Tube amp noise - Help

bratpack7

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Nov 28, 2003
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Blues Jr., just had my EL84's replaced. Brought it home and sounded great.
Today went to play and there is a distorted buzz every time I pluck a string. It lasts until the note fades a bit and then is gone. It happens whether or not the amp it set up clean or with gain.

Any ideas? Could it be a bad tube?
 

el84ster

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Sep 10, 2001
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Well a bad tube can cause almost any symptom yes.
To me this sounds like a vibration causing a solder joint or tube pin to make a momentary iffy connection.

I’m a tech and that’d be what I’d look at first. Sounds like you had the tubes changed professionally? Take it back.
 
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bratpack7

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Well a bad tube can cause almost any symptom yes.
To me this sounds like a vibration causing a solder joint or tube pin to make a momentary iffy connection.

I’m a tech and that’d be what I’d look at first. Sounds like you had the tubes changed professionally? Take it back.

Thanks for the response. I did have it replaced at the store I bought the amp from, but for anything more than tubes they have to ship it away. So that just tells me they just had a sales guy do the tube change. And I've read the pin sockets can be fragile on this amp.

Is it not advisable to to try replacing tubes singly? I could maybe eliminate a bad tube by replacing with my one good one, but I understand it isn't good to replace unmatched tubes.
 

bratpack7

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Also just noticed the amp had Groove Tube 7's originally and the shop put in 4's. Not sure if this is a problem in the Blues Jr?
 

el84ster

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Well... depends how you describe problem. That amp is fixed bias to it technically does need to be adjusted when you swap tubes, however if you look round back and they’re not glowing red and you like the sound you’ll be fine. Amps in many ways sort of self adjust to an extent and aren’t as fussy as many make them out to be.

You could pop in a new tube and see, but long term you should replace both and ideally readjust the bias.
 

bratpack7

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Well... depends how you describe problem. That amp is fixed bias to it technically does need to be adjusted when you swap tubes, however if you look round back and they’re not glowing red and you like the sound you’ll be fine. Amps in many ways sort of self adjust to an extent and aren’t as fussy as many make them out to be.

You could pop in a new tube and see, but long term you should replace both and ideally readjust the bias.

The tubes glow red just fine. The best I can describe the sound is like a real crappy distortion that lasts (after you strike a note) for about 2-3 seconds, and then the note rings normally after that. If I roll the guitar back to volume 1 or 2 it doesn't do it but anything higher is affected. I thought it might be a ripped speaker or something but it all looks good.

I've taken it to a shop to have it looked at. I'm wondering if the first store put in a defective tube? or if they weren't matched, could that play a role? The package the tubes came in had been opened.
 

Wally

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Feb 27, 2003
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Congrats on getting the problem fixed.


Also just noticed the amp had Groove Tube 7's originally and the shop put in 4's. Not sure if this is a problem in the Blues Jr?

The Blues Jr. is biased very hot. It is not uncommon to see 100% of max plate dissipation at idle I; this fixed biased amp. That is hard on t7bes. When signal is processed, the dissipation goes up 8n fixed bias. That is why many use 70% as a max setting although one can set an amp hotter than that with the down side being shorter tube life than if it were biased cooler. Imho, running. Grade 4 would be advisable. Also, imho and ime, it is of value to know what the Biasing numbers are. This is how one can maintain continuity of sound from one set of power tubes to the next in a non-adjustable bias situation as you have there. You need a tech. Or...you need a bias probe tool that reads voltage as well as current draw so that you can understand what the tubes are doing in your circuit.
So the question at hand now that the amp has been repaired, does the amp sound differently than it did before the tube change? Is there some difference in the breakup both in sound quality/difference and in the volume at which the amp breaks up. Or....if you never push the amp hard, is there a difference in the content of the clean sound? Ime, there should be due to the difference in the current draw and p.late dissipation difference between; the 7s and the 4s. Note the differences. A yearmor so from now when you replace those power tubes again, try a 5.
 
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