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(@scrtchy)
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Joined: 18 years ago
Posts: 106
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Well of course I never got the chance to fiddle around with my preamp tubes because something has gone seriously wrong with my amp's health.

This came on soon after I started using a more powerful boost pedal... here's what it is doing:

AMP = '70 to '73 Fender Pro Reverb, non master volume

*The two power tubes instantly give off a bluish glow as soon as I take it off standby

*A very loud buzz also starts right when I take it off of standby

Could this be the sound of a dead rectifier? The Rectifier is still lighting up. There are no abnormal smells. Any ideas?

http://www.daughtersandsons.net -Cincinnati CEA Award winners for best original RnB/Funk band! (Bragging is in the user manual and encouraged)(Hi Mom)


   
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(@dogbite)
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Joined: 19 years ago
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pedals do make noise. perhaps it is the pedal.
does the amp whack out like that w/o the pedal plugged in and on.?

http://www.soundclick.com/bands/pagemusic.cfm?bandID=644552
http://www.soundclick.com/couleerockinvaders


   
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(@scrtchy)
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Joined: 18 years ago
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True, but nothing was plugged in. I just went to check it to see if it still did it with volume knobs down and it wasn't doing it at all. Thoroughly confused. When it was doing it, it was very loud... like maybe like a guitar plugged in that had it's ground removed, and a distortion pedal on full.

It seems to be a bit thin and too bright sounding, all the tubes are new except the rectifier, preamps are JJ's, power tubes are Svetlana Winged C's. Maybe the bias has drifted? Going to check...

http://www.daughtersandsons.net -Cincinnati CEA Award winners for best original RnB/Funk band! (Bragging is in the user manual and encouraged)(Hi Mom)


   
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(@ricochet)
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Joined: 21 years ago
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Blue glow from the power tubes is fine.

A loud buzz and no useful sound is most likely from a dead filter capacitor in the power supply. They're big electrolytic capacitors, and they have a limited lifespan. They can either dry up, lose capacity and go open, develop leaks in the oxide dielectric film on the anode foil and short out, or both. (That last sounds paradoxical, but it happens.) Lots of folks won't start up an old amp without first replacing the filter caps, it's an expected thing to need to do anyway. If the cap is leaky and you let it keep running while humming like that, sometimes it'll "heal" itself by reforming the oxide film, but there's a risk that the cap will get hot and explode (which is a real mess!) and the excessive current can damage the rectifier tube. Good practice is to just go on and replace all the filter caps and any other electrolytic caps inside such as cathode bypass or screen bypass caps while you're at it.

I'll confess to having "resuscitated" old stuff by just letting it hum a while and re-form the caps, while keeping a fingertip on the outside of the filter caps to feel for excessive heating and watching the plates of the rectifier closely for reddening. But when they reform, usually they still hum excessively from the drying and loss of capacity, and it's a temporary "fix" at best. Better than just letting it run, if you want to reform caps, is to slowly bring the voltage up from about 70% to full voltage with a Variac or use a light bulb load initially in series with the power supply to drop the voltage.

Your caps may be on the edge of death, reforming under power for a bit, breaking down again when turned off or put on standby, then reforming again. They're about to quit for good if that's the case.

The rectifier tube may well be going out, too, they go out faster than power tubes on the average and hum, loss of power and excessive distortion can be caused by that. Certainly it's easier to plug in a rectifier tube than to change out filter caps!

"A cheerful heart is good medicine."


   
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(@trguitar)
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Joined: 18 years ago
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Scr@tchy ....... I noticed a series of posts where you and Ric were .. ummm ..... bantering back and forth. I was about to PM you when a resolution occured. I was going to tell you not to bee too "porky" with Ric as he is not only a very nice guy but a most valuable resource when it comes to tube amps. I myself don't know squat, but I think you can see from the above post he can be a life saver! He understands these things inside and out as he has been tinkering with them most of his life. I defer to Ric. Get new filter caps. If changing your rectifier tube doesn't help of course. 8)

"Work hard, rock hard, eat hard, sleep hard,
grow big, wear glasses if you need 'em."
-- The Webb Wilder Credo --


   
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(@scrtchy)
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Joined: 18 years ago
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I actually did replace the filter caps, I'm one of those replace them first types, but I'll go back and check to see if one of the new ones is faulty, or if my work is. Though I didn't change any of the other caps yet, I got a little impatient to use it. I have 4 new rectifiers coming in the post, so meanwhile I'm going to change the rest of the caps and anything else any of you think I should. The amp just made it through another rehearsal but it is thin sounding, the buzz has not returned and it makes me think that it was temporary wierdness. I did check and rebias it before rehearsal, and found that the bias had slipped what I would consider a large amount. The only thing I had changed since I last set it was a different rectifier tube, and a trial test of a more powerful booster pedal. I'm going to check the bias again in a few hours and see if it drifted again, because the amp sounded thin and brittle while on about 4-5 on the volume dial.

Anything else I should consider changing out while I have her opened up?

http://www.daughtersandsons.net -Cincinnati CEA Award winners for best original RnB/Funk band! (Bragging is in the user manual and encouraged)(Hi Mom)


   
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(@scrtchy)
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Thanks TRGuitars, and what a nice thing for you to do. Luckily, these types of things will forever work themselves out as reality is what, um, it is. I coudn't imagine how two nice people wouldn't end up hitting it off if given enough time to intereact and devulge.

http://www.daughtersandsons.net -Cincinnati CEA Award winners for best original RnB/Funk band! (Bragging is in the user manual and encouraged)(Hi Mom)


   
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(@ricochet)
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Scr@tchy, for an exhaustive list of all the other possible things to check, I'd suggest you go to http://geofex.com/ and click on "Tube Amp Tech Pages," then on "Tube Amp Debugging Page," then "Hum."
:D

Glad to hear you replaced your filter caps first. Here's another, not so nice thought: Sometimes caps sit on the shelf long enough to go bad when they're "new." I bought new caps a while back and put them in my old Hammond organ to replace the leaky screen bypass caps in the power amp driver tubes, which were "thunderstorming." (Arcing through the oxide, making a steady crackle and hiss with intermittent booming and roar.) The new ones were even worse, so bad I immediately powered off, went back and looked to make sure I hadn't installed them backward! I hadn't. After letting them re-form for a few minutes, they quietened down and have worked OK since, but it was a real attention getter. I could find no manufacture date codes on the caps or their packaging. They came straight out of a plastic package on the rack at the local electronics store. (Not a Radio Shack, they don't carry anything useful in a tube amp anymore.)

Thanks for the nice words, guys. Like I said, I was pretty jerky the other night. I've been really tired and cranky lately.

"A cheerful heart is good medicine."


   
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(@scrtchy)
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I will check those new filter caps. Here is the latest update in what is becoming what I believe to be evidence of a curse laid upon me.

I powered up last night to check and see if my bias had drifted. But what I found first was two microphonic preamp tubes in positions V1 and V2! I changed them and that got me back to square, I feel pretty silly for having not checked this earlier but in my defense I was concerned about that buzz. Ok, so after switching out the bad tubes I rechecked the bias and let it sit for a couple of minutes, nothing was plugged into the front. All of a sudden that loud buzz attacked again, so I quickly checked the bias to find it had jumped up insanely. I switched off the amp immediately and haven't turned it back on.

I am hoping that with the mention of the voltage surge coupled to the buzz it would provide a clue that would reveal the problem to those more knowledgable...

http://www.daughtersandsons.net -Cincinnati CEA Award winners for best original RnB/Funk band! (Bragging is in the user manual and encouraged)(Hi Mom)


   
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(@ricochet)
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Sudden out-of-nowhere strange noises and the bias voltage jumping up (I'm assuming you mean more negative) make me think of the amp oscillating. I mentioned that a few days ago in another thread about an amp that was intermittently losing volume and making noises. Any amp can start oscillating, an internal feedback of some sort causing it to produce a wave that's much like the squeal you hear with a mic or guitar too close to a speaker except that it's in ultrasonic or radio frequency. Draws power away from the desired audio amplification and can distort it, could cause hum by drawing heavy current from the filter caps so their smoothing effect is overwhelmed, and get this, can make your bias voltage go up. Here's how: (This is also going to explain that "blocking distortion" I mentioned, Musica123.)

An amplifying tube has an operating bias voltage that allows a certain amount of idle current to flow in the tube, and is the zero point that the signal voltage applied to the control grid fluctuates around to produce the amplified signal that shows up as fluctuations around that zero signal or idle plate current. (When the signal drives the grid momentarily more positive, more plate current flows, and when it goes more negative, less plate current flows. Normally the bias stays right around that average level.) If that voltage is an actual negative voltage applied to the grid, we have "fixed bias." (Never mind that you usually can adjust its level.) Otherwise, a resistor in series with the cathode makes the cathode develop a positive charge relative to ground, and the grid stays at ground potential, which is pretty much the same thing but it adjusts itself for tubes that tend to draw different amounts of current. That's the way all of our preamp tubes are biased, and lots of power tubes like the one in an Epi Valve Junior are, too.

Without going into a lot of explanation about how the amp works, I'll just point out that if the driving signal on the grid (which is an alternating current) ever exceeds twice the grid bias, the positive peak of it will drive the grid voltage to 0V, which is about the point where the grid starts "sucking up" electrons out of the stream going from the cathode to the plate. Those electrons will flow back to the coupling capacitor that links the AC signal to the grid from the plate of the previous tube. That stream of negative electrons keeps the side of the capacitor hooked to the grid from going much more positive on that signal peak. Clamps the voltage there, rounding off the signal peak. But when the signal swings back negative again, those electrons can't leave the grid. They have to flow to ground through a high-value resistor, so the negative peak now goes more negative than before. The bias has gone up. (In a negative sense.)

"Blocking distortion" is when the capacitor charges up enough from a sharply positive impulse to the grid that the excessive bias "cuts off" the tube for a time until the charge leaks away through the resistor. You actually hear cutouts in the sound. The time constant is the product of the values of the capacitor and resistor; if they're small, the cutouts are very brief. Make them too big in a misguided attempt to build a hi-fi bass amp with excellent low frequency response, while driving it to the point of grid current, and you'll get very disturbing breaks in the sound.

The inaudible ultrasonic or RF frequency from your amp's oscillation is driving your output tubes into grid current and running up the bias. It's also drawing lots of current from the power supply so you hear the hum as it's overloaded, and heating up your power tubes. Most likely cause is a bad preamp tube. Could be oscillating in the power stage itself, though. Anyway, look to the tubes first. If you have or can borrow an oscilloscope, you can quickly prove me right or wrong about the oscillation. But for fixing it, look to your tubes. New tubes can be bad, too.

BTW, I'm excited to find that the Music Electronics Forum is thriving, not down as I thought a week or so ago. http://music-electronics-forum.com/

"A cheerful heart is good medicine."


   
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(@scrtchy)
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Joined: 18 years ago
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Thanks for the information. I will be going through the preamp tubes to see if any of those are the causes. I haven't turned it on since it's last bout of 'possession'. Any thoughts on me getting a young priest and an old priest in there to yell at it for a while? Though I guess I should be happy that it hasn't levitated or thrown up on me yet.

Also, are any of these symptoms I am describing in anyway pointing towards a possibility of there being anything wrong with the transformer should the rest of the preamp tubes be good? What about the bias pot? Which preamp tube could have this drastic of an effect on the bias reading?

http://www.daughtersandsons.net -Cincinnati CEA Award winners for best original RnB/Funk band! (Bragging is in the user manual and encouraged)(Hi Mom)


   
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(@ricochet)
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I don't think it could have anything to do with the transformer or the pot.

Any tube in front of the power tubes, or in fact the power tubes themselves, if oscillating, would cause the rise in bias. The bias rise is a symptom of the power tubes being overdriven into grid conduction, not the cause of the problem.

Put a voltmeter on your bias voltage and whang on your guitar with the amp cranked. The bias will go up exactly the same way. This is what causes the distortion fans of cranked amps love. Grid current charges up the coupling capacitors, overbiases the power tubes, and in the case of a Class AB1 push-pull amp, this means that each tube cuts off before the next starts conducting for the other half of the sound wave. Makes a big glitch in the waveform at the zero voltage crossing. This crossover distortion (which hi-fi freaks regard as a terrible thing) is the "sweet power tube distortion" rockers love. It's all odd harmonics, mainly third harmonic, which is a perfect fifth musically. (One octave up.)

"A cheerful heart is good medicine."


   
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(@scrtchy)
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Joined: 18 years ago
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Thank you. You were absolutely correct, once I changed the phase inverter it quit that humming. That is three new JJ preamp tubes that have gone bad from the batch I just bought. So now I am going to go through the rest of theme as I suspect there is another that is acting up, or maybe the power tubes... the amp seems restrained and off colour in the EQ. But at least now I can hear and feel that possibilties ahead once I have settled the tube issues as best I can.

Do you still think it is a good idea to change out the original caps? How does this usually effect the character of the amp afterwards?

http://www.daughtersandsons.net -Cincinnati CEA Award winners for best original RnB/Funk band! (Bragging is in the user manual and encouraged)(Hi Mom)


   
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(@ricochet)
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Joined: 21 years ago
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It's surprising how many new tubes are bad, and during the first 100 hours or so is when most tube failures occur. If they make it through the early mortality, they last a long time and gradually fade away. That's why I never change tubes that are working as "preventive maintenance," because it's asking for trouble, and I never switch tubes around for "tonal" reasons, either.

The new tube makers get a lot of criticism for poor quality, but when I hung around the TV repair shop as a kid in the early '60s I watched the owner's teenage daughter test every new tube that came in, all American tubes of the sort now sold as NOS, when they were simply brand new. An awful lot of them went back to the distributor because they didn't pass testing. It may well be that the new tubes have more problems, but I've had good success with them.

Tubes were always regarded as a fungible commodity, as gasoline is. Every maker had the brand stamps of every other maker, and when one was running off a batch of a particular type of tube and another maker needed some for their inventory, they'd just call up for some to be made with their brand on them. As long as they met the type specs, it didn't matter who made them. I still regard them this way. I buy what's cheapest and works.

Electrolytic caps can only deteriorate with aging. Replacing them with similar values just restores the original character of the amp, it doesn't change it.

"A cheerful heart is good medicine."


   
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(@scrtchy)
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Replacing them with similar values just restores the original character of the amp...

You do realize that this means it will change it! That is to say, if it is not the original character, and mine sure couldn't be, then I would like to change it back to it's original character... I can hear it's real close at the moment but it just doesn't have the impact and attack, it's seems just, well, less. I think if I recapped it I would be less concerned about the preamp tubes like you. I just played through a Deluxe Reverb reissue and it sounded astounding with the Fender Groove tubes in that I hear alot of people bemoaning. I kept thinking that if my amp sounded like this I don't think I could do much to hurt it by changing tubes, or to help it. Really I mean why would I, it couldn't have gotten too much more astounding. I say this now, but I'll be searching for Mullard 12ax7's on eBay in a moment. What are the odds that at some point I accidentally open mouth kissed Eric Johnson and he passed on his sickness to me? I say accidentally because I usually request medical papers from celebrities I kiss. It would have to have been accidental, I feel I must stress this here.

http://www.daughtersandsons.net -Cincinnati CEA Award winners for best original RnB/Funk band! (Bragging is in the user manual and encouraged)(Hi Mom)


   
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