed this too... and then got busy with work and forgot that I noticed it. Thanks for the reminder!

[T Wilcox]

I also forgot that neither of you used DC heaters on my last post so my question for both BobL and Pullshocks is does your PT have a center tap for the 6.3vac that is grounded? or have you used 100R resistors to create an artificial center tap?

[BobL]

I also forgot that neither of you used DC heaters on my last post so my question for both BobL and Pullshocks is does your PT have a center tap for the 6.3vac that is grounded? or have you used 100R resistors to create an artificial center tap?

My PT has a center tap for the heaters.

[T Wilcox]

Then I am once again possibly barking up the wrong tree

[pullshocks]

PRR, I owe you one.  Your comment lead me to try  disconnecting the  lead to the grid of PI triode B.  (Easy to do with my mini-breadboard).  In the process of putting it back, the hum became intermittent  Investigating this, I discovered a couple  power supply ground connections were not up to my usual standards, at the Power Amp cathode resistor and the second filter cap.  I blame it on the brand x turrets I used.



After chopsticking the connections, the hum is gone, my voltages are up, but amp volume is way down.  Progress, but  I'm not out of the woods yet.  Tomorrow I will completely resolder the power supply and hopefully get my full volume back.


Once full power is restored, I will change the reverb recovery cathode resistor and experiment with the reverb return coupling capacitor.




T Wilcox, my PT has a heater center tap.  Right now I have it connected to the 6V6 cathode connection to elevate the heaters, as per Merlin, RobRob, and other info sources.  After I do the above mentioned rework, I will determine if it is worth the trouble to add the
DC heaters.  I am certainly open to doing that, and have parts on hand to do so if needed.  Right now hum is not audible but presumably will become audible again when I get the full amp volume back. 

[T Wilcox]

Thats great sounds like you are close
I bet elevated heaters will be plenty
I look forward to hearing your results after said changes

[BobL]

Good grief... I got this Sprague 4700uF 25v cap. It is the size of a baby's arm.  Not totally sure I can fit it in this tweed chassis... guess we'll see.

[Willabe]

Is the driver really powered from the screen B+ node? That's not enuff filtering, even all push-pull like it is. Note all 25uFd caps and only 1k to the screen filter. When Fender did like this he took 10K even 25k filter resistor.

I think PRR, ac427v and T Wilcox are right, at least part of your problem is not enough B+ filtering.

The PI, reverb and preamp tubes are all on the same filter cap node.

I'd add another filter cap node for the PI. If that doesn't fix it or at least make it better, then I'd try adding 1 more B+ filter node for the verb tube.

And your grounding wiring might need to be changed.

I don't know how well you understand ground wiring schemes, but this might help you greatly.

http://www.valvewizard.co.uk/Grounding.html

You might need to add at least another B+ filter cap node AND rewire the grounds.

[Willabe]

Maybe of note here is that my filtering is a little bit higher than the spec - I couldn't easily lay my hands on 25uF higher voltage caps for my filters, so I used 30uFs... but I suppose that is different than adding an additional filtering stage as you suggest?

A little higher value B+ ecaps is better, and yes it is different than adding another filter node.

Each B+ node 1; adds more filtering as the B+ filter string goes on and 2; each B+ filter node helps isolate what ever tube/circuitry from other tubes/circuits. 

[Willabe]

Are you power tube B+ nodes grounded separately from the PI, reverb and preamp tubes? If not, that often will cause an amp to buzz.

[sluckey]

Good grief... I got this Sprague 4700uF 25v cap. It is the size of a baby's arm.  Not totally sure I can fit it in this tweed chassis... guess we'll see.

Hoffman sells this one that would work fine in your amp. It's only 1" long and 1/2" diameter.



Or if you prefer axial this one is only 1" long and 5/8" diameter...

     https://www.tubesandmore.com/products/capacitor-16v-4700-f-axial-lead-electrolytic

[pullshocks]

OK, here's how I mounted the rectifier and cap.

[ac427v]

Reverb hum? The schematic shows no coupling capacitor between V1a and the input to the reverb driver 12DW7. Should the connection come between the .02 coupling cap and the 470k resistor? or does the driver need its' own cap such as the 500pf on a fencer reverb circuit?

[sluckey]

The reverb driver is just a direct coupled cathode follower. No input cap needed.

[pullshocks]

I wired this EDIT the DC heaters up last night.  I turned the amp on briefly.  Here is a preliminary report


To restate the issue:  With this amp , when I turn the amp on with nothing plugged in, and the volume turned all the way down, after about 30-40 seconds, there is an audible hum.  Still with nothing plugged in, when I turn the volume up past 5, additional noise becomes audible.  It has a different sound quality, a more complex sound.




Currently I have the first power supply cap grounded to the (steel) chassis at the PT end of the chassis.  The PT center tap is also connected at this point.The other 2 filter caps are grounded to a buss that is connected to the chassis near the input jack.


I wired in the bridge rectifier and cap as shown above.  The PT heater center tap is connected to the chassis at the same along with the first filter cap and High Voltage center tap.


So the heater circuit is now wired as shown in Merlin's diagram, see below.  Well, he only shows DC going to one preamp tube, I have it going to all 3 preamp tubes.


I finished the DC hookup fairly late.  I turned the amp on briefly, nothing plugged in, and volume on zero.  The heaters glowed, and the same initial hum as before became audible.


As I turned the volume up, additional noise came up as before.  Subjectively, I would say the quality of the sound is different, but still definitely there.


I'm really curious to hear a real one, to see whether a similar hum is present.  There is a Benson dealer in town.  They do not have any in stock.  So no go on that.  The guy on the phone said they are very quiet.

[pullshocks]

I still *think* the noise is coming in on the paraphase connection. 


This is based on the hum level going down when the PI balance control is turned all the way to the left, leaving the paraphase connection at ground potential.  also, removing the input tube and reverb tube do not change the hum.  removing the PI tube does eliminate the hum.


Amp volume also goes down, presumably because only one of the 6V6s is getting any signal.  Kind of like a single ended amp.


Based on the AmpBooks paraphase calculator, it looks like adjusting the balance control to about 4K ohms would give a balanced signal to both tubes, so I am using that as my baseline..


My next step is to completely redo the power supply with new caps.  After staring at the picture on Page 6 of the ampgarage thread, I believe the schematic represents how the real amp's PI is wired...and the available photos confirm only 3 filter caps, though the values cannot be verified from photos.

[pullshocks]

Just measured the DC heaters.  6.7 V DC between the 2 connections. Measured at the rectifier/cap assembly


So here's something that seems funny.  Measuring voltage to ground from each of the DC heater taps, I get 3.2 and -2.6,


The AC connection is 6.6 V ---each 3.3 V compared to ground

[Willabe]

Currently I have the first power supply cap grounded to the (steel) chassis at the PT end of the chassis.  The PT center tap is also connected at this point.The other 2 filter caps are grounded to a buss that is connected to the chassis near the input jack.

Your 2nd B+ filter cap feeds the power tube screens?

If so, you have to ground it with the 1st B+ filter cap that feeds the power tube B+ OT CT.

Read that grounding link I posted from Merlin.

[sluckey]

So here's something that seems funny.  Measuring voltage to ground from each of the DC heater taps, I get 3.2 and -2.6,

That's because you wired it like Merlin. If you wire it like the original, ie, PT CT not connected and negative side of bridge to ground, the funny stuff will go away. Just remember, you cannot connect the PT CT to ground ***AND*** the negative side of the bridge to ground. Doing so will destroy the bridge every time, maybe take the PT out too.

[pullshocks]

Thanks Sluckey.  Disconnected the heater PT center tap and ran the ground connection to the negative heater.  The hash I was hearing with the volume control turned up is reduced dramatically. 


Although the schematic shows it with one heater grounded, in the picture it appears the DC heaters are referenced to ground with 2, 100 ohm resistors, like an artificial center tap.


While I was in there, I replaced filter caps 2 and 3 with fresh  22 uF caps, and changed the ground connection on filter cap2 to the same chassis connection as filter cap 1. 


Unfortunately I still get the same initial hum.  I have  also tried it with all the filter caps connected to the same buss.  Hum is the same with all 3 arrangements


I sure hope I can get that figured out, because I LOVE the sound of this amp

[sluckey]

I would temporarily do away with the dc heaters and wire all heaters for ac and connect the PT CT to ground. Any better?

[pullshocks]

I would temporarily do away with the dc heaters and wire all heaters for ac and connect the PT CT to ground. Any better?

That's how I had it when I first powered up this build.  It hummed then just like now.  The second thing I tried ( before doing the DC heater change) was to elevate the heaters by connecting the heater center tap to the 6V6 cathodes.  Same hum issue that way.


I have read that mismatched power tubes can cause hum.  The Electro Harmonics 6V6 I have in there have cathode currents of 34 mV and 37 mV (measured via 1 ohm resistors from the cathodes to the cathode resistor).  Would those be considered mismatched?

[Willabe]

While I was in there, I replaced filter caps 2 and 3 with fresh  22 uF caps, and changed the ground connection on filter cap2 to the same chassis connection as filter cap 1.

Your power tubes cathode ground has to go to the cap 1 and cap 2 ground.

You might be having a problem with lead dress too. Like your tube grid wires, are they long?  Are any of your grid wires using shielded wire? And other too long of wires that would need to be trimmed shorter?

You seem to have (had) multiple noise problems?

Can you post a few full gut shot pictures of the chassis?
 
   

[Willabe]

In this picture, terminal strips, have pins sticking out. They are really close to the tube sockets.

[Willabe]

And what are these 2 yellow wires for in the red circles?

[pullshocks]

Thanks Willabe.  And thanks for your earlier input about the power supply.


My system-yellow goes from plate resistor to plates, EDIT-- and from the plates out to the power amp coupling capacitors.  Most layouts put the plate resistors between those coupling caps but I put them over near the tube.

Brown is cathode connections


Orange is grid connections

I have taken the DC heater unit out.  I am in process of replacing the PI tube socket and improving the wiring in the area.

[pullshocks]

Downloaded a spectrum analyzer app and determined the hum is 120 hz.


Changed back to AC heaters.


Rewired preamp heaters, and cleaned up PI wiring.


Baseline hum is still there.


Initial testing was all done with an old Celestion G12H30 in an open back cab.  Plugged in an Eminence Ragin Cajun speaker.  Hum is much less prominent but still there. 


Attached is a gut shot and layout drawing.

[sluckey]

I think your breadboard/barrier terminal construction is the biggest hum/noise contributor in that amp.

[pullshocks]

I think your breadboard/barrier terminal construction is the biggest hum/noise contributor in that amp.

Fair point, but it turns out the schematic was wrong as to the power supply filtering.  With the revised power supply shown below, and my AC heater center tap connected to the 6V6 cathodes, the hum issue is GONE.  Even with the breadboard/barrier terminal construction.


Thanks Sluckey, I appreciate all the help you provide.

[Willabe]

That's what we said, more filtering. Glad you got it fixed.

Is the driver really powered from the screen B+ node? That's not enuff filtering, even all push-pull like it is. Note all 25uFd caps and only 1k to the screen filter. When Fender did like this he took 10K even 25k filter resistor.

I think PRR, ac427v and T Wilcox are right, at least part of your problem is not enough B+ filtering.

The PI, reverb and preamp tubes are all on the same filter cap node.

I'd add another filter cap node for the PI. If that doesn't fix it or at least make it better, then I'd try adding 1 more B+ filter node for the verb tube.

[pullshocks]

Thanks.  Yes you guys were right, and it did look "unconventional" but I was locked into the schematic having been drawn up by somebody who knows what he is doing and had access to an actual amp.  No knock on him, we can all make mistakes.


If anybody felt ignored or disrespected, I apologize for that.  I truly appreciate all the help  members receive here on this forum.



Silver lining is I was doing it on the infamous barrier strip breadboard with plenty of room to add the node.  Also, let's just say some learning took place.