[Doug B]

I'm thinking of getting the 15 w. Monoprice AMP. It comes with 12AX7 tubes. If I replace them with 12 AU7 tubes, would that result in a cleaner less distorted amp? If not what is the difference between the two types of tubes?

Thanks

Doug

 

[vokey design]

http://www.guitaramplifierblueprinting.com/files/Phaseinverter.pdf

 

[Robert1950]

The 5751 has 70% of the gain of a 12AX7. The 12AT7 has 60% of the gain, The 12AU7 has around 30% (I think)

 

[GTmaker]

The 5751 has 70% of the gain of a 12AX7. The 12AT7 has 60% of the gain, The 12AU7 has around 30% (I think)

I think the 12AU7 has 20% the power of a 12AX7 ...

I tried a 12AU7 in my Super reverb RI once ( I think it was in the reverb tube spot.
Even there it sucked the living tone out of my amp. I ended up giving that stupid tube to a friend for nothing.
There is a reason why a manufacture puts certain tubes from the factory. You certainly can tweak it a bit with maybe a 5751 ( which I think sounds great in the one spot)
But going nuts with such a difference in tube strength makes no sense to me. I think if you try the 12AU7, your ears will tell you Im right.

Maybe its not my place to say but I'll say it anyways...
Why not go for a nice tube amp like a used Blues Junior...Save up if you have to.

good luck with your amp search...its a never ending process to most of us.
G.

 

[Robert1950]

I had a Gibson GA5 (like the tweed champ). I stuck a 5751 in and it tamed the gain nicely,

 

[vadsy]

I love messing with differently gained tubes and keep a pile just for this purpose. I’ve never tried a AU in place of a AX though, read somewhere that it was too low and too far apart in gain. Never actually used the AU in anything, only seen it listed as a tube in tape delays. I personally like the AY in Fendery amps.

 

[Player99]

Does putting a tube with different values cause any problems with other tubes or components?

 

[DaddyDog]

This old article by The Tube Store is good info for you: www.thetubestore.com - Preamp Tube Gain Factor
This recent article on Reverb is also good: How to Improve Your Amp's Tone with Preamp Tube Swaps

In short, it might work for you. Based on my experience, I suggest you change one tube at a time, and change in small steps. Going from 12ax7 to 12au7 is huge.

 

[mhammer]

Sage advice, from both yourself and the cited sources.

Note that the "gain" of a tube, or rather the gaininess of the preamp, is also a function of the cathode resistor and bypass cap values. We would normally see something in the vicinity of 1500-2200 ohms as a cathode resistor, as in this example of a Fender Vibro-Champ. The larger the value of the cathode bypass cap used, the more bass is retained by that stage. If a person wanted to make the bass "tighter" and less flubby, they could drop back to a lower gain tube like an AY7, OR they could simply drop the bypass-cap value from 25uf down to 10uf or maybe even 4.7uf, such that the max gain of that tube is applied more to the mids and highs than to the bass end. The bass still "gets through"; it's just not pushed as hard.

 

[High/Deaf]

The good news is all the 12A?7 tubes are interchangeable (as is the 5751). You can't hurt anything by trying them. And less common (less desirable) NOS tubes like the 12AU7 can be found for relatively cheap compared to 12AX7's.

 

[mhammer]

Yes and no. One can certainly sub lower-gain tubes for higher gain, but subbing higher-gain for lower gain can create problems in some instances. For instance, if a reverb pan is normally driven by a stock AY7, sticking an AX7 in the same spot may slap those springs harder than would yield a pleasing reverb. As well, cascading high gain stages is understood to deliver a lot more high end. If it runs out of headroom, AND you dime things, the output transformer may not be spec'd for handling that sort of output.

Like they say, moderation in all things.

 

[Hammerhands]

And there are certain things that an AT7 or AU7 can do that an AX7 can't, like the AT7 can output lots of power, for example as it is used in the reverb of a Fender amp, and the AU7 can run at tiny voltages.

12AX7 current 1.2ma @ 250V = 0.3W (per triode)
12AT7 current 10ma @ 250V = 2.5W (per triode)

 

[High/Deaf]

Yes and no. One can certainly sub lower-gain tubes for higher gain, but subbing higher-gain for lower gain can create problems in some instances. For instance, if a reverb pan is normally driven by a stock AY7, sticking an AX7 in the same spot may slap those springs harder than would yield a pleasing reverb. As well, cascading high gain stages is understood to deliver a lot more high end. If it runs out of headroom, AND you dime things, the output transformer may not be spec'd for handling that sort of output.

Like they say, moderation in all things.

You may have 'gain' issues but you won't damage anything. Driving a spring tank too hard won't melt the springs. it'll just sound bad. And driving power tubes too hard may wear them a bit quicker, but they can only put out so much power. They will limit and won't won't hurt the OT unless the amp was badly designed, IME.

Power tubes is a whole 'nother thing. Putting 6L6's in a 6V6 amp may not be a good idea (depending on the design, some will tolerate it). Forcing EL34's into an EL84 amp, even worse.

 

[mhammer]

From time to time, I futz around with my tweed Princeton, and stick a 6L6 in instead of the stock 6V6, and also pop the 5Y3 for a solid-state rectifier plug-in module, just for something different. The plate voltage is less than what a 6L6 wants, so there is no huge increment in output power. I've also experimented with lifting the negative feedback resistor for icepick-through-the-forehead tone. The combination of no feedback resistor, solid-state rectifier, and 6L6 gets a bit more output and more bark, less smooth.

The qualifier is that it's a single-ended amp. The nice thing is that everything is reversible.