Re-Bias Pre-amp tubes?

[Jamingguitarist]

Quick question If I where to replace my 12AX7 preamp tube in my V1 position with a 12AT7 tube do I have to have someone re-bias them, or is that only for power tubes?

[Matt Syson]

Hi
Strictly speaking they should be rebiassed but this is mainly to get the operating point nearer to 'optimal' for that kind of valve. In terms of it's safety or reliability most small valves of the correct pinout will work mostly giving a change in gain and distortion characteristics which is usually the reason for changing them.
On some amplifiers the anode resistor is rated a bit close so increasing it to same value but 1 Watt or maybe 2 Watt rating would allow swapping without fear of a burnout.
Matt S

[jamsmith]

Its been a while, but I think most amps use self-bias (cathode bias) on their preamp tubes. Ampeg did this on power tubes also. You can read more on this than. Bias adjustment is more critical on power tubes in a AB pull-pull circuit to eliminate crossover distortion. If you are going to do anything with tube amps, this is where you start:

The Tube Amp Book: Deluxe Revised Edition by Aspen Pittman

[Wes Kuhnley]

You absolutely DON'T need to re-bias pre-amp stages in a guitar amp. In 99.99% of guitar amps, these are cathode biased stages, which bias themselves essentially.

Power tubes are a different story, and depend amp to amp whether biasing is necessary. One thing that is really nice about using Groove Tubes is their proprietary power tube rating system. When you have an amp biased with (for shits-n-grins) #6 EL34's, you don't HAVE to have your amp re-biased if you replace those tubes with the same type (EL34) and number (#6), though you can go to 5's or 7's as well.

Anyone who tells you to pay them to re-bias your pre amp tubes is full of shit.

[DamnYankee]

Originally Posted by Matt Syson

Hi
Strictly speaking they should be rebiassed but this is mainly to get the operating point nearer to 'optimal' for that kind of valve. In terms of it's safety or reliability most small valves of the correct pinout will work mostly giving a change in gain and distortion characteristics which is usually the reason for changing them.
On some amplifiers the anode resistor is rated a bit close so increasing it to same value but 1 Watt or maybe 2 Watt rating would allow swapping without fear of a burnout.
Matt S

I've never re-bias'ed preamp tubes in my life...

[Starang]

also worth noting, groove tubes is a RESELLER. they rebadge sovteks and the like. remember where tubes come from? eastern europe and china. there was a website (years ago, long forgotten) that did the same thing, i told them my GT #, and i got the same tubes for waaay cheaper than matched GTs.

[Wavebourn]

Cathode bias resistors are selected such a way so a voltage drop on a cathode resistor is a negative grid 1 bias. This voltage depends on a current, so it is kind of a negative feedback by DC current, and the tube during it's age will live happily without needs to be rebiased. But for different types of tubes you need different cathode resistors, because it needs different anode current and grid 1 voltage to perform in so called "sweet spot", i.e. when it performs the best. Each tube has own sweet spot, and even famous Telefunken tubes have it, and change with age, so if you want to get a reliable stage from the tube cathode bias may be enough, but rebiasing may squeeze the best from it, though it requites skills and careful measurements. Especially, when you swap tubes that are known to have different characteristics from design.

Originally Posted by DamnYankee

I've never re-bias'ed preamp tubes in my life...

It means, you never designed a stuff that will be "over-biased" when tubes are fresh, but "under-biased" when they are dying. Designing a tube stuff you would had to remember about tolerances and aging. Cathode bias helps a lot to simplify exploitation of a gear without attention to details, but it does not mean that completely different double triodes with the same pinouts may be freely swapped without a need for adjustment.
What may be more significant, other elements of a gear may need to be adjusted more than cathode bias resistors. There are well known issues when real NOS tubes in a gear designed with modern Chinese replacaments starts to oscillate...

Originally Posted by Starang

also worth noting, groove tubes is a RESELLER. they rebadge sovteks and the like. remember where tubes come from? eastern europe and china. there was a website (years ago, long forgotten) that did the same thing, i told them my GT #, and i got the same tubes for waaay cheaper than matched GTs.

It is not bad if when relabeling they are measuring them; the real problems is when tubes with the same name have spread of parameters from Kaluga to Petropavlovsk.