Fender '65 Twin Reverb amp

Superpool · 01-13-2002, 11:23 AM

I have a 1965 Twin Reverb amp and it sounds really good but the tubes in it are still the original one's and they need to be replaced. the only problem is that I wanted to do that but when I went to my local music store to get the tubes, the guy told me that it needed to be biased and my best bet was to just bring it to them and let them send it off to get it fixed. I was wondering if anybody new about these kind of amps or had any better ideas for me to change the tubes.

01-13-2002, 03:23 PM

You can do it yourself, but keep in mind there are leathal voltages stored in a tube amp even after it is turned off. If the amp still has the original tubes, it would probably be a good idea to have it biased. This should make your amp sound better, plus you can customize the tone by the type of preamp tubes you choose, and how you set the bias. Any time you change tubes (after a long time, or when changing tube types), it is a good idea to adjust the bias. A hotter bias setting will give a "hotter" sound, and a lower bias will give a "looser" sound. The range you can adjust is small, so unless you have a way to measure the bias of the tubes, it is best to let some one else do it. It would also be a good idea to have the shop go over the amp and check everything out. Sometimes capicitors and things need to be changed as they get old.

Ask the guy if you can watch over his shoulder when he is doing it, so yo can get an idea of what is requiered for the next time you change tubes.

GodsStratMan · 01-14-2002, 06:26 AM

Uhm ....

Is this a reputable music store? Cause I think they're taking you for a ride.

I say this, cause I'm a Navy Electronics Tech (22 yrs and counting) and am currently the lead instructor at a 6 month Advanced Electronics School ... where we teach tube/transistor theory.

The bias on a tube (or transistor) is set using fixed resistors and capacitors. The only things associated with a tube/transistor that can (normally) be adjusted are the supply voltage(s) and (for tubes) the filament voltage. Increasing the filament voltage will make the tubes conduct harder ... but will shorten their life expectancy.

Also ... why do you feel the tubes need to be changed ? And, if you have a '65, are the tubes still in production ? Chances are, the repair shop will reach into a box of tubes that are as old as the ones you already have. The golden rule of electronics is ... "if it ain't broke ... don't fix it".

Good luck ...

LWatford · 01-14-2002, 02:10 PM

Lots of companies are making all manner of tubes for guitar amps. Groove Tubes, Mesa/Boogie, and Electro-Harmonix all make pretty good tubes. Watford Valves in England sells lots of new old stock tubes, as do lots of other retailers.

Fender amps do have a variable bias control, but it is fairly simply to set and it shouldn't cost very much, or take that long to set.

Here are some articles with some information you might find interesting:

http://www.mesaboogie.com/US/Smith/biasadjust.html
http://users.erols.com/bluestat/FAQ/FAQ.html
http://www.duncanamps.com/technical/lvbias.html

mesabooger · 02-11-2002, 06:14 PM

Tubes go microphonic after a while and generally in guitar amps it's a good idea to change them after 1 year of hard playing. Now a 65, yeah I would get them swapped. But keep the originals if they really are from 65. Also, Most amps need to get re-biased after you swap tubes with the exceptions of Boogies, the don't require it.