SFDR Preamp Tube Question

[Robsocal]

My tech installed a 12AX7 in V1, and I have a "Fender" 7025 in V2. The amp sounds great, but I want to know if anyone thinks this is the best tube complement for these 2 slots (e.g., should the 2 preamp tubes match?). I plug into the Vibrato channel almost always because I love the reverb and Tremolo. I tried removing V1 to see if I could hear any difference compared to leaving V1 in. I may be in the minority, but I like the sound better with V1 in -- it seems smoother and cleaner.

All that being said, I love my SFDR.

[milocj]

A 7025 used to be a lower noise version of a 12AX7 but it had the same properties. For the most part now I think they are identical (if they're new production). Fender used to spec the 7025 because it was the first tube in the signal chain and the lower noise meant you wouldn't have that little bit of extra noise amplified multiple times through the signal chain.

V1 only works in the normal channel and V2 only works in the vibrato channel so having matching tubes makes no difference at all...it only matters which tube you like in that slot. If you hardly ever use the normal channel you could put a crummy sounding 12AX7/7025 and should have no effect on the vibrato channel.

I can't remember exactly why, but when you remove the tube from V1 or V2 and use the other channel you get some added gain (less drain on the amp with one less tube?). That's why the amp sounds smoother and cleaner to you when you have both tubes in. You would get the same effect if you plugged into the normal channel and pulled V2. You could try swapping the V1 and V2 tubes to see if you hear a difference when using your vibrato channel and then you would be hearing any differences between the two tubes.

[Robsocal]

Hi milocj,

Thanks for your thorough, and insightful, response. These Fender tube amps are great. I just want it to be the best it can be.

Rob

Originally Posted by milocj

A 7025 used to be a lower noise version of a 12AX7 but it had the same properties. For the most part now I think they are identical (if they're new production). Fender used to spec the 7025 because it was the first tube in the signal chain and the lower noise meant you wouldn't have that little bit of extra noise amplified multiple times through the signal chain.

V1 only works in the normal channel and V2 only works in the vibrato channel so having matching tubes makes no difference at all...it only matters which tube you like in that slot. If you hardly ever use the normal channel you could put a crummy sounding 12AX7/7025 and should have no effect on the vibrato channel.

I can't remember exactly why, but when you remove the tube from V1 or V2 and use the other channel you get some added gain (less drain on the amp with one less tube?). That's why the amp sounds smoother and cleaner to you when you have both tubes in. You would get the same effect if you plugged into the normal channel and pulled V2. You could try swapping the V1 and V2 tubes to see if you hear a difference when using your vibrato channel and then you would be hearing any differences between the two tubes.

[Singin' Dave]

Good article here

http://www.kcanostubes.com/content/newsletter_details.asp?ArticleID=5

[PhatTele]

V1 and V2 share a cathode resister. The regular (if you will) value for those cathode resisters is usually 1.5K. Since this particular one is shared, the value is halved to 820 ohm to keep the performance "normal" for both tubes. When you pull V1 or V2, the lower 820 ohm resister causes the remaining tube to run a bit hotter and sound a bit overdriven.