2004 SLO w/Deyoung Transformers versus New Bad SLO vid.

[fusedbrain]

It's simply for noise. And mostly for preamp tubes, not counting the PI position. Power tubes don't benefit from DC heaters, as far as I know.

I think what you're talking about might be DC elevated heaters. This is something different. And it's usually done to reduce the heater-to-cathode voltage differential in cathode follower preamp tube positions. Mostly in Marshall-style circuits. The cathode follower can be hard on preamp tubes, causing modern tubes with spiral filaments to die quicker.

The heater circuit's center tap is typically referenced to 0V (chassis ground). When you reference the heater's center tap to some higher, elevated DC point (like 45v DC, typically an already filtered/smoothed node), the AC heater circuit "floats" on top of a DC-referenced voltage instead of 0V/ground. This elevation helps reduce the strain on tubes in cathode follower positions. It can help reduce some noise, but it's really not intended to do that and the heaters are still AC.

Think of a sine wave. It's oscillating from +3.15v to -3.15, centered at 0v.

When you elevate the heaters, 0v is no longer the center reference. You reference the wave to... let's say +45v DC. Now your "center" is +45v. So the 6.3v AC heater circuit oscillates from +48.35v DC to -41.85v DC, with 45v DC being the new "0". The AC wave is still only 6.3v, it's just floating higher.

Because the new reference is +45v, the DC voltage differential between the cathode and the heater circuit is 45v lower, thus lowering the stress on the tube. The maximum voltage differential is listed in the tube's data sheet. So it's easy to calculate if you need to elevate the heaters and by how much.

Yes, this is correct, and coincidentally the cathode follower voltage at the fx loop send in the OG SLO is around 210v, and 12ax7s with spiral filaments will hum like a mofo and die a quick death in this case.
Max heater to cathode voltage for those tubes is typically 180v.
The BAD SLO solves this issue.

Many Marshall style circuits benefit from higher preamp voltages, so using elevated heaters in these designs will allow for the use of most current production tubes without issue.

 

[RedPlated]

Am I wrong in thinking a NFB knob would help open up the SLO, shifting the mids?

Yes, absolutely. I add a NFB control to all my SLO mods. It makes a massive difference and really opens the amp up. Once you turn it up, and then set it back to stock value, it sounds super dark and woofy in comparison. SLO’s have a lot of NFB stock. 39k NFB resistor and it’s on the 8 ohm tap.

I use the line out level knob hole to add the NFB. So no holes get drilled with the mod. I also add a 3 way depth frequency switch.

How many have you modded now?

That’s a good question. Actually been a lot that I’ve done now. Both 100’s and 30’s.

 

[scottosan]

Yes, I hear a difference in sound, but why would anyone assume the difference is due to the transformers? These two amplifiers have so many differences in their components, and then there's the aging process to consider. When you also consider that different tubes can color the sound differently, the transformer hypothesis sounds rather far-fetched.

Why is it far fetched? 2 transformers wound to similar specs be different companies will sound different