Dumble Champ Schematic

[Bwmitc9285]

I've been toying with this idea for a while now and finally decided to sit down and draw up a schematic. Its basically just the clean preamp from a 70's ODS (https://schematicheaven.net/dumbleamps/preamp.pdf) mated to the power section of a champ. I'll be using a Weber WSE15 output transformer and it will be diode rectified. This will be my 10th amp build but my first time designing a schematic so If someone wouldn't mind to check this schematic for me I would greatly appreciate it!

[shooter]

looks like ur boat floats.  The switching complexity wants to be carefully considered when building.

[jjasilli]

I didn't scour the schematic, but your Presence control won't work.  All NFB will bleed to ground through the K bypass cap.  To correctly use NFB AND a bypass cap look at the VibroChamp schematic, e.g.

[PRR]

Plate power comes from.... ?? (I know you know, but good to draw it.)

Agree the Presence is no-go as drawn.

[Bwmitc9285]

Thanks for everyones input. I'm still learning! After doing some reading I think I've come to the conclusion that presence controls aren't possible for SE amps, so I think I will remove it and add a variable NFB control in its place.

[sluckey]

I've come to the conclusion that presence controls aren't possible for SE amps, so I think I will remove it and add a variable NFB control in its place.

Presence is very doable in a SE amp, but not as you have drawn. You'll also have the same problem with NFB in your circuit. Look at the Vibro Champ schematic, or just remove the 5µF cathode bypass cap.

[Bwmitc9285]

I think I see now. The NFB should be connected after the bias resistor instead of at the cathode?

[sluckey]

Just remove the cap...

[jjasilli]

Sluckey, PRR & I are saying the same thing.  Sluckey's drawing works.  But it removes the bypass cap.  If you want the bypass cap for the gain boost, then use the VibroChamp schematic.

[PRR]

> Just remove the cap...

Or cut a switch into it. Between switch and pot you have three cases.

[jjasilli]

Or do it the VibroChamp way, see schematic.  This way the NFB is lifted from ground by the 47 Ohm R.  (So are the cathode R & bypass cap.  But the 47 Ohm R is so small it has essentially no effect on them.)  The NFB travels UP through the bypass cap, around the 1.5K cathode R, and into the tube where it EDIT: "boosts" gain reduces the hi frequencies in its bandwidth.


Your way: The NFB bleeds down through the bypass cap to ground.


(This does not address values for a more complex Presence control to tweak NFB.)

[Bwmitc9285]

Thanks for the explanations PRR and jjasilli. I've updated the schematic to match the Vibro Champ. I also attempted my first layout using DIYLC. The software is fairly limited so it doesn't look as great as the ones you pros upload. I'm sure there might be some mistakes so I am going to keep going over it but I figured I could post it here and let you all look at it in case there are some obvious errors. Thanks again for everyones help! 

[jjasilli]

Now you have the idea; but it's not clear your Presence control component values will deliver as hoped for.   You've cut & pasted a PI style Presence circuit into a different setting.  It has significantly different component values, and a higher source voltage, than the VibroChamp NFB circuit. 

What I would do 1st is get a simple NFB circuit working; maybe make it variable.  Note the NFB voltage -- or if variable, the NFB voltage range -- that I like.  That way there's a benchmark for further tinkering.  Then add a Presence control to the NFB circuit.

[shooter]

You might find on SE amps there's enough "presence" tweaking the TS.  I only used NFB in 1 build, none had presence.  They are both worth trying, then un-trying and compare

[jjasilli]

My VibroChamp (hotrodded with 6L6, etc.) had a fizzy sound with every speaker I tried.  I got the fizziness to go away by "increasing" the global NFB voltage & re-installing the local NFB cap.  (By "increasing" I mean reducing the amount of voltage division.)


N.B.:  The VibroChamp has both global NFB; and local NFB @ the power tube: cap from G1 to K.

[DummyLoad]

presence with bypass cap. tune 1K for desired results. parallel 5K pot with 4.7K for finer control, or use a 2K pot. 


--pete

[Bwmitc9285]

Made some tweaks: Removed the presence control (I agree it may not make enough difference), removed the local negative feedback on v1b (More gain isn't a bad thing), removed the deep switch, added a master volume and added an overdrive channel. I may need to add a voltage dropping resistor between the diodes and B+1. What do you guys think?


 

[VMS]

Overdrive channel is inside the nfb-loop so that could cause some problems, not sure.

[Bwmitc9285]

Thanks for catching that VMS. I've updated the schematic and layout.

[tbn]

I Recently built a dumble-champ (with OD, as in your latest schematic), only with a switchable 470 / 330 ohms cathode resistor on the powertube, so I can run 6L6's as well.

I left out the nfb-loop from the champ, but I did put in the local negative feedback on the second preamp stage on a switch, so I could hear the difference. I can recommend putting that LNFB in, since it makes both the clean sound as the OD-sound much more defined.

There is plenty of gain, even when using an 1Meg resistor to the OD-trim pot I keep the trim in the first quarter of its range and get marshallesque drive.
I can also recomend putting that trim pot in at the input of the OD-channel for the same reason :)

[HeavensThunder]

Any video/sound samples of one that was built?

[HeavensThunder]

Has anyone confirmed this build?

[tubenit]

https://soundclick.com/share.cfm?id=9089004


Check out Mattie's mini-HRM.   More info about it on The Amp Garage.


Or his EF86 Special   


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You'll have to a search on both forums for more information.


with respect, tubenit