Using tubes from old organ in my amp?

[elenore19]

Alright, so I had this huge ass organ...Finally got rid of it, but I kept the 12ax7's that were in it.
I also have a SCXD. While I've heard that tube change doesn't do really much at all on the Super Champ...I feel like messing around anyways.

The SCXD has a 12AX7A if I put one in there without the "A" would it work fine?

Just wondering.

Thanks guys.


Elliot

 

[antichef]

I would definitely try those out -- they won't hurt your amp, and if they're not all used up, they might sound great.

 

[elenore19]

I would definitely try those out -- they won't hurt your amp, and if they're not all used up, they might sound great.

Awesome. I'll do that in a couple hours or so

 

[ermghoti]

Heck, you can try any 12A_7 in there, won't hurt anything. You probably want a 12AX7 of some sort, or you won't have much gain available.

 

[Gear_Junky]

now, you must also do a fair comparison - play a few of your favorite settings, write them down, then play those same settings with same guitar, etc. with new tubes.

you must let us know what you hear (and more specifically what sort of tubes those are).

 

[JCH]

Vintage tubes are my favorite form of recycling. What brand are they?

I've got a small assortment of old preamp tubes. It's fun to compare them against each other. The tubes in the picture are re-branded Amperex.

 

[JCH]

Sorry

I forget the picture

 

[elenore19]

now, you must also do a fair comparison - play a few of your favorite settings, write them down, then play those same settings with same guitar, etc. with new tubes.

you must let us know what you hear (and more specifically what sort of tubes those are).

I'll most Definitely do that....Once I get my guitar working...if you look at my other thread, my amp is breaking...

Vintage tubes are my favorite form of recycling. What brand are they?

I've got a small assortment of old preamp tubes. It's fun to compare them against each other. The tubes in the picture are re-branded Amperex.

Yeah, I can't wait...although, I do have question...

All these tubes from the organ (3 of them) are the same size and everything.
Only one actually has "12AX7A" on it. It is a General Electric tube. it also has ECC83 on it. And "NJ" with "188" below it.

Next one...Also a General Electric "Electronic Tube" All it has is "SK" with "188-5" below it.

The last one is a Sylvania. "K?" (?=non legible letter/number of some sort) and under that is ABS. Now looking closer I see that this one also has "12AX7A" and below that it has "EC83" and blow that "7025" and then "USA" beside those.

So there's info on the tubes...the one without any indication of 12ax7...is that automatically a "12a_7" due to it's shape/size? thanks for all the help guys.

 

[JCH]

Look at the tubes very closely. The glass may be etched. Sometimes it's difficult to see. The unknown tube is just that. There are many tubes that will fit that socket that aren't compatible with a 12AX7. If you had a schematic for the organ you could identify it that way maybe. GE's are good, although not my first choice in a preamp I like them for a Phase Inverter. Sylvania 12AX7's are a rare, and beautiful thing. I've got one that sounds great. They're not very common. ECC83 is a different name for the same tube.

 

[elenore19]

Look at the tubes very closely. The glass may be etched. Sometimes it's difficult to see. The unknown tube is just that. There are many tubes that will fit that socket that aren't compatible with a 12AX7. If you had a schematic for the organ you could identify it that way maybe. GE's are good, although not my first choice in a preamp I like them for a Phase Inverter. Sylvania 12AX7's are a rare, and beautiful thing. I've got one that sounds great. They're not very common. ECC83 is a different name for the same tube.

So, is that Sylvania worth about...you know...like $400 or something? (I wish)

Here are some pics of the tubes. Like 10 pictures...But yeah. I couldn't find any etching on the "unknown" tube. There is no manual to the organ either.
Is there a way to put these pictures in a spoiler type deal? Or no? There are a bunch of them.
Let me know,and I'll edit this post. Otherwise...Here they are. (no organized order of any sort really.)

 

[dementedchord]

12ax7-ecc83-7025... all the same tube... 12ax7 is US... ecc83 is british/european... 7025 is military spec...

 

[Light]

Heck, you can try any 12A_7 in there, won't hurt anything. You probably want a 12AX7 of some sort, or you won't have much gain available.

NOT TRUE!!!!!!!!!!!!!!



Some of the different 12A_7 tubes have very different voltage and current requirements from each other, and using the wrong one without changing certain things in your amp could cause serious problems.

For instance, a 12AT7 draws 15mA, while a 12AX7 draws only 8mA. They are NOT interchangeable.

To the OP's question, yes, you can try those tubes in your amp. The only one which is likely to be worn out is the PI tube, so if you can figure out which one that is keep track of it.


Light

"Cowards can never be moral."
M.K. Gandhi

 

[dgatwood]

NOT TRUE!!!!!!!!!!!!!!



Some of the different 12A_7 tubes have very different voltage and current requirements from each other, and using the wrong one without changing certain things in your amp could cause serious problems.

For instance, a 12AT7 draws 15mA, while a 12AX7 draws only 8mA. They are NOT interchangeable.

Both a 12AT7 and a 12AX7 draw 150 mA (@12V, 300 mA @6V), assuming you mean the heater current. Even subminiature tubes draw way more than 15 mA....

 

[Light]

Both a 12AT7 and a 12AX7 draw 150 mA (@12V, 300 mA @6V), assuming you mean the heater current. Even subminiature tubes draw way more than 15 mA....

I'm talking about plate current, and at 300vdc that is a real difference.


Light

"Cowards can never be moral."
M.K. Gandhi

 

[ranjam]

OK, it's just about time for my medication and beddy-bye, so I'll make this quick.

1. The current dgatwood is listing for 12AT7 vs. 12AX7 is based on Plate resistance and typical Plate voltage. I=E/R is still true today as it was in 1812. The 12AT7 has a lower Plate resistance.
2. The filament current is exactly the same for either tube.
3. You won't hurt anything by swapping any 12A_7 tube, but the tone may suffer.

What can happen in theory is the 12AX7 draws about 1mA or 2mA of Plate current. The 12AT7 draws about 10mA or 12mA or so, depending on Plate voltage. Now if the Plate supply is a real piece of poop the voltage may sag. But I think we have all seen 912 Marshall heads in our day that had an extra tube added for gain or an FX Loop. Everyone who owned that head or the technician who added the tube presumably lived a long and fruitful life. The extra 8mA ain't gonna cause huge cracks to appear in the Earth's surface or large boulders to fall from the sky. In many a Fender, you would swap the 12AT7 phase inverter for a 12AX7 and get more gain. Some tube gurus advocate replacing the first 12AX7 with either a 12AY7 (3mA) or a 5751 (1mA) to lower the preamp gain and sweeten up a reissue Bassman. Actually, those are low current tubes, so ignore that. But the Plate impedance is different. And here is how that can change things.......

The RC coupling network from one stage to the next also takes into account input impedances.

Raising Plate voltages, changing the tube Plate impedance, etc. all means the RC time constant has changed, and the frequency response will change. The immediate affect will be more/less gain, but the frequency response also is affected, and will be mistaken for......... gosh, I am not sure . But the 'average' person will say the 12AT7 had less bass or whatever, when in reality the frequency response changed. Increase the coupling capacitor, and they'll sound identical.

 

[dgatwood]

What can happen in theory is the 12AX7 draws about 1mA or 2mA of Plate current. The 12AT7 draws about 10mA or 12mA or so, depending on Plate voltage. Now if the Plate supply is a real piece of poop the voltage may sag.

Well, if the amp does something idiotic like use low wattage resistors between B+ and the plate, the resistors could overheat. That said, that's grounds for a product safety lawsuit, IMHO.

Plate dissipation max for a 12AT7 is 2.5W. Max for a 12AX7 is 1.1W. So yeah, if you used a 2W resistor, you'd be out of spec, but probably not enough to cause a flameout, I wouldn't think. If you used a 1W resistor, that's out of spec even for the 12AX7, and the designer should have his/her head examined.

There's just not a lot of things you could do wrong in a design that wouldn't open up the manufacturer to a product safety lawsuit. They should be using 5W resistors in those spots anyway.

 

[ranjam]

Regardless of what either the 12AX7 or 12AT7 can draw, the plate current is going to be limited to the value of the resistor in series with the plate if it's a typical gain stage. The schematic for my Blues Jr. shows the supply voltage for the 12AX7 preamp tubes at 242 vdc. There is a 100K 1/2w resistor in series with the plate. Using Ohms law the maximum current that could possibly flow in that circuit would be 2.42 ma, even if the plate to cathode were shorted. Plugging in a 12AT7 could not exceed this current value.

Man, are you in trouble . I keep a tube manual near my computer. I also keep one in the outhouse, but that's another story. Let's see how much I can relate what I read into what I type. Are you ready, fingers?

1. The Plate resistor is in series with the tube.
2. The Plate impedance/resistance (whatever you'd like to call it is OK for now) Rp + Rl =Rt. Rl = your load resistance, and Rt = the total series resistance. On the other end of that load resistor is a big juicy filter capacitor that is also in parallel, but for now ignore that.
3. What matters more is that 100K Plate resistor is a lot bigger than the 11K Plate resistance of a 12AT7 with a B+ of 250VDC.
4. In a series circuit, the current is the same, but the volatge will be distrubed proportionately to the resitance value.

Now, if you have say a Bassman, and the B+ node is 380VDC to the first preamp tube, and 235 right on the Plate of the first 12AX7, the load resistor 'dropped' 145-volts. Now 145-volts through a 100K Plate load resistor equals just under 1.5mA by my math. It is not 2.42mA, although the difference is minor. The point is the tube sees 235VDC, and we know for future reference a 12AX7 draws about 1.5mA or so. Drop a 12AT7 in there, and the voltage at the Plate itself will be much lower. The Plate resistor will drop more, and the current will be higher. Most Fender schematics will show a B+ node to the phase inverter of 410VDC, and at the tube Plate itself as being 220VDC . Now 410-220=190VDC across the 82K Plate resistor going to the 12AT7. 190/82000=2.3mA or so. The Cathode resistance also matters, and it is much higher in a phase inverter. So the comparison isn't perfect, but should demonstrate a point. In the end, you see (hopefully), the Plate voltage supply was higher, but the voltage right on the tube was lower because the Plate impedance was lower, and the current was higher. At a B+ of 250VDC, most tube manuals list the Plate impedance of a 12AX7 as being about 62K, as opposed to 11K for a 12AT7. Big difference. But my position hasn't changed; you won't blow anything up, but the RC time constant will change, and the frequency response will change. Say you look at an AB763 Twin schematic. Fender put a .001uF capacitor to the phase inverter, and the Grid resistors are 1Meg. Now, in the AA769 and AA270 they dropped the Grid resistors to 330K, possibly to aleviate blocking distortion. However, this also affected the frequency response, so for the later Twins it is raised to .01uF whilst keeping the Grid resistors at 330K. Clear as mud? People pay through the nose (what a strange place to keep your money ) to have technicians 'Blackface' their CBS Fender. And then some people read Gerald Weber telling us to use a 5751 for the first tube in your reissue Bassman to get a more vintage tone. This is because the higher Plate voltages in the Reissue lowers the Plate resistance, and the RC time constant get mucked with again. Copying the original Bassman values doesn't give you the copied tone.
Hey, try a 12AT7 for the phase inverter, and then the first preamp stage. You may like it. If not, put 12AX7's back in there. No one will lose an eye, and you'll be further ahead then reading my babble.

 

[ranjam]

I am one of these fellows that before I have a bowl of Corn Flakes I might read the history of the spoon, the John Harvey Kellogg biography, how milk is pasteurized, etc. I have no life. So now you make me dig out all my college electronics text books, and study Fender schematics. Thanks.

 

[Gear_Junky]

I am one of these fellows that before I have a bowl of Corn Flakes I might read the history of the spoon, the John Harvey Kellogg biography, how milk is pasteurized, etc. I have no life. So now you make me dig out all my college electronics text books, and study Fender schematics. Thanks.

well, i have just the cure for that:

Genesis 1:1 - "In the beginning..."