Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by smolder, Oct 25, 2008.
Anyone know how a 12AZ7A (GE NOS) differs from a 12AT7 or 12AX7?
Less gain. The gain factor of a 12AX7 is 100, and I think the 12AZ7 is 60.
I did find out from another source that is is basically an 12AT7 (with similar gain) but that it has a better heater. Not sure how that would effect tone. My guess is that is will not sound significantly different than the T.
I use 12AZ7s' in place of 12AT7's in my vintage Twin Reverb...they are nearly the same as T7's...in truth I bet 98%+ poeple couldn't tell any difference
I am using a GE 12AZ7 in my Phase Inverter and from what I have been told it uses a bit more current and is a bit slower warming up, otherwise gain is the same. I have a RCA and Raytheon as spares for later.
I have to say it really added a ton of clean headroom to my amp. Jack "the builder" turned me on to it as an alternative to a 12AT7. The plus is people aren't asking for body parts in payment for them yet.
wow I thought I knew about tubes. never heard of a az7. thanks for the schoolin!
The 12AZ7 is generally regarded as too microphonic for amplification use. Original use was for TV tuners. However, I am sure there are exceptions. I actually have a NOS one in my box. Never bothered to try it. Now I may give it a whirl in the old PI just for kicks and to test the microphonic theory. I wonder if mine is balanced...
I popped my 12AZ7A (NOS GE) into V1 of my Dr Z Carmen Ghia and moved the 12AX7 into V2. I did an A/B with it and a (also NOS GE) 12AT7... both reduced the gain and added some brightness. The Z7A seemed slightly more defined... clearer (the opposite of muddy) to me. I know the (toneQuest guys) pros do this sort of thing for hours back and fourth and over a period of weeks... so YMMV... I spent about a half an hour.
In a combo that might be an issue for sure. I have a head and I actually place some foam between the head and the cabs reducing any microphonic which could happen. I have a much better time now doing that, plus it extends the tube life.
The AZ in the Phase Inverter sure seems to be the trick. I like the clarity and headroom. It had a 5751 in that position until now.
Can't believe I have'nt heard of this tube before. If you want more gain try the oddball 12bz7. Its a taller stacked gain ax7 it seems.
I have a few and yes they tend to be too micro to use in any V1 spot at least. I have so many 12AX7's,5751, etc I just put them aside. Bob
I have several of these around here. I agree that they are too microphonic for most positions, but seem to do well in the PI position.
I used to think a tube being microphonic made it completely worthless for use in a guitar amplifier until I discovered high-temperature O-rings at my local hydraulics shop. I pay roughly a dollar a piece for them. Even the worst of microphonic tubes see a drastic improvement.
I make sure I find a set that fits tightly and put two at each mica and one in the middle. Then install as normal. Guarantee you could probably make good use of a 12AZ7 in a guitar amp with a few of these O-rings.
Yes, that's true. Or you can use a microphonic tube as the phase-inverter, without the rings.
Can you show pictures of those O-rings and where do you put them ?thx
Try these... The reddish ones are what I have. I haven't found placement to be that important. And, some tubes are to far gone, it's worked most of the time for me.
You do realize that the post you quoted is almost 3 years old, right? Just google "tube dampers" and you'll find out more than you ever wanted to know about the subject. Oh yeah, welcome to TGP!
The red silicone dampers work wonders. They make tubes that might otherwise be thrown away useful.
For a lark try a 12BZ7 - two 12AX7's in one bottle - Your PT needs to be up to snuff (enough current for another tube) In the right spot these are over the top -
I was going to mention that one. Fun tube in the right place.
I've been using the red ones for a couple of years and they're great. If you buy 'em from a tube/music gear place they're about 2 bucks each. Get them from a local supply shop or http://www.oringwarehouse.com/ and they're about 15 cents each.
EDIT: Looks like these guys went out of business. However, you can get them on Amazon now, among other places.