Fender Bassman Head Preamp Tube woes!

AndyD

Hi Guys. I was playing my amp tonight and started noticing a weird overtone happening as I played certain notes (almost sounded like the famous stratitis ). Some of the notes had a slight fuzz to them and others sounded as though I had an octaver pedal activated except the octave was out of tune. I checked out Matts and karels post re preamp tubes going on the blink and I decided to take off the covers and check it out, pencil at the ready. I couldn't believe it! She still has the original 7025's x 3 and a 12at7.44 years of playing Brilliant! I have nos phillips 12 ax7 x1 and 2 nos ecc 83 and a new phillips 12at7.
1)Do I replace the offending tube only?
2)Would the above tubes work as replacements?
3)Should I just Replace the whole lot and be done with it as 44 years is a Fair innings to say the Least.
I must mention that the power tubes are 5 month old jjs and the sound great in this amp.
Hopefully I will get some advice from the gurus on the forum as I must use the amp for a band practice tomorow evening.
Cheers All
Andy

AlanRatcliffe

I couldn't believe it! She still has the original 7025's x 3 and a 12at7.44 years of playing Brilliant!

Yeah they don't make 'em quite like they used to, although the quality is getting better again every year. So hopefully we'll see Mullard/Telefunken quality valves again in the near future

1/3 - The phase inverter (the 12AT7) should always be changed when you change the power tubes. So if it wasn't done when the JJs were fitted, replace it now. Otherwise replace faulty pre valves as needed (especially when you want to get it up and running quickly to play). However, when you have the time and you already have a selection of good NOS valves, it's often worth rolling them (trying them in different positions to see what sounds the best), especially with the classic amp designs. If you end up with a full set of the unused NOS valves, so much the better for reliability's sake.

2 - 12AX7 / ECC83 / 7025 / CV4004 are all interchangeable.

matta

Hey Andy,

Yes, what you are hearing are called 'Ghost Notes', it can happen when tubes go microphonic but can also happens when filter caps go bad (not a problem there because your amp has been recapped).

RE your tubes, 7025's are military grade 12AX7's/ECC83's and fully interchangeable. If I recall correctly the first 2 7025's are for the BASS channel, the 3rd for the NORMAL channel and the 12AT7 is your phase invertor, so if you DID replace all the tubes use the pair of ECC83's for the first 2 valves, the Phillips in the 3rd socket and then the 12AT7.

I'm not sure you need to replace all of them, turn the amp up loud and gently tap on the tube with a non-conductive instrument, chop sticks or an artists paintbrush work well and listen to them, if you hear a ringing or microphonic sound, replace the tube.

Hope that helps!

Cheers

Matt

EDIT: Alan beat me to it! LOL

AndyD

Hey Matt and Alan. I thought you Guys would be awake.Thanks for the info.Amp looks Brilliant now that the Cab has the original Grill Cloth.Will Change tube in the am and give feedback.

matta

AndyD wroteWill Change tube in the am and give feedback.

Feedback? For that you want to turn the amp REALLY loud and stand close to the cab ?

Matt

AndyD

Yeah Yeah.Microphonic feedback as per tube depot video! It is the tube next to the phase inverter.Will reolace and give comment.Hows that.
Cheers

matta

AndyD wrote Yeah Yeah.Microphonic feedback as per tube depot video! It is the tube next to the phase inverter.Will reolace and give comment.Hows that.
Cheers

Yip, that is the preamp tube for the 'Normal' channel, which i'm guessing you use more than the 'Bass' one ?

Matt

TomCat

44 years.....Don't tubes have a 50 year guarantee..... ? ?....Just kidding. 44 years is a good run.....

Rob-dos

wow thats decent.. shop around a little, with a beautiful amp like that, spoil yourself ?

id love to hear it play