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Experiments with "UltrPath" connection

Posted by Kurt Strain on March 6, 2001 at 00:01:39

I added an "UltraPath" connection to my three DIY tube amps: 45 SET, KT88 SE, and 845 push-pull. I added 40uF from B+ to cathode on my 45, 20uF from B+ to cathode on my KT88, and only 9uF from B+ to cathode on my 845. It seems not to matter that I only used 9uF or 40 uF, they all work very similar.

The sound of all of these improved in almost all areas except noise and hum, which got worse. Hum appears to increase proportionately to the gain of the stage, at least the amount of B+ caused hum. It will really tell you how well filtered you are and if you have some buzz to remove. You can increase filtering to remove it if it's bad enough. On my 845 PP with its relatively light filtering it really increased the buzz and 120 Hz hum. I added a large 200 ohm 20W resistor (should be a 50W to be safe) in between SS rectifier and input cap. Here's where resistive tube rectification shines, but adding the resistor seems to me as good as a tube.

All amps improved very much in bass on up, first noticed is the bass improvement. But the articulation increases in the midrange with no penalty to the harmonic structure and all the way up through the highs. Microdynamics increase in speed pretty dramatically, or put another way the pace really quickens.

I had the 845 on the Avantgarde Duos tonight and with 50W/ch class A power it was just slamming rock and roll and classical. I have never heard such agility with such volume - got a little carried away.

But the really good amp is that 45. Best timbre, best liquidity, most "correct" pacing, not slamming. No noticeable "horn sound" going on at all with that amp. With the 845 you know it's a horn, but a real good one.

With the UltraPath I can achieve PP bass and treble extension with a SET amp. But with UltraPath on a PP amp I get extreme dynamics and control, seemingly too much and a bit forced, but it really rocks. I'm sold on this concept. It really works, and it's an easy mod.

See the link below for a schematic of my 45 output stage with two paths of "positive feedback" - one from the parafeed and one from the UltraPath connection. Best output stage I ever heard.

Kurt

• 45 Parafeed Output with UltraPath

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