Tube Substitution

[unclej]

i had a customer bring me a fender blues de ville yesterday that needed a little work. guy plays the harmonica through it. after he described the symptoms he told me that someone had told him that if he replaced the 12ax7 closest to the power tubes with a 12ay7 that it would help eliminate some of the feedback problems that he was having because it would change the gain levels. i was honest and told him that i didn't have an answer but that i would find some info and let him know.

so that's what i'm doing..trying to find some info.

what would happen if he replaced just that one tube?

thanks

[Paul Marossy]

I have a Hot Rod DeVille amp, but I play guitar thru it.

12AY7s are a little hard to get, but a 12AT7 would also work. Check out this page for a comparison between different preamp tube gain factors: http://www.thetubestore.com/gainfactor.html

[unclej]

thanks paul..as usual that's the exact information i needed. i've saved the page so i won't have to ask again.

john

[Paul Marossy]

I'm glad that I could help.

[unclej]

just a quick follow up on this post. i tried a little experiment based on the information that paul provided. on the little gretch amp that i've been working on i installed an 12ax7 and got some of the greatest overdrive and feedback that a rocker could hope for. when i replaced it with a 12au7 and played an epiphone dot through it i got a great george benson, clean sound. then i put in a 12at7 and got a good in between sound..a little overdrive at high volume and pretty clean at the lower settings.

you probably all knew this already but it was fun, new infor to me.

thanks again.