Rick Davis
673 posts
Sep 14, 2012
1:14 PM
|
I know there are some who try the 12AT7 as a method to lower the gain in the preamp section of a tube amp, and I've heard from some players who swear by it. But the 12AT7 tube is a different animal than the 12AX7. It is intended for a different purpose and is not really compatible in circuits designed for the 12AX7 tube.
Here is an article from Guitar Amplifier Blueprinting that deals with the rectifier tube. (scroll to the top of the 2nd page.) Fascinating stuff, but it also explains why the 12AT7 is more suitable as a phase inverter or device drive. It's standing current output is 10 times greater than the 12AX7, and it's transconductance is 3 times greater. Everything downstream of it will be effected negatively.
A much better choice is 5751 tube. It has 70 percent of the gain of the 12AX7 with the current and transconductance being more closely matched. I've tried both tubes and I can hear the difference. The 5751 is a MUCH better tone generator in a tube amp.
----------
-Rick Davis
|
9000
115 posts
Sep 14, 2012
2:48 PM
|
I don't really understand the electronics/physics involved but my successes with a 12AT7 have been reserved for the phase inverter.
This is worth looking into if you have an issue.
All the best,
Jay
----------
Music speaks where words fail.
|
Rick Davis
674 posts
Sep 14, 2012
3:00 PM
|
yeah, the 12AT7 is great as a phase inverter. My point is that it is a poor tone generator in V1 of the input section.
----------
-Rick Davis
|
MJ
473 posts
Sep 14, 2012
2:57 PM
|
I have read in the past that the 12AT7 was a way to go when wanting to tame the gain. I tried it for a while until I read the same article that Rick has posted some time ago. I have since 5751s in the #1 slot. Some prefer a 12AY7 which is also fine and a matter of taste. It seems that the platitudes of using a 12AT7 are urban legend and unfortunately, urban legend all too often carries the day. Thanks for posting this article Rick.
|
walterharp
936 posts
Sep 14, 2012
6:26 PM
|
It seems like the main point in the article is what I have found.. and figured out should be true. If most of the breakup is in the preamp section, and you want that tube distortion, then you should use high gain tubes in early position to drive the later tubes harder. It is counter productive to use a low gain tube early... it will mainly tame feedback because it gives you more sweep on the volume knob.. but not much breakup because you hit the later tubes not so hard and the power tubes don't contribute much of that tube distortion. Sure you can crank a bassman up to 12 with a 12au7 in the first position (ok 8 maybe...) but it gets more breakup if that same tube is in phase inverter and the higher gain tubes are earlier. Try it.
|
Greg Heumann
1784 posts
Sep 14, 2012
6:33 PM
|
I agree - a 5751 is a much better choice. I also agree with Walter - the 1st tube should be a high gain tube. I use 12AX 12AU 12AU or 5751 12AU 12AU. Works great.
----------
/Greg
|
Rick Davis
675 posts
Sep 14, 2012
8:40 PM
|
Greg, I use 5751, 12AU7, 5751 in my Bassman. I tried every possible combination (I think) and this mix sounded good. Or maybe I just got tired of it all, so that's where I left it.
I'm happy with the tone and volume. Actually, I needed a bit more amplitude because my band has a rather high stage volume, so that might explain the higher gain tube in V3.
----------
-Rick Davis
|
Jehosaphat
295 posts
Sep 14, 2012
10:49 PM
|
Do you guys think that your tube configs for the Bassman would be a good choice for a Blues Deville 410 RI too?
|
Rick Davis
678 posts
Sep 15, 2012
8:36 AM
|
Jehosaphat, I have never tried swapping tubes in a Blues Deville, but I would think it would be a good idea to try a combination of the 5751 and 12AU7.
My favorite place to buy tubes is Tube Depot in Memphis. http://tubedepot.com/
-The 5751 I recommend is the NOS JAN Philips, $34.95.
-The 12AU7 I like is the NOS Sylvania Triple Mica 5814A, $19.95.
I would suggest you buy two of each (sorry, I know, kind of expensive) so you can try different combinations in your amp and have a backup tube.
Good luck, and let me know how it sounds.
----------
-Rick Davis
Last Edited by on Sep 15, 2012 9:07 AM
|
Frank
1151 posts
Sep 15, 2012
8:55 AM
|
I like the tubes high and hot and so do my mics...I'd rather turn volume to "3" and have the BITE I like -- rather then with tubes low and warm, volume to "6" but toothless in comparison.
|
Fingers
208 posts
Sep 15, 2012
1:24 PM
|
Strange!! i tried a 12at7 in v1 and then in v3 but went back to v1 because it was too clean in v3 in my hot-rod deville!! the guy at watford valves also said this would happen, i did use a very hot mike and love the tone!
v1 12at7 v2 12ax7 v3 12ax7 works for me i guess all amps respond in different ways.
i have also heard that 12au7 in v3 could cause damage!!!
Last Edited by on Sep 15, 2012 1:27 PM
|
5F6H
1360 posts
Sep 15, 2012
2:34 PM
|
@Fingers "v1 12at7 v2 12ax7 v3 12ax7 works for me i guess all amps respond in different ways." Very true, a 12AT7 as a preamp tube won't be great in all amps, but it's too much of a generalisation to rule it out completely...I have 2 amps where it sounds better than anything else as an input tube, Sonny Jr used them in his cathode biased SJ1 & SJ2 too.
"i have also heard that 12au7 in v3 could cause damage!!!" The flame proof 1W PI plate resistors in aDeville fail with a 12AX7 sometimes...the 12AU draws around 3 times the current of a 12AX, so this is more stress...it's a weak spot either way, you migh decide the tone is so good that you'll risk it, it's not an expensive fix, but I find a 5751 is about as low as I go in the PI in these amps.
@Jehosephat, the circuitry & therefore the tube function in a Deville/Blues Deluxe is different to a bassman/Sonny Jr...doesn't mean that similar set ups won't work but they won't give the same result as they do in those amps. I tend to just change V3, next to the power tubes, if any at all, in the Deville/BluesDeluxe, I always prefer a 12AX7 in V2, middle preamp, in a Deville/Blues Deluxe. So I'd get a 12AT7, 5751 & a 12AY7 and switch them about in V1 & V3 (as well as the original 12AX7) until you get what works for you.
----------
www.myspace.com/markburness
http://www.facebook.com/markburness
|
walterharp
937 posts
Sep 15, 2012
3:13 PM
|
I am with Frank and Fingers for my bassman clone, the higher gain tubes give the amp more spank, but less sweep on the volume, so harder to find that sweet distortion spot but mine sounds better when you do than when 12au is in the circuit anywhere
|
Libertad
122 posts
Sep 16, 2012
7:11 AM
|
I have to admit I know very little about this but as I modded my VHT Special 6 last week I have been looking at valve choices. I was confused by the description of the 5751:
"The Philips 5751 is a superbly warm and rich sounding 5751 type.
The valve has a lower amplification of 70 mu as opposed to a 100 mu of a ECC83/12AX7. This makes it harder for the valve to distort and therefore an ideal choice when the best clean sound is required.
It has more midrange and less top end than The G.E 5751 which also makes it the ideal choice for blues and harmonica players."
I am not after a clean sound, I want distortion, so is the 12AX7 a better choice for me? I have reduced the gain with resistors.
Thanks
Martin
|
5F6H
1362 posts
Sep 16, 2012
7:58 AM
|
@ Libertad - A harp & mic will push any 12A#7/5751 type tube to distort, that "best clean sound" relates to guitar use. I think the vast majority of people would prefer the 5751 over the 12AX7 in a VHT Special 6, it will still distort but in a less "spikey", harsh manner.
----------
www.myspace.com/markburness
http://www.facebook.com/markburness
|
Frank
1156 posts
Sep 16, 2012
8:03 AM
|
You can still get a "clean" sound with an AX and still get "distortion" with a lower gain...In my experience - it's HOW I choose to play the harmonica that distinguishes between a clean or distorted tone/sound...But, you'll be the best judge of which is better for what YOU want to hear and produce as a final tonal product, So try different tube and mic combos and trust your ears!
Last Edited by on Sep 16, 2012 8:07 AM
|
Rick Davis
683 posts
Sep 16, 2012
8:26 AM
|
Frank, exactly right.
It takes time to learn to hear the differences: Part of the distortion you hear is generated by your amp, other parts by your mic, and still other parts by your technique on the harp. Truly, all of these elements are part of your instrument. So many moving parts...
----------
-Rick Davis
|
Libertad
123 posts
Sep 16, 2012
9:04 AM
|
Thanks guys I have ordered one to try with my VHT. I will report back!
|
Rick Davis
685 posts
Sep 16, 2012
9:20 AM
|
Libertad - the Philips 5751 is beautiful in V1 of the VHT Special 6. I like a JJ 6V6S in the power tube socket. Installing an Eminence Lil' Buddy speaker makes it an absolute killer little amp.
----------
-Rick Davis
|
Tuckster
1149 posts
Sep 16, 2012
9:54 AM
|
I'm a bit confused. In a previous thread,there was a vid of a guitarist demonstrating a master volume amp. From that thread,I concluded that you wanted power tube distortion. This thread seems to say the opposite-you want preamp tube distortion. Is it a matter of taste? In that other thread,I definitely thought that power tube distortion was "sweeter" than preamp tube distortion.
|
5F6H
1364 posts
Sep 16, 2012
10:59 AM
|
@Tuckster - In an amp, whether using harp or guitar, by the time you hit "power tube distortion" they are most likely already amplifying a certain degree of preamp distortion. In fact, in most cases with harp, you will be getting preamp disortion before anything else. In the real world it isn't always possible to determine which is which purely by ear, or when one takes prevalence over the other. You start getting "power tube distortion" when your power tubes are asked to make more power than their clean W RMS rating...again, not easy to determine by ear.
Preamps are where the flavour, or character of an amp is decided. As you go farther down the chain of amplification the idea is normally to reduce distortion at each stage a little, so as to let the amp make good power.
There is certainly value in the previous thread about preamp & power tube distortion...excessive preamp distortion can be overly buzzy, what folks call "wasp in a jam jar" distortion. This is where lower gain preamp tubes can help, by limiting the amount and tone of the distorted sound from stage to stage, also keeping gain controls low and pushing up the master volume.
When the power amp is pushed, distortion can be richer, smoother & incorporate some sag. A balance is key, power tube distortion on it's own, with little preamp flavour can sound bad too.
I'd recommend going by ear and not really worrying about where the distortion occurs. In some cases amps used with harp will never overdrive their power tubes, but they can still sound good.
----------
www.myspace.com/markburness
http://www.facebook.com/markburness
Last Edited by on Sep 17, 2012 1:15 AM
|
Tuckster
1151 posts
Sep 16, 2012
2:53 PM
|
Thanks,5F6H!
|
