w5m 12ax7 swap with 12au7

[bunk]

I'm curious to know if it would be safe to run two 12ax7 tubes in place of the 12au7 set in my heathkit w5m?
I'm sorry if this was already asked, I'm a noob and I'm still learning the forum. Thanks!

 

[ictwoody]

I'm not an expert, but I'm pretty sure the answer is no. They have different levels of gain, you need to run the tubes the circuit was designed for generally. Someone will be along shortly with a way better answer, but play it safe and don't insert the 12AU7's until you have a definitive answer.

- Woody

 

[cademan]

You can, and it won't cause any damage, but the 12AU7 is a medium MU triode and the 12AX7 is a HI MU triode so you might experience too much gain and/or distortion with the 12AX7 in the circuit that might be originally designed for the 12AU7 so I would stick to the 12AU7 if the original circuit calls for it.

I have an amplifier that I found that didn't come with any tubes and no schematic was available so I used a 12AX7 in the preamp tube spot and it was too overly bright and majorly distorted so I threw in a 12AU7 and it worked perfectly.

 

[gadget73]

my guess is you won't get enough current drive out of a 12ax7. This is a Williamson circuit, which was originally based around a 6SN7. If you'd like to try other tubes, maybe a 12BH7 is what you're after. Just make sure there is enough current capacity available since a 12BH7 requires more heater current. I vaguely remember mention of marginal power transformers on some Heathkits, though i have no idea if the W5M is one of them. If you really want to try a 12ax7, try it in the second spot. Thats a straight voltage amplifier. The first tube is actually the phase inverter, and it uses a directly coupled grid. I dont know what the voltages are like on this, but its possible that you'd exceed the ratings on a 12ax7 in that spot.

 

[bunk]

Thank you for the replies. I think I'll just keep the 12au where it's at.

 

[BmWr75]

Here is a useful tube substitution chart.

http://www.thetubestore.com/Gain-Factor

 

[jaymanaa]

Thank you for the replies. I think I'll just keep the 12au where it's at.

There are some special quality tubes that will drop right in; the 5963, the 5814, among others. I recommend an RCA Command 5814 with black plates being better than grey plates.

 

[sony6060]

Biasing is wrong for 12AX7s in a 12AU7 circuit, but will not cause damage. I thought 12AU7 are classified as low mu although some call the tubes medium mu. Best sounding 12AU7 type tubes are Brimar 13D5s.

 

[cademan]

I thought 12AU7 are classified as low mu although some call the tubes medium mu.

http://www.mif.pg.gda.pl/homepages/frank/sheets/093/1/12AU7A.pdf

 

[sony6060]

http://www.mif.pg.gda.pl/homepages/frank/sheets/093/1/12AU7A.pdf

Ya, I know 12AU7 amp factor of 20 whereas 12AX7 is 100. 12AT7 is 70.