ampage Tube Amps / Music Electronics	For current discussions, please visit Music Electronics Forum.

ampage archive

Vintage threads from the first ten years

Fender mod using 12SL7's or 12SN7's

:

10/7/1997 6:56 PM

Fender mod using 12SL7's or 12SN7's

Hi,I'm curious if this will work.
I want to change the 12AX7 to either a 12SL7 or 12SN7.
I know this will also lower the gain compared to a 12AX7.
I don't know if a octal tube was made with an amplification factor of 100.
I know it will require a bigger 8 pin socket.Will I have enough room in the chassis to do this.
What other changes curcuit or other will this involve.
I want to do this because I like the tone of octal tubes better than 9 pin tubes (12AX7)etc.
Thanks for your time and help!

10/8/1997 6:46 AM
J Fletcher	6SL7 or 6SN7 since you don't have a 12 volt heater supply.Tubes appear similar enough in specs to work ok as a sub.Haven't tried it, though I did once make an adapter to put a 12AX7 in a 6SL7 socket in a Ampeg Jet.Worked fine.Borrow a chassis punch from an electrician to enlarge the hole.Does a nice neat job.

10/8/1997 7:15 AM

12AX7 actually uses 2, 6.3V heater connections (just expanding a bit on what J Fletcher said.)
They CAN use a 12V supply if the two heater sections are run in series, but this is NOT the way it is usually implemented in practice. The two 6.3V heaters share a common point, with 6.3 V in parallel on these two heaters you get a "humbucking" improvement in the noise from the heaters, which is why the tube is almost always wired in this way. I beleive the two heaters are shaped as a double helix (like DNA!) to get this advantage.

In general you can make this sub - if you just drop-in substitute a 12AU7 or 12AV7 you can get a preview of the lower gain although it won't sound exactly like the octal tube I guess. I tried this drop-in experiment and went back to the 12AX7 butI did like the sound of (I think the *U* was the one I liked best) the lower-mu tubes pretty well.

-j

10/8/1997 7:26 AM
Whit	George, Check out Angela Instruments' new schematic at http://www.angela.com/catalog/how-to/Super_SE_6V6/ which describes a circuit which will apparently allow for swapping between 12AX7 or 6SN7, by installing one or the other. Should be some DIY info available from that project when the promised updated schematic gets posted. ... Whit

10/8/1997 10:19 AM

I took a quick look at the page. In a quick read through the page, it states that the parallel 6V6 will be "twice as loud as a single 6V6 amp".

Acoustic theory says that this is not strictly the case. It will have twice the output power if you do everything right in building, but even at twice the power, the human ear's logarithmic loudness response means that it will be perceived as only 3db louder, which is a just perceptible increase in loudness. Theory suggests that you need TEN times the power to be perceived as twice as loud.

Nothing to do with 12AX7/6SN7, but I thought it was appropriate.

10/9/1997 11:07 AM

R.G.: I think perceived acoustic level, or volume, is related to power by a square function. For instance, if you want to double the sound pressure level, you need four times the electrical power for a constant speaker cone area. Or stated another way, you can double the loudness with the same electrical power by using four spekers instead of one. Back to Angela's parallel 6V6 amp, twice the power would result in the square root of that, or 1.4 times as loud (3db, as you pointed out).

Best regards, Doc

10/8/1997 11:55 AM
John Martin	Yes, octal tubes do sound sweet. There is only one common octal triode that I know of with a amplification factor of 100, but it is a single triode, not double like the others you mention. Would be a space problem for sure. JM

Page 1 of 2