Dumb question re 12AX7 vs. 12AT7 and clean headroom...

Steve Lipsey · From: Portland, Oregon, USA

Amp: Matchless HC-30, but the question is valid for any amp, I think...
NOTE: Gig with master volume in "BYPASS" position, tested with it engaged at medium low level to ensure no final amp distortion. using Input 1 (low level).

I want the most clean headroom possible.....certainly with the 12AT7 I can turn the VOLUME up further before getting preamp distortion....but is that really helping my clean loudness?

Isn't the second preamp tube (where the volume control is) just distorting at exactly the same loudness level as before, just requiring more input to get there due to reduced first stage gain?

How is the classic "substitute 12AT7 for 12AX7" helping here? I could see it helping if the first tube was pushed into distortion by the input signal, but using the "low" input, that doesn't happen...
Thanks...

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Dave Grafe · From: Hudson River Valley NY

In this scheamatic, if you put a 12AT7 in the first socket it serves BOTH stages of the preamp circuit. With Fender amps the setup is different. Either way, it is usually the first stage that overdrives with the following stages being designed for clean(ish) gain at the max output of the first stage. Once a device, tube or solid state, begins to clip only the distortion increases while the output level remains the same, so whatever happens there is replicated throughout the amp circuitry..

On a side note, the 12AT7 is a 20kHz tube, the 12AX7 has a bandwidth of less than 7kHz, so there are other differences in the characteristics besides simpley the gain specs.

Does that answer your question?

Tim Marcus · From: San Francisco, CA

Short answer - no

a 12AT7 in V1 position will gain you more real estate on the preamp volume knob and potentially eliminate any chance of preamp distortion (which is not a good sound IMO)

However, some guitar amps are designed to distort at the output - so no matter what Tube you have in V1 you will eventually drive the phase inverter to distort the output tubes.

If you do not have enough voltage gain in the first stage you may eliminate the distortion at the output as well (read: 12AU7) however you will seriously lower the overall wattage of the amp.

The way I look at it is a 30W amp is a 30W amp *including* the distortion. A 60W amp may have a clean 30W in there somewhere. You can confirm this with an oscilloscope.

My Milkman amp, for example, will do about 55-60W with a 1k sine wave before clipping and distortion begins to occur - but its an 85W amp. There is a distortion threshold in there somewhere. It does not matter which preamp tube I use - eventually the 6L6s will distort, and its usually around the same 55-60W point. That is because there is another gain stage in there, and a long tail phase inverter which also adds gain.

You need to decide where you want distortion to come from - the front or back end of the amp. My experience is to clean up the front end and push with the back end, however, you will always get the same general volume, or less, depending on what is in the preamp.
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Steve Lipsey · From: Portland, Oregon, USA

OK...I guess I haven't been completely clear on my question....bottom line, should I do the swap?

Will I get more clean headroom going into the final EL84 stage if I use a 12AT7? Or will I just be turning the volume up higher and getting the same output characteristic of loudness before distortion occurs? I think Tim is saying no to the first part of that question, and yes to the second....so why is this swap always recommended as a clean headroom boost? Because Fenders are different?

Certainly turning the volume up can get the amp into preamp distortion....but will I just have a different knob setting with a 12AT7 for the same goal - which is turning it up to, but not beyond, where distortion occurs - due to the lower input from the 12AT7 first stage?

I don't quite get how the second preamp stage will interact with what is coming out of the first....the 12AT7 will just lower the output of the first stage, but that stage isn't being overdriven now, so I don't know why the 12AT7 will change anything... (and yes, I hope to end up with Milkman amp at some point, but for now I have to get what I can out of the Matchless)

Sorry for being thick here...

It does do what Tim suggests, gives me more of the full range without distortion, but I just turn it up to where it just starts to distort on pretty loud stuff anyway...(and of course the volume pedal impacts all this, at the end of note decay, the amp can be set real loud and not distort)

And the added bandwidth will be interesting....I wasn't listening for that while A-B'ing the tubes....that EL84 Matchless puts out so much high end anyway....or is there also additional range in the lows from that?
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Brad Sarno · From: St. Louis, MO USA

The Matchless thing in many of their amps is to tie both halves of the first tube in parallel. It has a cool sound, cleaner, quieter, double the drive capability, maybe smoother.

Dropping a 12AT7 or 12AU7 in that first position is not necessarily a great idea. You would gain the ability to hit the input of the amp with a hot steel pickup and not overdrive that first stage because of the lower gain. But that stage is definitely not optimized for those tubes since the plate resistor and cathode resistor values are high, definitely for a 12AX7. So you could alter those values to get the best performance out of the 12AT7 or 12AU7, but those tubes may not sound all that great in that slot.

The trick I like to do for steel since the signal is sometimes TOO hot for preamp inputs is to disconnect the cathode bypass capacitors. You lower the gain and greatly increase clean headroom and still use a 12AX7. You also lower the distortion quite a bit. In this amp they have two cathode bypass cap's. Simply disconnect them and just leave the 1.5k cathode resistor in place.

The removal of the bypass cap's creates a "local degenerative feedback" situation. It's essentially negative feedback with virtually zero phase shift. Very clean, lots more headroom, sounds great for steel, and very easy and reversible. You could even put the cap's on a toggle.

Also, by raising the clean headroom there, you can then know for sure that the entire preamp section is running clean, and then when you hit distortion, you know it's power amp distortion, NOT preamp distortion. Since the gain pot is AFTER the input tube, it's the input tube that may be at risk of distortion from hot input signal. If you turn the gain way, way down and the thing is dead clean, then you know you're not clipping the input and no need to mod. Turn it up and get distortion, then you know you've maxed out what the amp can deliver cleanly. Power is power, and you can't get more clean power by altering the preamp.

Brad

Earnest Bovine · From: Los Angeles CA USA

What is "clean headroom"?

Brett Lanier · From: Madison, TN

I tried a 5751 tube (70% gain) in my twin and the result was just earlier power tube distortion. I ended up going back to a 12ax7 and using the 5751 for the guitar channel.

Another thing to consider is the height of the pickups. If they're kind of high, they may sound ok with a solid state amp, but could distort the preamp of a tube amp even at a low volume.

Steve Lipsey · From: Portland, Oregon, USA

Most important - I want to thank all of you for your time and input...once again, you guys (and somehow my questions always need Dave and Brad and Tim) have really helped, and cleared up something that has confused me forever...figuring out what parts of the "conventional wisdom" really is wise in any particular situation is often difficult, and it is really easy to get it exactly wrong...

Brad-
Thanks, you suggested that change when I was having huge hum problems with my Matchless Lightning (which has a single 25mf cap on that cathode), I did it, and it helped the hum and in general (I put a toggle on the back panel)....the Lightning only has a single, high-gain input, very hot, and the Trutone was just sending too much into the first stage.... The HC-30 also has a low gain input, and using that, I guess it is fine the way it is (and I've switched to Telonics, no hum)...just keep the volume at a level that doesn't mush everything up, until I can get an amp with more clean power in the final amp...

Brett-
Same answer, the low gain input tames the pickups, I was just looking for more if there was a way to get it....I'd heard about that 12AT7 and 12AY7 tube swap for years, didn't see how it would help, but tried it anyway, and it looks like all it does is leave the volume knob in a slightly different position....no actual change in how it sounds...

Ernest-
By "clean headroom" I meant I was looking for a way to get a louder signal out on peaks without distortion...room for the output to be driven harder without going over the edge into distortion....mainly when hitting it with a spike of input, like a big, sudden, loud chord...The amp is actually plenty loud enough in general, but peaks are messy...
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