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Dirty-South Blues Harp forum: wail on! > preamp tubes 12Ax7 5721 12At7 12Ay7 questions
preamp tubes 12Ax7 5721 12At7 12Ay7 questions
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528hemi
125 posts
May 29, 2010
5:20 PM
In general can you describe what would be expected tone wise when for example changing the pre-amp tube
in my 8 watt amp from a 12At7 to a 12AX7 or a 12Ay7? WIll going from the 12At7 to a 12X7 make the amp break up sooner?
Does going from a 12At7 to a 12Ay7 make the amp cleaner?

How does this all work?

Thanks 528hemi
rharley5652
151 posts
May 29, 2010
11:31 PM
Right from the Guru himself Dave @ greenbulletmics.com
Tube: Gain Factor:
12AX7 100
12AT7 70
5751 60
12AY7(A) 40
5814A 22
12AU7(A) 17

Go to Dave's site an have a look around,...great info !!!!
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Simply Unique Kustom Mic's By Rharley

Last Edited by on May 29, 2010 11:33 PM
LittleJoeSamson
295 posts
May 30, 2010
5:38 AM
I think you will notice better results from a swap to a 12AY7 from a 12AT7 more than any other. the AT's have a transconductance twice the AY's.
The AX's have more juice, but feedback can become an issue. The 5751's are sweet, but pricey.

I've had luck also with oddball 6211's...@ 26-28 gain and milspec 6829's. The 6211's usually have extra long plates, which seem to add a musicality/chiminess.
The 6829's are bolder @48 gain and have better presence. The one thing to know is that their bottle is a little taller, so you have to check the chassis to see if it will fit.

The other tube to consider is the mismatched plates 12DW7, or if you can find and afford them: 7247's ( I got lucky and fleabayed two Mullards for $12 @ . Recently bid on two others, and they went for over $100 for the pair. ) One side is a 12AX7 100mu and the other side is 12AU7 20mu...GREAT tube for harp! You can clip the 5 pin and reverse it, too! Just mark where the original 1 to 9 space was.
Cisco
129 posts
May 30, 2010
7:01 AM
Actually the greenbullets web site has two of the gains marked wrong. The correct gains are 12AT7 = 60%, 5751 = 70%.
Greg Heumann
493 posts
May 30, 2010
8:18 AM
The gain info is important to know but the question was about tone.

Theoretically, tubes don't really have "tone" - they simply take an input signal and produce an output signal of the same frequency, just with more amplitude. The main thing you're doing with tube swaps is changing the gain of your amp. This has a profound affect on the amp's tendency to feedback, but it also changes the level at which everything you haven't swapped operates (in particular, the power tubes) so tone does change.

At the lowest gain end of the spectrum, expect muddiness. There will be more distortion, so there will be more bass overtones and the bright portion of the signal won't seem as pronounced. At the highest end, expect harshness (and hairy edge feedback) as there will be considerably less harmonic distortion. In between the changes are more subtle, but there is some tonal change, even between two tubes of the same value - because tubes simply vary like all other components.

If you have a 3x 12AX7 pre-amp circuit typical of the larger Fender amps, in my experience changing one tube from 12AX to 12AT will NOT make a significant difference. My two SJ amps have 12AU's in V2 and V3 positions (more "inboard" on the amp.) Then 12AX7, 12AT or 5751 in V1.

If for no other reason than to really experience a significant change so you know what you're listening for, I encourage you to buy a couple of 12AU's and start there. If it is too much you can always head back up the scale.
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/Greg

BlowsMeAway Productions
BlueState - my band
Bluestate on iTunes
5F6H
160 posts
May 30, 2010
9:24 AM
In addition to what Greg says, it can sometimes be overkill to use 12AU/12AY7.

In a tweed bassman style amp, each tube only really works as if it is half a tube, so in terms of dropping gain, fitting a 12AU7 in place of a 12AX7 effects a 80% drop in gain. However in a champ style amp (I'm assuming your 8W amp is similar to a champ, with 1 preamp?), or V1 in a tolex Fender, where one 12AX7 tube has both triodes in series in the circuit, subbing a 12AX7 for a 12AU7 drops gain by over 95%...the amp might still work OK, it's just the effects are more profound than in a Cruncher/SJ410/tweed bassman.

Also, a 12AU7 can draw an awful lot more current than a 12AX7, increasing plate dissipation accross the resistors & occasionally burning them up. Easily fixed, but just be aware.

But if you only have 1x preamp tube, it only takes a few minutes to go through the various permutations (so try them all), unlike a SJ/tweed bassman where there are 179 possibilities.

Little Joe - does clipping pin 5 of a 12DW7 not adversely affect hum? Is the tube still wired in humbucking mode? JJ do a version of the 12DW7 that has the triodes reversed without any pin clipping.
528hemi
129 posts
May 30, 2010
3:38 PM
Thanks..Yes it is a single 7591 power tube and a single 12AT7 the way I bought it. I figure I will just buy a few tubes to play around and see how it changes. Any particular brands to look for in a preamp tube? Do the different brands make a big difference?

Thanks 528hemi
528hemi
130 posts
May 31, 2010
6:31 PM
Actually I was told it was a 12At7 but I just checked and it is a 12Ax7 so I will try the other tubes to see how they compare.

Thanks again
TheATrain
1 post
Oct 24, 2012
9:25 AM
To Greg, 5F6H and others:

I have recently bought a Fender Excelsior (2 12AX7s in the preamp, 2 6v6 in the power section) through a 15". I was hoping that changing to a lower gain factor tube would improve this amps "harpability". I dont know alot about Tube Amps but I have trying to pick up as much as I can. My primitive understanding of my amp in particular is that the two preamp tubes both play a role in driving the amp's tremolo... I said all of that to say this (thaks for those who are still reading!) there are two preamp tube sockets (both with 12ax7's stock) should I change them both to the same tube? If so which one? If two different tubes in those positions would produce a more appealing harp sound, which combos should I try?

It has occured to me that I haven't defined the problem :). Basically, way too much clean headroom, and with the amp maxed out the breakup is decent , but not great. Also feedback (every harp players enemy) is horrendous. Final piece of info, I am playing through a ~6 or 7 year old CAD mic which is essentially a JT30 with a volume control...


Thanks for any advice!
Rick Davis
853 posts
Oct 24, 2012
10:00 AM
Lot's of great comments here. I can't add much.

The "Rick Davis Rule" is to always first try a 5751 tube in the preamp socket of any harp amp project. It is most likely to get you where you want to be. The 2nd part of the rule is to never use a 12AT7 tube: They are evil.

That is all.


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-Rick Davis
The Blues Harp Amps Blog
5F6H
1398 posts
Oct 24, 2012
10:06 AM
"...there are two preamp tube sockets (both with 12ax7's stock) should I change them both to the same tube? If so which one? If two different tubes in those positions would produce a more appealing harp sound, which combos should I try?"

There is no need to run 2 of the same type of preamp tube, only the 12A#7 farthest from the inputs has anything to do with the tremolo. It's really a case of trying the options and seeing what works for you, if only one tube works well for the trem (which you use) then that decision is pretty well made for you ;-) V1 (under the inputs) is two series stages in one tube...so I'd try fairly good gain tubes here to start with, like 5751?

Ultimately, I think the Excelsior can only be taken so far with tube subs, there are other factors in the circuitry that will have a significant effect. You might try getting a tech/suitably qualified person to snip off C7 or C32 and see what tonal effect that has (both caps boost gain, try removing one at a time, both can be replaced easily if it doesn't work out)? Also try removing C2.

The circuit is based fairly heavily on the 6G2 Princeton, with a bigger first stage cathode resistor, cathode biased (as opposed to fixed) & without a NFB loop.

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Last Edited by on Oct 24, 2012 10:32 AM
TheATrain
2 posts
Oct 24, 2012
10:19 AM
Thanks for those that have posted info!

I was afraid that a simple tube swap wouldn't get me where I wanted to be. I guess I either need to sell it off and score one of Greg's 'zoos (if available) or try to trade it for something more suitable for harp. I guess I just didnt do enough reseach on this one...
LSC
327 posts
Oct 25, 2012
12:41 AM
@Greg- Have you tried anything other than the stock tubes in your Avenger and if so what was the result?
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LSC
jbone
1094 posts
Oct 25, 2012
4:37 AM
ATrain, i have used a 1482 for about 10 years now. speaking of tube swaps i run p1- 12ax7......p2- 12au7......p3- 12au7. when i got this amp the speaker was ragged so i put a Jensen Special Design ceramic speaker in its place. that's it. i have used this amp a LOT. for big room or loud band mates it's not going to keep up so well but for medium and small rooms and reasonable volumes it's a wonder to hear. the added feature of tremolo is nice, i usually barely turn it up. just for added warmth.
coupled with a harp delay pedal this is a great amp for me at least. i use a hot dynamic or cm powered mic usually but a jt30 type crystal is also a good choice for say, chromatic or jazz in general where you want warmth with clarity.
durability wise i have had no issues with this amp. mine has a grounded plug and cord but i don't recall if i put it on or if it came that way. note we did get a matching 1482 for my wife to play guitar with and it also has a 3 prong.
i think in 10 years this amp has been to the shop once. can't recall specifically what was done to it but it didn't work and then it did. it was like 5 years ago and i've used it a lot since then with no issues. the covering typically has some tears but i'm not worried about looks. it does all i want in a duo and a smaller but distorted 4 piece. we play open tuned slide stuff with drums and bass behind us when we're not doing guitar and harp with vocals only.

check out Bluesboy Jag with Jawbone on youtube for samples.

one other feature- these amps weigh maybe 20 lbs as opposed to over double that for a bassman 4x10. at my age lighter is definitely better!
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PeterG
8 posts
Mar 11, 2013
10:53 AM
To TheATrain:

What did you end up trying? I'd like to know for use in my Excelsior.
Bruce S
27 posts
Mar 11, 2013
11:31 AM
I clipped out C7 in the Excelsior, and kept 12AX7's in both preamp and PI. There is a reduction in feedback, and more usable sweep on the volume control. I also changed the tone switch to a treble cut tone control. I'm currently toying with removing C32, but I will put a bypass cap in at C7 again if I try it.
PeterG
9 posts
Mar 11, 2013
2:19 PM
Interesting. Did you try different tubes first, or just go directly to the mods? Are you able to use the guitar input or bright channel? I use the mic input and dark setting only (guitar input and bright being unusable due to feedback). With the harp commander I can adjust tone, gain and output to reduce feedback, so I'm still interested to hear what benefits, if any, tube swapping might yield. Otherwise, I gigged with last night and had no problem being heard against loud drummer and multiple guitars, keyboard, etc.
FMWoodeye
596 posts
Mar 11, 2013
8:38 PM
Let me premise my question by simply saying "duh."
No one has been able to explain to me (to my satisfaction) what electricity even is. My wife told me, when trying to get me to climb up on a vanity in the bathroom and wire a light fixture, that the human body does not conduct electricity. I could not reconcile that statement with the knowledge that people are executed via the electric chair. In any event, this is my question. If you can go through a pedal, i.e., the LW Harp Attack, directly into a PA and get a "good" sound, can you not use that pedal (or another) to make a marginal amp like the Excelsior sound decent? I will thank you all in advance for not making disparaging statements with regard to my intellect.
1847
593 posts
Mar 11, 2013
9:52 PM
woodeye, electricity is the shit at the end of ben Franklin's kite
last i checked he was still alive, so obviously the human body does not conduct electricity. and lastly,disparaging is a big word for someone of your intellect.
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tipjar
Bruce S
28 posts
Mar 12, 2013
7:14 AM
@PeterG
I changed out the tone switch for a tone control. This was fairly straightforward to do. I can dial in the treble, rather than having the fixed settings.
I tried a 12AY7 in the V1 slot, but the amp seemed brighter and I did not care for the tone. I prefered 5751's in both V1 and V2 with the stock setup. After removing C7 I went back to 12AX7's in V1 and V2.
This brings it a little closer to a 6G2 Princeton. Breakup is still quite late, but the amp is more usable. Typically I use the mic input with CM/dynamics but the guitar input is usable now too, especially with crystals.
Removing C32 will reduce gain in the output section. I haven't tried this, but suspect if both C7 and 32 are removed the amp's character may be a bit "dull/lifeless". If I remove C32 I will put a 22uF or larger cathode bypass cap in for C7 and see how it sounds.
Many of the vintage amps with similar circuit designs didn't have a cathode bypass cap on the output stage, and having tried some amps with and without bypass caps on the power stage I think it's a worthwhile experiment- will probably have to wait for the weekend though!

The 15" speaker is another limiting factor IMO. Much as some folk love them,(and I do like the Excelsior's 15" speaker tone) I think they do feedback earlier than a 10" or even a 12". I ran my Excelsior through an Eminence C-Rex 12" speaker-with a higher efficiency; quite impressive, more aggressive sounding. I may well make up speaker adapter rings for a 10" and a 12" just to see how they perform.
PeterG
10 posts
Mar 12, 2013
8:36 AM
Thanks, Bruce. Your responses have been the most useful information I've found thus far regarding using this amp with harps.
HawkeyeKane
1537 posts
Mar 12, 2013
8:47 AM
"The "Rick Davis Rule" is to always first try a 5751 tube in the preamp socket of any harp amp project. It is most likely to get you where you want to be. The 2nd part of the rule is to never use a 12AT7 tube: They are evil. That is all."

Amen on both counts. I do believe that's a creedo for a number of amp gurus.

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Hawkeye Kane

Last Edited by HawkeyeKane on Mar 12, 2013 8:49 AM


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