Benson Monarch

[BobL]

Well... then it is correct.

I think that I went through most of the thread, but I haven't seen that stated by Mr. Benson

Still think that xFB is a better fit for the circuit and intend to keep it.

EB are more suitable for op-amp (~TL072) driven circuits.

To each its own I guess.

Niki

Yeah, that was information that I got outside of this thread.

If it works, it works!

[mikeywoll]

Thanks for posting. Enjoyed the Soundcloud.

Thanks

Interesting that you are using an xFBxxx tank, as I am. I ordered the "correct" xEBxxx, it should be here in a few days.

I don't really see how xEB is "correct"?
Swart amplifiers use very similar (if not the same) reverb arrangement with xFB, Traynor (albeit EL84 driven - same principle) and Magnatone (early models with single triode of 12AU7 driven) to name another few I didn't run the calculations, but based on these - xFB seemed like the right one from the beginning, hence that's what I ordered and installed. And it sounds quite right to my ears.

Could you elaborate on your grounding scheme? I found it hard to pick it out in the photos.

I would be sincerely amazed, if you would in this mess (PR friendly statement would be - built in the best yesteryear traditions of Magnatone, Supro, etc...) The facts are that it sounds great and is dead quiet.

Here it is in short:
Reverb driver/recovery to the common (pots) bus and input ground.
1st, 2nd and PI stages to the respective tag strips' chassis lugs.
Supply to its own tag strip chassis lug (and HT center tap goes there).
I also used the DC filaments supply for the preamp tubes and referenced it to (a separate strip chassis lug) ground via two 100R resistors (prior to the small DC rectifier, filaments transformer secondary CT is not connected).

Hope that's of some use.

Will be looking to post some single lines samples as well. To me, it's more important how the sound plugs into a full piece, but I often find that some may see single-line samples more valid.

Niki

I think it's an impedance matching issue, which gets real confusing really quickly. The output impedance of that cathode follower is 450-ish ohms. If the input impedance of the reverb tank is lower you lose voltage, which really does not work in this context. Higher input impedance (like 2 to 3 X in this case) sheds current without any significant improvement in voltage transfer. I'm not sure how/if that effects things here. I thought long and hard about it, then decided if it sounds good, it is good. I would wager that either an E or F tank will work fine.

Mike

[Synchu]

You're both of course right and that's my bad - apologies - I am all the time talking standard anode coupled stage, while looking at a cathode follower...like hundred times while building it - idiot me.

Kids started going to school again so might get my focus back with just the day job

Still it sounds good...

Niki

[pullshocks]

Thanks for posting. Enjoyed the Soundcloud.

Thanks

Interesting that you are using an xFBxxx tank, as I am. I ordered the "correct" xEBxxx, it should be here in a few days.

I don't really see how xEB is "correct"?
Swart amplifiers use very similar (if not the same) reverb arrangement with xFB, Traynor (albeit EL84 driven - same principle) and Magnatone (early models with single triode of 12AU7 driven) to name another few I didn't run the calculations, but based on these - xFB seemed like the right one from the beginning, hence that's what I ordered and installed. And it sounds quite right to my ears.

Could you elaborate on your grounding scheme? I found it hard to pick it out in the photos.

I would be sincerely amazed, if you would in this mess (PR friendly statement would be - built in the best yesteryear traditions of Magnatone, Supro, etc...) The facts are that it sounds great and is dead quiet.

Here it is in short:
Reverb driver/recovery to the common (pots) bus and input ground.
1st, 2nd and PI stages to the respective tag strips' chassis lugs.
Supply to its own tag strip chassis lug (and HT center tap goes there).
I also used the DC filaments supply for the preamp tubes and referenced it to (a separate strip chassis lug) ground via two 100R resistors (prior to the small DC rectifier, filaments transformer secondary CT is not connected).

Hope that's of some use.



Niki

Niki, thanks for the grounding info. Lines up with the available pictures and what other PTP builds in this thread have used. Right now my Monarch build is in breadboard form, I will do a permanent build soon. All my other builds have used single chassis connection or 2 chassis connection buss grounding, so this multi-point grounding is new ground (yuk yuk) for me.

As far as the reverb tank, yes, the FB tank works. I will be interested to hear the difference with the EB tank. It will not be an apples to apples comparison, the FB is 2 spring long tank, the EB will be 3 spring short tank. But I will post my results. I also plan to try the FB tank with the 12DW7 plate driven reverb as discussed in the Guitar article https://guitar.com/guides/diy-workshop/ ... amplifier/. Supposedly it is copied from Swart.

[pullshocks]

The EB tank arrived today, sooner than expected. (Yay StewMac!)

I swapped back and forth between the 2 tanks a couple times, and at a few different reverb settings. My conclusion: both FB and EB tanks work fine in the circuit. I would say there is a bit more verb with the EB, and maybe a little brighter, and the 3 spring gives it a bit different character. But not a big difference at all, and I would be fine with either.

FWIW, my FB tank is an old Accutronics that is not coded so I am uncertain of the decay time. But the decay time sounds similar to the medium decay EB tank.

When I do my permanent build, I need to add a mute or standby switch, since sound gets through even with the gain on 0.

[mikeywoll]

When I do my permanent build, I need to add a mute or standby switch, since sound gets through even with the gain on 0.

I think we have something subtly wrong with the reverb circuit, maybe. I have this bleed through using both BobL's layout and using a point to point layout. There have been a handful of different point to point builds that report the issue as well despite having different lead dress.

Any thoughts?

Mike

[T Wilcox]

When I do my permanent build, I need to add a mute or standby switch, since sound gets through even with the gain on 0.

I think we have something subtly wrong with the reverb circuit, maybe. I have this bleed through using both BobL's layout and using a point to point layout. There have been a handful of different point to point builds that report the issue as well despite having different lead dress.

Any thoughts?

Mike

Just an observation but with the reverb tank not connected the volume shuts all the way off on my build. I added the reverb kill footswitch jack per the original pic posted by Aaron, it still gets through with reverb defeated but tank still connected
I have moved on as this is not a problem for me since with guitar connected and guitar volume all the way down I cannot even tell the amp is on
Still a head scratcher so will be interested to see if you guys figure this out

[Synchu]

This bleed seems to be expected, looking at how the reverb is connected and mixed back.

Niki

[mikeywoll]

Just an observation but with the reverb tank not connected the volume shuts all the way off on my build. I added the reverb kill footswitch jack per the original pic posted by Aaron, it still gets through with reverb defeated but tank still connected
I have moved on as this is not a problem for me since with guitar connected and guitar volume all the way down I cannot even tell the amp is on
Still a head scratcher so will be interested to see if you guys figure this out

This bleed seems to be expected, looking at how the reverb is connected and mixed back.

Niki

My initial thought was that it's a ground loop between the two (volume and reverb) pots or something to do with the insertion of the reverb onto the volume wiper and second pre-amp stage grid. Then I played around a little....

If I pull the reverb return cable, keeping the send attached, there is bleed through and there is no reverb.

If I pull the reverb send cable, with either the return attached or not, there is no bleed through and there is no reverb.

The signal to the reverb recovery stage is interrupted when either cable to the tank is disconnected, as expected.
There is no reverb on the bleed signal and the reverb knob does not effect the bleed signal.
The tone knob does effect the bleed signal.
The American/British voice switch does not effect the bleed signal.

It seems to me that the bleed signal originates after the first stage output and junction to the tone knob, before the voice switch. This basically only leaves the reverb cathode follower circuit as the source. The bleed signal is rejoining the circuit at the grid of the phase inverter, probably via the 10k split plate resistor. This route was suggested a while back (page 17). Back then, I disconnected the volume pot from the grid of the second preamp stage and grounded that tube grid. The signal was able to bleed through even with the second pre-amp stage effectively turned off. So, the signal is skipping the reverb recovery stage and the second stage of the pre-amp.

I think the signal is bleeding through the plate of the cathode follower, into the phase inverter through the shared power supply node, via the 10K split-load resistor. Why does this happen only when the reverb tank send is connected? Any ideas how to shut this down?

Mike

[Synchu]

I will run it through the o-scope next week.

Niki

[CraigGa]

I think the signal is bleeding through the plate of the cathode follower, into the phase inverter through the shared power supply node, via the 10K split-load resistor. Why does this happen only when the reverb tank send is connected? Any ideas how to shut this down?

Mike

Mike

How many power supply nodes do you have?
Later versions of the schematic have four and the PI and reverb are on seperate ones.

Alternatively what about trying this?
https://ampgarage.com/forum/viewtopic.p ... 06#p426406

Craig

[mikeywoll]

I think the signal is bleeding through the plate of the cathode follower, into the phase inverter through the shared power supply node, via the 10K split-load resistor. Why does this happen only when the reverb tank send is connected? Any ideas how to shut this down?

Mike

Mike

How many power supply nodes do you have?
Later versions of the schematic have four and the PI and reverb are on seperate ones.

Alternatively what about trying this?
https://ampgarage.com/forum/viewtopic.p ... 06#p426406

Craig

Hi Craig -

I have 4 nodes (22uf,47uf,22uf,22uf). I can't find anything on the image of the pre-amp we have that suggests we have anything wrong with the circuit, but I can't imagine that the real deal behaves this way. I have been searching around for a clear image of the reverb model power supply or the reverb pot wiring. Another power supply node would probably fix it, but I'm not sure there are 5 nodes in the real deal.

Looking around the web I found a couple other shots of production amps... A chimera reverb (4 6v6 monarch with different tone stack):

https://www.thegearpage.net/board/index ... t-24614188

And this page with what looks like a monarch reverb in assembly:

http://guitaristmarkmarshall.com/anatom ... tory-tour/

Both photos are a little blurry... there are two dual capacitor cans and an additional cap. It's hard to tell, but it looks like one of the can caps is wired in parallel, so a total of 4 nodes, like we have. I also only account for 2 power supply wires heading into the pre-amp.

Another approach I may try... Put the PI and pre-amp tube on the same node, and the reverb tube alone. The photo we have really does not look like that is the answer though.

UPDATE I rechecked my voltages tonight. They are just too spot on to the voltages on the schematic. I find it really hard to believe that there is a PS issue. The lefthand column are my direct measurements, the righthand column is corrected for the VVR circuit dropping a couple volts.

Screen Shot 2021-05-20 at 11.46.45 PM.png

Mike

[CraigGa]

I don't have the experience of a lot of you guys so I'm just throwing ideas in.

If the assumption is that the cathode follower is loading the B+4 then two things spring to mind,

Have you tried strapping an extra capacitor on the B+4 node?
What impedance reverb pan are you using?

Craig

[mikeywoll]

I don't have the experience of a lot of you guys so I'm just throwing ideas in.

If the assumption is that the cathode follower is loading the B+4 then two things spring to mind,

Have you tried strapping an extra capacitor on the B+4 node?
What impedance reverb pan are you using?

Craig

Great thought. I tried a bypass cap across the B+4 node a while back. It didn't effect things. Last night, I clipped a 30uf filter cap between the 10k split load resistor and ground. It reduced the hum in the reverb circuit, but did not extinguish the bleed signal. I haven't tried an independent (B+5) PS node for the reverb tube.

Another thought... Could the second stage triode be acting as a grounded grid amplifier?
https://www.tubecad.com/2014/11/blog0312.htm

If so, there would be a traceable signal on the cathode with the volume pot all the way down. That would mean signal sneaking in via ground. Unfortunately I don't have a scope to check.

I'm using an 8E tank. It's 800 ohms.

Mike

[CraigGa]

I'm using an 8E tank. It's 800 ohms.

Mike

How about putting 1K in series with the reverb tank to see if the problem is current draw through the cathode follower?

Craig