doylemich
1 post
Sep 19, 2009
8:33 PM
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I need help here...I bought a nice little '79 Fender Champ to play harp. I am using a Green Bullet mic with it, but I am having a lot of issues with feedback. I have the mic volume set at 3/4, and I cannot get the amp volume any higher than 2 (out of 10) without major feedback. How can I resolve this?
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Bluzdude46
183 posts
Sep 19, 2009
9:52 PM
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Don't worry Doyle we'll get ya through this. To help us help you you need to do a little research. Take a look at the Fender specs and verify in your amp the tube compliment. The 70's Fender amps while not as desirable as the early 60's Blackfaces can still be transformed into pretty good Harp amps. Find out if it's a solid state rectifier (fairly certain it is) and find out if you can change out Pre Amp tubes of the same type without re-biasing get back to us with that and we'll begin.
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doylemich
2 posts
Sep 20, 2009
5:48 AM
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Inside the amp there are two tubes...one is a 5Y3GT, one is a 6V6GT, and there is something else labeled a 12AX7A. I am not sure how to check if I can change out pre-amp tubes? Where do I go for that info.?
Last Edited by on Sep 20, 2009 5:49 AM
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Cisco
15 posts
Sep 20, 2009
7:19 AM
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Change out the 100% gain 12AX7 to a 40% gain 12AY7. It will help immensely. You can even go to a 18% gain 12AU7 if you need more room. My 57 Champ uses the 12AY7 when I use a green bullet.
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doylemich
3 posts
Sep 20, 2009
7:32 AM
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I am a newbie to this...what volume level should I expect to be running my Green Bullet, and at what level should my amp be set under normal gigging/practice conditions? Will these tube changes get rid of my feedback?
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Bluzdude46
188 posts
Sep 20, 2009
8:05 AM
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Ok Doyle the 5Y3 is your rectifier tube. Your amp has Tube rectifier!! This is great news!! The 6v6GT is your Power Output tube and a fairly good one if it's one of the older Groove Tubes. The quality of newer ones are suspect. I think you are good here. You SHOULD be able to change out the 12AX7( which is your pre-amp tube) for the 12AT7, 12AY7 or 12AU7 without re-biasing, but you should check with either Fender or an amp tech first. Guys who have done it here can advise but make sure they have done it with your year box. I would, as Cisco advised, start out in the middle with the 12AY. The reason you are doing this is to cut the gain that is fed to the Power circuit. This will allow the amp overall a better range in the power stage without a higher gain signal to feedback.
Ok before the more technical among us jump me for not being precise with that explanation I know it's slightly different but best way for me to describe in lay terms. I feel you should have a basic idea of what works and how it works. Chances are if Doyle continues with harp he will buy another amp and now he'll have a better idea what to look for and how sound is effected by changes. Just because I had to buy 12 different amps before I figured it out doesn't mean we should punish newbies. Also remember, Doyle, sound is a relative thing. You may be after something totally different in "your sound" Good Luck
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Cisco
17 posts
Sep 20, 2009
8:23 AM
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The Champ has no bias adjuster so you don't have to worry about that. You should be able to crank your amp at least half way or a little more so that you're driving the power tube into distortion. The exact volume control on your mic (no feedback) will depend upon the room dynamics. You want to adjust to just a hair under where you start to get feedback.
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kudzurunner
702 posts
Sep 20, 2009
9:03 AM
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I should step in here and say that Doyle emailed me privately and I encouraged him to post here. I'm glad he has! I told him there were folks here who had much more knowledge than I do about preamp tubes, mic/amp matching, and the like.
Changing out preamp tubes can make an amazing difference. I recently had my Premier Twin-8 completely rebuilt by Skip Simmons. When it came back, it was physically in MUCH better shape, and it didn't sound bad, but something was missing. Recently I sat down with Brian Purdy and he changed out the pair of 12AY7s for 12AX7s. Pow! Suddenly the amp was EXACTLY right. And, to his credit, Skip had told me that I might want to try exactly this tweak. He'd left in my old 12AY7s because I'd asked him to, but I probably should have let him put in the stronger tubes. Right now, the amp is better than it's ever been. His rebuild really shines. But I needed those stronger preamp tubes in order to make that happen.
Now, here's why it worked: I use a Shure dynamic mic that doesn't have nearly the output of a Green Bullet. So with 12AY7s, my mic was UNDERdriving the amp. When we swapped in the 12AX7s, suddenly the preamp stage was putting out a lot of heat. And that made the whole amp come alive.
The Green Bullet, matched with the Champ, presents exactly the opposite problem. It puts out so much heat that the 12AX7s are too strong. This all makes sense to me. Thanks for the tips.
Last Edited by on Sep 20, 2009 9:05 AM
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doylemich
4 posts
Sep 20, 2009
9:35 AM
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You guys are great! All of you...Thanks so much for the info...I'm gonna give this a shot and I will let you know what happens!!
Thanks again!
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doylemich
5 posts
Sep 20, 2009
9:50 AM
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I forgot...one quick one. Is there a preferred place to buy the tubes?
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Scrapboss
22 posts
Sep 20, 2009
10:55 AM
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+1 for tubedepot.com fast shipping and great customer service.
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"I have a high tolerance for boredom
as long as it has a groove" - Scrapboss
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Jfllr1
42 posts
Sep 20, 2009
11:54 AM
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I experiance the same problems with an all tube champ 600 reissue(one12AX7 tube; One 6V6 tube) i just cant get the amp loud enough with feedback! Any suggstions on tube swaps?
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"Blow as thou pleaseth"
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Cisco
19 posts
Sep 20, 2009
8:51 PM
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12AY7 or 12AU7 in place of the 12AX7.
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