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Can a rebias alter the sound of the tubes?

I sent my Sonic Frontiers Line 1 back to the factory recently for a 'SE' upgrade. Rather than 'upgrade' to their current choice of tubes I sent along my carefully-chosen nearly-new set of Tungsrams and Electro-Harmonix and asked them to rebias the preamp to these tubes. Which they apparently did.

This tube combo gave a wonderful weight and holography to the sound, subtle but noticeable. Now it's gone. Careful A/Bing with a good solid-state pre I also own I can hear no difference in the sound. While this may not be a bad thing, it renders all the time, expense and results of tube rolling moot, and removes the sonic attributes of tubes, despite SFI gear sounding non-tubey by design. The improvement from the handful of upgraded SE components doesn't play into what I'm hearing here.

So can rebiasing of any tubed pre alter the sonics of the particular tubes? Would the factory bias settings return any tube to the neutral SFI 'house sound'? And if so how do I get back to the sound I once enjoyed? It seems wrong to intentionally set the bias off to get 'better' sound. I don't know the biasing procedure for the unit (yet), and I sure don't want to adjust my way into a black hole. What would Chris Johnson do? Or any other knowledgeable tube-heads?
128x128sdecker
sdecker's avatar
sdecker OP
143 posts
 
Well, I'll answer my own question for the benefit of those who didn't know:-) There are no tube bias adjustments on SFI preamps, and small-signal gear in general. On the fully-balanced Line X preamp series, the only adjustment is one pot per channel to balance the positive and negative phases of the waveform for minimum distortion. As I'm using the single-ended ouput, this should have zero sonic effect before or after as the phases are summed, despite the factory rebalancing the pots after my upgrade.

Nonetheless, *something* has changed to lose the characteristics of the tubes I had been using for a year prior. SFI suggests one month of my audio memory. While I agree with Jea48 if it ain't broke don't fix it, the SE upgrade does sound better, but is a minor improvement relative to how ones preferred tubes improve the sound.
jea48
4,446 posts
 
In your original question you said said you sent your preamp in to SFI with tubes of your choice and asked SFI to reset the bias for your tubes.Just because there are not adjustable pots the bias of tubes can be changed by changing the fixed resistance of the cathode bias resistor. I found it odd that SFI would do this for you.
sdecker's avatar
sdecker OP
143 posts
 
Jea48, I misspoke on my first post wrt SFI actually rebiasing the tubes. They simply reset the phase balancing pots to match my own tubes. I readjusted them recently as the 'SE' components they replaced seemed to have settled in further. I did put my OE sovteks back in for a baseline listen, and found the Tungsrams and EH tubes to in fact sound similar to how they used to, the difference was just not as dramatic. SFI suggests my 'audio memory' after a month w/o the preamp had faded...
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