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I'm a newbie to the tube game and recently came upon several amperex bugle-boy 6922 tubes, including one telefunken and one mullard as well. My question is, can I place these in a Fender Vibrolux Reverb (blackface) in the pre-amp section (normally 7025/12AX7) without blowing anything up? Would I need to modify anything on the amp for it to cater to these tubes? Because I have several, I'd like to utilize them for something. If not this amp, perhaps some suggestions on finding some schematics for amps that do use them?

My background in electronics is rudimentaryto intermediate at best, but I'm catching on pretty quick (other than deciphering a datasheet for these things.) and I love this kind of gear.

Thanks for any information you may have.

-z

Hello and welcome!

The 6922 will not work without rewiring the socket - the heater connections for the 12ax7 series of tubes tie pins 4 and 5 together and feed the heater to 4/5 and 9, while the 6922 has its heater connections of 4 and 5 (with pin 9 being a shield between the sections). Essentially the heater wouldn't work. This is VERY BAD for the tube because
1) applying high voltage to the tube plates without the heaters being on is not a very good idea (it will survive for a while, but can become damaged if the exposure is prolonged), and
2) the 6922 has an absolute maximum plate voltage of 250 Volts, which would be exceeded in this circumstance.

You could modify the socket connections to accept the tube, but I am against doing this mod to your amp not only because of the max. voltage issues, but also because you can't plug a 12ax7 series tube back in without having the mod undone (there are some other tubes that would plug into the socket, though). Not to mention that those are highly sought after tubes for Hi-Fi, and quite valuable - and that's a valuable amp too!

As for schematics, there aren't many for guitar that use it (not that it couldn't be used), but many for Hi-Fi (it's a higher quality version of the 6DJ8 or ECC88) if you're interested.

jonnyeye-

Thanks for the info-- I had no idea that the heaters were different and I would have prolly fried me some tubes. I'm currently building my first tube guitar amp (learning as I go) and if that is succesful I will build a hi-fi amp for my entertainment center, so I'll save my 6922's for that. Thanks again for the info.

-z

zebrostripe wrote:jonnyeye-

Thanks for the info-- I had no idea that the heaters were different and I would have prolly fried me some tubes. I'm currently building my first tube guitar amp (learning as I go) and if that is succesful I will build a hi-fi amp for my entertainment center, so I'll save my 6922's for that. Thanks again for the info.

-z

The "12" in the 12AX7 part number is for "12 Volts". But the 12A*7 series of tubes are designed so you can run two halves of the heater in parallel at 6.3V. Think of the 12A*7 as having either (1) two 6V heaters sharing a common return path or (2) a single 12V heater with a center tap.

You might be able to rig a switch or jumpper that would allow you to use either kind of tube. Your tubes are the "standard" type