Microphonic amp...or tube? What's going on?

TheGoodTexan · February 8th, 2005, 11:22 AM

Last night I'm cranking my new tweed Bassman RI LTD....just guitar + Maxon OD9 + amp. I've got the channels jumped.

I hear a little uncontrolable high pitched feedback, so I turn the tubescreamer down....then off. Feedback is still there....so I turned the guitar all the way down....feedback is still there...

So I unplugged everthing from the amp....and the high-pitched feedback is still there...with both volumes around 4 or 5 (which is loud).

As I unplugged the jumper cable, I noticed that the top panel...the control panel...is microphonic. I can tap on it and hear it clearly (and loudly) through the speakers.

What's going on? Pre-amp tube? Worse?

I just bought this amp (new) before Christmas.

Jakedog · February 8th, 2005, 11:29 AM

I would guess pre-amp tubes. Tap 'em all lightly with a pencil with the amp set the same way. if one of 'em sounds really loud through the amp when you tap it, that's your cuprit. If it ain't that, I have no idea.

Jake

TheGoodTexan · February 8th, 2005, 11:45 AM

Quote:

Originally Posted by Jakedog

I would guess pre-amp tubes. Tap 'em all lightly with a pencil with the amp set the same way. if one of 'em sounds really loud through the amp when you tap it, that's your cuprit. If it ain't that, I have no idea.

Jake

Yep...that happens...but is that a true test? or just the lay-person test?

What's the prefered 12AX7 du jour?

I usually just by an EH 12AX7 or a Sovtek 12AX7LP or LPS.

Jakedog · February 8th, 2005, 12:26 PM

I'm kind of like you are. I just buy whatever is readily available. When my amps start sounding like rice krispies, or sqealing like yours did, I start tapping tubes. when I find the one that's acting up, I slap a new one in there. Usually whatever is handy. I've been lucky, in that I've always been able to fix whatever problem I had this way.

I will say, that when I bought my peavey head last summer, it was a wreck, so I had the whole thing gone through and fully serviced. The tech recommended JJ tubes, so I went with a full set of those, in the pre and power sections. I haven't tried the amp with anything else, so I really can't compare, but to my ears these JJ's are awesome. They are fairly cheap too.

Jake

bo · February 8th, 2005, 01:15 PM

In fact I'd guess that it might be the first 12AX7, as that seems to be the one that acts up on my Bassman. Hope it's a simple fix.

Daddy Elmis · February 8th, 2005, 01:46 PM

Certainly any 12ax7 will work, but what you put in that first (v1) position will have more affect on your tone than anything else you can/will do to that amp.

The original 59 Bassman's used a 12ay7 in the first position, which lowers the gain a bit for a more "vintage" sound.

Other than that, you might want to get some good preamp tubes, as other than obscure NOS RCA's or Mullards they are relatively cheap.

FYI, I really like NOS Jan/Philips 12ax7's which can be had for around $12-15 each. I've also recently tried the new Groove Tube 12ax7-M (the "Mullard Reissues") and they sound really nice in the Bassman.

jjmantele · February 9th, 2005, 10:56 PM

I had a similar problem with my Harmony. Sometimes a note and always a tap on the input area caused a noise. I had assumed it was a pre-amp tube or something close to the inputs area for a few weeks before I considered the output tubes. It was a microphonic JJ EL-84. The tapping-on-the-tube test revealed it and the replacement (pair) confirmed it.

February 8th, 2005, 11:29 AM	#2 (permalink)
Jakedog Friend of Leo's Join Date: Mar 2003 Location: Cleveland,OH But my heart's still in TE Posts: 3,227	I would guess pre-amp tubes. Tap 'em all lightly with a pencil with the amp set the same way. if one of 'em sounds really loud through the amp when you tap it, that's your cuprit. If it ain't that, I have no idea. Jake __________________ Oh, the fear I've known That I might reap the praise of strangers And end up on my own All I've sown was a song And maybe I was wrong -Emily Saliers

February 8th, 2005, 12:26 PM	#4 (permalink)
Jakedog Friend of Leo's Join Date: Mar 2003 Location: Cleveland,OH But my heart's still in TE Posts: 3,227	I'm kind of like you are. I just buy whatever is readily available. When my amps start sounding like rice krispies, or sqealing like yours did, I start tapping tubes. when I find the one that's acting up, I slap a new one in there. Usually whatever is handy. I've been lucky, in that I've always been able to fix whatever problem I had this way. I will say, that when I bought my peavey head last summer, it was a wreck, so I had the whole thing gone through and fully serviced. The tech recommended JJ tubes, so I went with a full set of those, in the pre and power sections. I haven't tried the amp with anything else, so I really can't compare, but to my ears these JJ's are awesome. They are fairly cheap too. Jake __________________ Oh, the fear I've known That I might reap the praise of strangers And end up on my own All I've sown was a song And maybe I was wrong -Emily Saliers

February 8th, 2005, 01:15 PM	#5 (permalink)
bo Tele-Afflicted Join Date: Mar 2003 Location: Arlington, VA Posts: 1,794	In fact I'd guess that it might be the first 12AX7, as that seems to be the one that acts up on my Bassman. Hope it's a simple fix. __________________ www.bourbondynasty.com

February 9th, 2005, 10:56 PM	#7 (permalink)
jjmantele Tele-Holic Join Date: Aug 2003 Location: New Jersey Posts: 851	Micro I had a similar problem with my Harmony. Sometimes a note and always a tap on the input area caused a noise. I had assumed it was a pre-amp tube or something close to the inputs area for a few weeks before I considered the output tubes. It was a microphonic JJ EL-84. The tapping-on-the-tube test revealed it and the replacement (pair) confirmed it. __________________ JJman If it says "Vintage" on it -it isn't.

February 8th, 2005, 01:46 PM	#6 (permalink)
Daddy Elmis Tele-Holic Join Date: Jun 2004 Location: USA Posts: 961	Certainly any 12ax7 will work, but what you put in that first (v1) position will have more affect on your tone than anything else you can/will do to that amp. The original 59 Bassman's used a 12ay7 in the first position, which lowers the gain a bit for a more "vintage" sound. Other than that, you might want to get some good preamp tubes, as other than obscure NOS RCA's or Mullards they are relatively cheap. FYI, I really like NOS Jan/Philips 12ax7's which can be had for around $12-15 each. I've also recently tried the new Groove Tube 12ax7-M (the "Mullard Reissues") and they sound really nice in the Bassman.