250KA Presence Control - Mahalo

[sonicmojo]

Aloha,

I recently acquired a Mahalo DR20 combo amp - made around 2014 (no Hawaii affiliation as far as I can tell). It has a 250KA presence control attached to the PI output with a simple .0047uF cap on the Presence pot. I was a bit confused by this design as I have never seen a Presence control implemented this way. Thought I would share and ask if anyone has seen this presence implementation before and what you think. I am probably modifying this amp a bit as there is an unused triode on V2 and it is too low gain to my taste so wanted to soup it up a bit. The transformers can definitely handle more power. It came with 6v6s but I'll probably put in 6L6 or EL34s. I believe the Mahalo folks made this amp as either a DR20 or DR40 with the same circuit but maybe some slight cathode bias mods to handle different tubes. Well built amp for sure and I lived in Tampa for a long time where is which I think they are based!

Also, any suggestions on what I should do to beef this guy up would be appreciated.

Mahalo!

[sluckey]

It has a 250KA presence control attached to the PI output with a simple .0047uF cap on the Presence pot. I was a bit confused by this design as I have never seen a Presence control implemented this way.

Most people call that a cut control.

[martin manning]

Yes, cut control as seen on Vox AC15 and AC30.

Do you have a schematic of this Mahalo amp?

[Stevem]

Swapping in output tubes capable of higher wattage output will buy you nothing with out a bigger transformer set!

In fact bigger output tubes would just give you more clean head room if anything.

Both the 6L6 family and the 34s need more heater current then it’s likely that your power transformer now can provide, and the 34s need a lot more heater current so at that point you wound at least have to add a filament transformer just to power those.

Right now for more preamp gain you could add a 25 uf 25 volt cap across that other 1.5k resistor in the preamp section.
If you find that this adds too much overall bottom then you can roll off the use a 5 uf, or a .068 uf to kick up the mids.

[sonicmojo]

Yes, cut control as seen on Vox AC15 and AC30.

Do you have a schematic of this Mahalo amp?

Thanks! I don't have a schematic.

Swapping in output tubes capable of higher wattage output will buy you nothing with out a bigger transformer set!

In fact bigger output tubes would just give you more clean head room if anything.

Both the 6L6 family and the 34s need more heater current then it’s likely that your power transformer now can provide, and the 34s need a lot more heater current so at that point you wound at least have to add a filament transformer just to power those.

Right now for more preamp gain you could add a 25 uf 25 volt cap across that other 1.5k resistor in the preamp section.
If you find that this adds too much overall bottom then you can roll off the use a 5 uf, or a .068 uf to kick up the mids.

Thanks for the ideas on gain.

I considered the bigger current draw but it has a Classictone 40-18073 PT and 40-18006 OT so I think I'm safe with bigger tubes. I emailed Mahalo and they confirmed it should be fine. I'm pretty sure their DR40 model amp uses the same components but just different cathode biasing. It is a brand new amp but it sat around for years. GC kept dropping the price over time and I finally could not resist it when it dropped to $879. Apparently nobody in Hawaii but me would buy a "Mahalo" amp....go figure. It has a very nice 1x12 cab with WGS ET65 speaker.

[roberto]

Without changing the supply voltage nor the impedance, you will get absolutely no benefit from a different tube.
In AB1 PP pentode mode, the anodes already swing down to around 50V, and it will do it with higher wattage tubes as well.

On the other side, IF YOU VERIFY WITH THE BUILDER THAT THE TRANSFORMERS CAN ACCEPT IT, you can half the primary impedance of the OT and get more power from it even if the B+ is the same.
Typically 6V6 work with 8 to 10k Raa, and you can run bigger octals at half that and squeeze more power.

Do you really need it?

[Stevem]

Boy.
I hope the Anodes don't swing down as low as 50 volts!

When figure out voltage and current needs I like to have no less the 90 volts left to keep the tubes from fully cutting off.

[roberto]

Hi Stevem,

I don't know if you are used to draw loadlines, but a EL84 or 6V6 with standard 8 to 10k Raa will easily reach 50V on its anode staying in AB1.
Same happens with an EL34 and 4k Raa as written above, with standard Marshall voltages around 460V.
If we use 3k4 it will be around 70V in AB1.

Pentodes have been invented to be more efficient than triodes.

The fact that you have a personal preference, doesn't mean that everyone needs to follow it hoping that it won't be different.

[sonicmojo]

I appreciate all the ideas. I left the power section alone and incorporated some ideas from the Dirty Shirley details out there. It is Cathode follower before the tone stack. I used the extra triode on V2 that was available. It is mostly like a Dirty Shirley on the first 3 gain stages and the master/gain controls with a hardcoded cathode on V1B instead of any switch. There is something about the Friedman design that I like that enables the Volume control to not affect the gain control too much when you turn up or down. I think it has to do with the couple of voltage dividers in the mix on the Volume pot and the 2x470K divider before the third gain stage. Anyhow, I am very happy with the result. Much smoother distortion, more gain, and doesn't really need a pedal in front which I typically don't use. Rolloff on the guitar volume cleans up nicely.