Another pre-amp tube question.

[zeegler]

I've been messing around with the settings on my Randall, and getting rather frustrated. I started to think I should change out the pre-amp tubes since I already changed the power tubes.

My amp has 1 - 12AT7, 3 - 12AX7A, and 1 - 12AX7-7025 low noise.

Can someone enlighten me as to the reason for the different tubes, and the difference between them?

The 12AT7 and the 7025 were in the wrong positions, and I switched them back to where they were supposed to be. It made a noticeable difference to the sound of my amp.

What would be the best way to get more gain out of the amp? Will different brands (I'm thinking either JJs or EH)provide better gain response?

[CZSER]

Re: Another pre-amp tube question.

Well Zeg, me to the rescue!
The 12AT7 is a less gain preamp tube.
The 7025 low noise usually goes for V1, the first & more amplified.
So, and I'm just guessing, if the AT7 were at V2 & the 7025 at V1, I would say someone did that to lower general noise an to reduce lead gain.
I supposed that position because V1 & 2 are the ones that affect the amp sound. In second place, V5 (the closest to power tubes). It's important to use a balanced one.
V3 & 4 doesn't affect to much overall sound.
In your place, I would buy 2 high gain JJ 12AX7 S (Bob @ Eurotubes selects them)for V1 & 2 an a balanced one (also Bob does) for V5. Leave the existing V3 & 4(for saving some bucks).
Tell me more about the tubes positions. Have to go now. See ya. SERGIO

[toejam]

Re: Another pre-amp tube question.

I ordered four 12AX7 EH tubes yesterday from http://www.tubestore.com/ and the UPS driver should be here any minute with them. [img]/images/graemlins/smile.gif[/img]
Their review said they're very low noise and help quiet microphonic problems in noisy amps and that they work great in all positions, so that's one of the reasons I decided to go with them. I'll report back when they're here and in my amp. http://www.tubestore.com/ehx12ax7.html

[zeegler]

Re: Another pre-amp tube question.

[ QUOTE ]
Well Zeg, me to the rescue!
The 12AT7 is a less gain preamp tube.
The 7025 low noise usually goes for V1, the first & more amplified.
So, and I'm just guessing, if the AT7 were at V2 & the 7025 at V1, I would say someone did that to lower general noise an to reduce lead gain.
I supposed that position because V1 & 2 are the ones that affect the amp sound. In second place, V5 (the closest to power tubes). It's important to use a balanced one.
V3 & 4 doesn't affect to much overall sound.
In your place, I would buy 2 high gain JJ 12AX7 S (Bob @ Eurotubes selects them)for V1 & 2 an a balanced one (also Bob does) for V5. Leave the existing V3 & 4(for saving some bucks).
Tell me more about the tubes positions. Have to go now. See ya. SERGIO

[/ QUOTE ]

Thanks Serg! The pre-amp tubes are in this order looking at the front of the amp: 7025, 12AX7, 12AX7, 12AX7, 12AT7. Following the schematic, Channel 1 goes: V1A, V2A, V2B. Channel 2 goes: V1B, V3A, V3B. Then, both channels go into V4A, V4B. I don't even see V5 on the schematic.

[CZSER]

Re: Another pre-amp tube question.

is the AT7 the closest to the power tubes? IF so, should not be safe to replace it with an AX7, because the AT7 handles twice current than the AX7(with 70% of the AX7 gain). If it's designed so, you may screw up an AX7 placed there

[zeegler]

Re: Another pre-amp tube question.

Well, now that I have switched the 7025, and the 12AT7, yes the 12AT7 is closest (in the circuit) to the power tubes. Physically speaking, all the pre-amp tubes are lined up in front of the power tubes. So is it possible that my 7025 is damaged? What would happen if I replaced the 7025 with a regular 12AX7? More gain? Less gain? More noise? Fire? Volcanic eruptions?

[Shawn Lutz]

Re: Another pre-amp tube question.

The 7025 is the same thing as a 12AX7 so they should be able to be changed out without any problems, same tube different name. usually a 12AT7 is a lower gain tube used on the clean channel of a 2 channel amp. Some people use them to tame the gain on certain amps as well and just us them in place of 12AX7's

Spend some time on tubstore.com they have some handy info on there http://tubestore.com/gainfactor.html they also have great customer service. I bought some THD Yellow Jackets and some JJ EL-84's from them and got them the very next day [img]/images/graemlins/smile.gif[/img]

I would go for the same tubes that Toejam got http://tubestore.com/ehx12ax7.html

[CZSER]

Re: Another pre-amp tube question.

Always can replace a 12AX7 for a 12AT7. The result will be less gain IF the change takes place at any of the channel's gain stages(usually V1 & 2). In the other way, replacing a 12AT7 for a 12AX7 (which handles half current than first)is safe or not depending of the current flowing there. In the worst case, you harm 1 preamp tube (the 12AX7 in 12AT7's place).
Other thing, do your amp have spring reverb? If so, the 12AT7 is the reverb recovery tube for sure.

[zeegler]

Re: Another pre-amp tube question.

Yes, the amp has a spring reverb unit. I figured one of the tubes was something to do with that. So I'm wondering, since the 12AX7 and the 7025 are the same tube, why does my handbook specify both tubes in different locations? The 7025 says "low noise" on it.

[Soloist1]

Re: Another pre-amp tube question.

The 7025's are just top of the line 12ax7's that are less likely to be microphonic. They are extremely high gain. They are often used in the first socket because they really hammer the other tubes in the channels gain stage. You can put the 7025 where ever you like. You could put all 12ax7a's or just 12ax7's. It's a matter of taste. I have a mix of all different brands and type 12ax7's in my preamp. If you wanted to tame an amp that is a bit to harsh gain wise you could use a 5751. Like the 12at7 it is 70% the gain of a 12ax7 but has the same plate resistance like a 12ax7. The 12at7 has a different plate resistance.

[fett]

Re: Another pre-amp tube question.

Check out Vacuumtubes.com. lots of info on numbers and characteristics of tubes. You guys ought to start using vintage tubes. [img]/images/graemlins/smile.gif[/img]

[Soloist1]

Re: Another pre-amp tube question.

I agree with Fett. The tubes in my preamp are used tested vintage tubes and they blow any of the new tubes away tonally.

[fett]

Re: Another pre-amp tube question.

Hey! Somebody agreed with me!!!! [img]/images/graemlins/headbang.gif[/img] [img]/images/graemlins/toast.gif[/img]

[toejam]

Re: Another pre-amp tube question.

[ QUOTE ]
Yes, the amp has a spring reverb unit. I figured one of the tubes was something to do with that.

[/ QUOTE ]
Not all spring reverb tanks are driven by a tube. My Mesa has a long-spring Accutronics tank, but is NOT run off of a tube. Some other amp companies do this, too, but I can't think of who offhand. Don't know if this helps or not.

[zeegler]

Re: Another pre-amp tube question.

I'm gonna pick up a few 12AX7s on the way home from work, and try a few things. I hope I don't blow nuthin' up. [img]/images/graemlins/laugh.gif[/img]