Tube Amp driver tubes question

This is kindof a stupid question, but I thought I'd see if someone might provide some insight

In my amp, I have four driver tubes and four power tubes. (I have a Golden Tube Audio SI-50 MKII)

I read in a forum post that to maximize my sound, I should have two 12AU7 tubes in the PRE-AMP section and to place two 12AX7s in the driver.

There are TWO tubes by the volume (at the front of the machine) and TWO tubes on the same circuit board as the power tubes.

Which ones are the PRE-AMP? The one by the volume?

 

Pretty sure that would be correct, but this makes it looks like the 12AU7 are the driver tubes and the preamp tubes. I wouldn't think increasing the gain on the driver tubes would not be a good thing, but maybe I'm wrong.

 

If dolsey01 has posted the correct schematic, you should have 4 of the 12AU7 and no 12AX7 at all. Of course, you say that you have a MKII and his schematic does not indicate that. No idea if there were any changes to the tube compliment with the revision. Also, most designs that do use 12AX7 and 12AU7 tubes do it the other way around, with the 12AX7 as the voltage amplifier in the preamp section, and the 12AU7 as the phase inverter in the driver / output stage. It would hurt nothing to try it that way and see if the sound was better. Using a 12AX7 on the board near the volume control will result in higher gain overall. That may make the sound more dynamic and have greater soundstage, or it may just add noise and diffuse the sound if you are not using the additional gain. If you can use 4 of the 12AU7 tubes and not have to turn the volume control past 12 o'clock, that is most likely the most transparent and warmest configuration. I'd not install 12AX7 as a phase inverter unless the design specifically called for it, and that would be a bit odd in a modern tube amp design. Many times the circuit board is actually stenciled with the tube type right beside the tube socket or in a location that provides a key, such as v1 is 12AU7, etc...
-Bill.

 

Thanks for some of the feedback! I am a newbie to all of this.

Here is what I found on an audio forum:



If you happen to own this entry-level tube integrated, please do yourself a favor:

1) Triple up the power supply’s capacitance from 2 X 470 mf to 6 X 470 mf. Do not forget to shunt 470s with the smaller capacitors to reduce noise. If you do not have that optional phono stage installed, there is the spot available.

2) Replace 12AU7 drivers with 12AX7s (Sovtek will go). KEEP 12AU7s in the pre-amp section.

3) Roll stock EL34s with the matched Svetlanas, and let the entire mod to burn-in for 20-30 hours.

4) Enjoy.

That "signature" SI-50 mkII veiling, soft dynamics, and decent lack of focus should go away now, making your unit a serious contender among push-pull integrated amps

 

It is well meaning but perhaps not accurate. The 12AU7 is used as a power amplifier phase inverter (some people call this a driver) in many tube amplifier designs as it is capable of delivering much more current than a 12AX7. It's function is to balance the AC signal and to supply voltage and current for the power tubes. The preamp or actual voltage amplifier section of tube amps often use the 12AX7 as it offers much more voltage gain and current demand is very low in that position. So the tubes are chosen based upon their function and location in the overall amplifier circuit for their operating characteristics. The author of that post may not realize this or may have just liked the way a particular pair of 12AX7 tubes sounded vs the originals. Or he may have done several mods at once and attributed one behavior to another source. The comment about the capacitance increase is perhaps OK as the unit appears to have a solid state rectified power supply. You would never want to do that with a tube rectifier. Basically my take on this is that the amp designer probably knew best and made their choices based upon their design goals and desired sound output. The more popular SE40, I believe, was a single-ended design which had a rather soft and warm sound signature. Not everyone's cup of tea, but it was optimized to do what it did. My response to hearing this amp and not favoring its characteristic sound would be to sell it and buy another amp that I preferred. Modding is usually trouble for subsequent owners as it isn't always (seldom in my experience) done well or correctly and often makes performance worse, no matter how proud the butcher, ah... "modder" feels. It has been a while since I repaired an SE40, as even that wasn't a very common amp.

You can try swapping any of the 12A type tubes with each other, likely without dire consequence, but usually with performance drawbacks. The tubes in this configuration are rated for voltage gain and current and the amount of each is the inverse of the other. In other words, higher voltage gain means lower current delivery, and vice-versa. In order based upon the voltage gain from highest to lowest, they are 12AX7 (100x), 12AT7 (60x), 12AU7 (20x). Conversely, these same tubes have plate dissipation or current output capabilities that are rated in the opposite direction; 12AX7 (1w), 12AT7 (2.5w), 12AU7 (3w). So you can perhaps better see which one is used where and why. It is possible to design an amp that uses 12AX7 in all positions or 12AU7 in all positions, etc. You can thus try swapping these and listen for effects but my advice is to leave the ones that the designer intended where they belong. With some designs, the amp could be at the border of a particular spec and using the wrong tube could either severely limit a performance parameter or in the worst case cause damage to the amp. While modding may be fun, it can also have consequences. A little bit of knowledge is a dangerous thing and I am a fairly dangerous man.

-Bill

 

Bill - your knowledge and insight is amazing. Thank you so much for taking the time to share your thoughts on the tube situation. I am going to stick with 12AU7s in my amp. I like the sound that the amp has, so that hasn't been a problem. It's more of the "the grass is always greener" mindset......I read the post and started thinking "Oooh....maybe I'm missing something."

Now the question for me to explore is which brand of tubes to go with! Hmmm......