Darkvoice and Crack Mods

I am ressurecting this threat thanks to the generosity of @roshambo123 giveaway and also allowing me to purchase a couple of upgraded tubes for this DV. You see, I got rid of my DV 336SE long ago in other audio pursuits. Now I have a better job and this is not my main amp and can afford to be out of commission a few days for modding.

I am also just now realizing that looking at the circuits of the Crack and the DV; straight out of the box, the DV is starting from somewhat higher place than the Crack. Mostly:

• Higher overall power capacitance
• Film caps already in the audio path for AC coupling

The Crack only has three 220µf caps on the PSU B+ (660uf total), while the DV has three 800µf on the PSU B+ (2,400µf), plus the DV added two more 200µf caps on the B+ line -one just for the power tube, one for the preamp tube. So 3,200µf total in the DV vs 660uf in the Crack - that is like 4.8x the overall capacitance.

Then, the Crack as electrolytics in the audio path on the final output for AC coupling and also setting the final output impedance. The DV uses film caps for this out of the box. Yes, you can mod the Crack for this, and probably more room to roll more exotic caps as well. However, I am not in any hurry as these are already propylene caps in the DV.

The most damming things and the greatest weaknesses in the DV is

• the transformer dumping 7V on the heaters causing hum for many tubes
• the crap linear taper volume pot with bad channel imbalance areas down low, and with it being linear, it means you are almost always in this area.
• cathode bias resistors on the preamp tube

The reason for the 7V on the heaters is because the DV 336SE has a 110V power transformer. It is no secret. Go look it up on the Drop page or elsewhere. It is clearly listed. When I put the DV on my variac and dropped it down to 110V on the output, I saw 6.3V on the heaters almost exactly, and no hum on the tube I was using.

So yes, I plan on:

• putting LED's on the cathode on the preamp tube with 2V forward voltage LED's
• get the 7V down to 6.3V on the heaters by using a voltage divider network of two resistors (100ohm and 1K)
• change the volume pot to Alps RK27 50K.

One side of the heaters is tied to ground, but there is another trick you can do with the heaters because there is no center tap on the heater is to create a fake one with a couple of resistors and tie it to the cathode bias voltage of one side...I did this in a guitar amp with much success. I will need to look up this circuit again and figure out how to implement this in the DV. If I do this, then I would need I think a 4k resistor for the power LED since this has two 2K resistors now on both legs.

 

So I think I fixed the hum issue without changing the bias or whatever. Even without fixing the high voltage.

I realized they are grounding one side of the 6.3V heater rails. It appears there is no center tap for ground. So I disconnected the ground on that one leg that is close to the front LED, then I did the virtual ground with two 220ohm resistors.



I have a new production EH tube that will not work in a stock DV. Hum city. Doing this works. Quiet as can be. And I am still measuring 7V on the heaters.

Now I could also do this and instead of float the heater ground on one of the bias of the cathodes.



But not sure that is necessary in this small simple amp. I’ve done this in guitar amps that have much more going on and more sensitive to hum. The ground seems to work great.

Now I racked my brain on how to combat the higher unregulated voltage with a 110V input transformer without having to change out the transformer. I could use a couple of diodes to reduce the voltage on the heaters by 0.7V, which makes it right at 6.3V. But that does not address the higher plate voltages and higher cathode voltages for proper operation point.

So I figured why not drop the mains by an inline resistor or two to drop 10V so the transformer sees the proper 110V. Well, there is a certain balance between power draw and voltage drop to figure what exact value you need. With some experimenting, it seems I need 33ohm resistor. It will need to be 5W resistor or more, and I better go even higher wattage than that really. So more on that later when I get the right resistors and get this completely figured out.

In the meantime, also enjoy the amber cathode bias LED’s.