D'Mars ODS EL84 quad

[Glennjeff]

Thought this may be of interest to someone. It's my D'Mars build schematic.

PLEASE CRITIQUE.

EDITED:  Power supply resistors and voltages changed. Made the cathode resistor a tiny bit larger for cooler bias

No cathode bypass on 5879. Changed trim and OD pot setup to tubnit's new D'Mars specs, thus less max drive with better pot control range. replaced 1M grid grounders in PA to 2M2.

I'm running a dual tap 5k2 / 6k6 primary on the output, set to 5k2. 6k6 primary would be more conservative on the valves. 4k primary looks like it might be a bit too hot and the EL84's cathode resistor would need to be increased ( ? 82 Ohm ? )

Will drop some sound clips in a day or so.


SECOND EDIT .

Put in a negative feedback circuit.

As NFB and PRESCENCE controls have the most profound effect of any control on the amp, I decided to make it variable with pots and switches.

Note the added 1000pF HV capacitor across PRIMARY of OPT XFORMER, to control radio frequency oscillation.

[tubenit]

I think your schematic looks fine overall.  I think Doug's Stout uses 470k grid resistors to ground. So, I am not sure about the 250k dual gang pot and 1M resistors being paralleled?

IF the OD is too buzzy/distorted for you consider trying some of the values in red around the OD section.

In the clean section, I might consider smaller values regarding the mid boost switching. But if the clean sounds good to you , then I'd leave it alone.

To the best of my memory, the different reviews of EL84  Dumblish type builds have mixed.  Some have been positive but comment that it sounds great but different and less Dumblish.

As a side note, I think a pair of 6V6's with a pair of EL84's could be a very cool sounding amp.

With respect, Tubenit

[Glennjeff]

I think the amp sounds good now. I have not heard enough Dumblishers to really know what others expect.

This amp does have characteristics quite similar to tubenits D'Mars sound clips.

Not sure the coupling caps need to be made much smaller, maybe a little.
Finding that turning the BASS control down as volume is turned up settles any bottom end flappiness.
Tweak to taste I guess.

Next I need to do a complete rebuild using a Hoffman layout and hopefully getting it on a Hoffman board. I'm hoping a few more turrets can be strategically placed on that board to accommodate the components for FX valve.

REGARDING pair of 6V6's with pair EL84's. That is how I usually run it.
The amp is set up for ( 2 X EL 84  ), ( 4 X EL84 ), ( 2 X EL84 + 2 X 6V6 ), (2 X 6V6 ), or ( 2 X 6L6 ).

Will post schematic soon of all cathode bias EL84 / 6V6 version. In the future, might prefer to fix bias the 6V6's and cathode bias the EL84's with some tweaking for even load sharing.

[DummyLoad]

Finding that turning the BASS control down as volume is turned up settles any bottom end flappiness.

add NFB from one of the speaker taps to 47R or 100R added under LTPI tail. copy deluxe reverb plan.

--pete

[Glennjeff]

pete

Had feedback circuit in there on a switch, as per original D'Mars, but it was causing some mains hum problems at high PPIMV. Might have to try again.

Thanks
Glenn

[tubenit]

it was causing some mains hum problems at high PPIMV.

I had that happen on my latest D'Mars.  I reversed the OT wires and it went away. Apparently at lower settings on the PPIMV, it's not an issue and then shows up as you increase the settings.

Geezer has a Dumblish amp that uses 100R & 4.7k
Deluxe Reverb uses 47R & 820R
Tweed BluezMeister is 47R & 10k
Tweed Overdrive Special is 47R & 4.7k

I had a NFB switch on my previous D'Mars & actually removed it to leave the NFB in place because I felt like the amp sounded best with it the NFB engaged. I felt like it was part of "the" tone.

Glenn, I like it that your amp can do the EL84, 6V6 thing!  Pretty cool, IMO.  Sounds like it would be a very versatile amp.

Looking forward to your sound clips.

with respect, Tubenit

[Glennjeff]

Well, well, well.

That was an adventure lost at sea. 

Took me three days to figure out and fix the mains hum caused by the NFB circuit.

It was actually a 250 kHz oscillation heavily amplitude modulated by mains hum.

Small high voltage capacitor across output transformer resolved this problem.

Now the amp sounds several different versions of awesome.

Thanks heaps to sluckey for the tech help. 

Now to work on the sound clips.

[Willabe]

It was actually a 250 kHz oscillation heavily amplitude modulated by mains hum.

Small high voltage capacitor across output transformer resolved this problem.

Wait a second, applied across the primary or across the secondary?


                  Brad     

[kagliostro]

I've seen Zobel Network applied at the primary and also at the secondary, give a look here



K

[Willabe]

Yes, OK, but, he said only 1 cap.


                  Brad   

[kagliostro]

One cap or one cap with an in series resistor isn't much different (to me)

EDIT: Glennjeff added this info on the first post

Note the added 1000pF HV capacitor across primary of OPT XFORMER, to control radio frequency oscillation.

Franco

[Glennjeff]

Willabe,

One cap 1000pF 600V ( 2 kV if you've got one ) across the OT primary, as per the edited first post and schematic.
If you think this is in error please explain why.
Perhaps a cap with series resistor would work just as well.
This cap has no effect in audio frequencies, unless you are a domestic cat.
The cap gives a low pass -3dB point of approximately 30 KHz.
With a resistor in series it would be a high shelf filter.
I just wanted to stop it being a radio frequency oscillator.

As I make improvements or solve problems I change the first post and update the schematic accordingly.
That way there doesn't end up being 500 different schematics scattered around all over the place.
If this practice upsets people please let me know.

All the best
Glenn

[sluckey]

My little Pee Wee amp (converted Hammond AO-44) has a 10K and .001µF across the OT primary. It was on the original so I just left it in place.

     http://home.comcast.net/~seluckey/amps/PeeWee/PeeWee.pdf

[Glennjeff]

SOUNDCLIP Clean Neck Single Coil (Tom Anderson Hollow Drop Top)


https://soundcloud.com/glennjeff/a-spanish-minor

[tubenit]

OK, I had a chance to listen to this thru some good headphones.

VERY nice tone!  Sounds remarkably transparent and clear.  Very musically sweet harmonics in your sound clip, IMO.

IF you have an OD sound clip that you like of this amp, I hope you will share that also.

The EL84's are a little bit chimier then the 5881's.

With respect, Tubenit

[Glennjeff]

A four piece band context for EL84 X 4 rhythm and lead guitar sounds.

https://soundcloud.com/glennjeff/quick-skip-shuffle

[tubenit]

Really nice playing!  I thought that had great tone and the tone around :40 is really fantastic.  Nice pinch harmonics with the OD stuff.

THANKS for sharing it!   With respect, Tubenit

[Glennjeff]

Happy Days,

My shipment of board and parts just arrived from Hoffman's so now I can start the rebuild.
Ordered some text books as well so I can do all the math and make sure the engineering is OK.

Here is a tentative update to the schem, tried some MOVs instead of capacitors to quell the OT oscillation, blew a couple up before got it OK. Still need to work on this part a bit.
Twerked a few values here and there to my taste as well.