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hum that probably ISN'T a ground problem


MattC

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My amp- my GK800RB- has an annoying little hum that is present at the same relative volume no matter how high I drive the amp. I've narrowed it down to the power amp- all the electronics up to the effects loop out are very quiet (including the direct out), but the amp still hums, even with nothing in the input (i.e., without the bass plugged in). It is not a ground loop problem; I've tried several grounding solutions including opening the amp and removing the ground cable from the power supply (only temporarially, for testing purposes).

 

At high stage volumes, this isn't really a problem (depending on the preset I use on my effect processor, although I usually turn the amp level control all the way down between sets). It is, however, rather annoying when I use this amp at low volumes to practice. The usual culprits, such as a noisy effect preset and flourescent lighting can exacerbate the problem but it is still present (again, only in the power amp) even in ideal conditions. Any ideas as to what component might be in need of replacement?

...think funky thoughts... :freak:
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No, not a combo, it's a head, and it's exhibited this problem with various sets of speakers. It's also not a loose solder connection; I've checked the amp several times. I do know for sure that it is a power amp problem.
...think funky thoughts... :freak:
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I have experience in this kind of stuff. Here's a few clues.

 

If the hum sounds like:

 

Low Bb (A string, first fret) on your bass, then it's a 60 Hz hum. Likely ground loop or external hum pickup, maybe from a broken shield in your guitar cable. It could also be a bad rectifier tube or a problem in the power cable ground to your amp.

 

If the hum sounds like:

 

Medium Bb (D string, third fret) on your bass, then it's 99.999% likely power supply filter hum (120 Hz). It could be due to gassy/grossly mismatched power tubes. Check and change electrolytic caps and check for leaky interstage coupling caps.

 

How to check for leaky interstage coupling caps? (Do this ONLY if you're comfortable with working on LIVE tube amps, since they have hazardous and deadly voltages to contend with. Play safe here....have a friend stand by in case you get into trouble and need the power killed immediately.)

 

Locate the "Control Grid" pin on the power tubes. For most power tubes of the 6L6 variety, it's pin #5. For other power tubes, look it up on the Internet. There's a resistor going from this pin to either chassis or bias supply, usually equal to or greater than 100 kOhms.

 

Pull all of the power tubes*, then turn the amp ON.

 

Use a digital multimeter, set on microamps, and shunt this resistor with the ammeter.

 

If the capacitor is leaking DC, the meter will show it this way.

 

After about 5 seconds or so, the reading should be no more than about 0.25 microamps for paper/wax capacitors, and much less than 0.08 microamps for Mylar/Polyester (or any other non-paper) capacitors.

 

Ideally all current reading should be zero microamps. That's the ideal to aim for.

 

*Why pull the power tubes? A gassy power tube WILL source current from the Control Grid, and if the tube is bad enough with gas, it could overheat, and it could damage your meter.

Beware the lollipop of mediocrity; one lick and you suck forever.
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GeorgeR -

 

Wow - impressive. I'll remember that you know how to do this - you're a brave man!

 

But the 800RB is all solid-state, so perhaps the tubes won't be much help.

 

My first thought was that it's the bass - except that it stays constant volume regardless of the level.

 

Does the hum happen when you bi-amp - meaning, do you get the same hum out of both the 100W "high" amp and the 300W "low" amp?

 

Have you tried replacing your speaker cables? (seriously!)

 

Does the hum change in volume as you vary the "boost" level? Try plugging in a "boost" footswitch and turning it on and off - it may be a problem with that circuit ...

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I couldn't say based on the model (I'm not familiar with models but with technologies)....but when I read the word "Vintage" I assumed it was a tube amp.

 

Solid state amps are less prone to these problems, but certainly not immune.

 

These are just a few things I would check if I had the amp on my bench and had to diagnose it. There are many other things to check into, assuming everything else I've said here checks ok.

 

Most common in solid state amps are decaying solder joints (so called "cold flow" due to years of heat/cool cycles) and oxidizing connections, especially around metal shields and ground connections. Any of these will be capable of contributing to hum.

 

Coupling capacitor leakage might be a problem if Tantalum or Electrolytic caps are used for signal coupling, but that's usually not the case. Besides, the input impedance of s.s. devices (FETs and op-amps excluded) is usually so low that leakage can be in the order of many microamps and no difference will be noted.

 

Closer to areas where significant heat may be generated (power supply, power amplifier) and especially because of the higher currents in s.s. amps, deterioration in the filter caps will result in a higher hum level (120 Hz) due to the cap's diminishing "ESR" (Equivalent Series Resistance) as the dielectric deteriorates.

 

If the amp was ever "repaired" (read HACKED) in the past, it might also be a "repair" which is gradually falling down on it's face.

 

An open in any pot in the signal path, or a lifted ground, will cause some 60 Hz pickup in the signal path.

 

Some older s.s. amps have adjustible bias on the power amp. If this is severely out of adjustment, due to hacking or age, (especially if it's a "bridge amplifier") then you can get some 120 Hz hum at no-signal conditions.

 

One more thing....check the screws for the power transformer. If they're loosening off, and IF at the same time the transformer's binding varnish is deteriorating in the magnetic core, you might get 60 Hz hum directly from the effects of the transformer's internal magnetic field.

 

The basis of all of this, is that assuming the amp was running perfectly when it left the factory, it's necessary to just return a number of key components to their factory-new condition to get new performance.

 

Anyway, I hope something here helps?

Beware the lollipop of mediocrity; one lick and you suck forever.
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Wow GeorgeR. Amazing.

 

So, here are some questions. Can you tell the rated power of a tube amp based on number of tubes? I have an old Peavey guitar amp with 4 jumbos in there...I think they are 6L6's.

 

Also, one fine day I was playing really loud rock and roll with the thing, and I noticed it was starting to smoke...I looked at the tubes and they were glowing bluewhite hot! I was seconds away from committing a terrorist act. So I shut the thing down. After it cooled off, I fired it up again, and never had another problem with it. I use it from time to time...but I'm frightened now.

"Let's raise the level of this conversation" -- Jeremy Cohen, in the Picasso Thread.

 

Still spendin' that political capital far faster than I can earn it...stretched way out on a limb here and looking for a better interest rate.

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So, here are some questions. Can you tell the rated power of a tube amp based on number of tubes? I have an old Peavey guitar amp with 4 jumbos in there...I think they are 6L6's.

 

A pair of 6L6's running class AB2 can yield about 50 watts (47 watts according to the RCA tube manual which runs them at 360 volts).

 

The class AB1 config, which is more commonly used in most guitar amps (due to it's simplicity), yields only about 26 watts per pair of 6L6's, again running at 360 volts on the plate.

 

You could get more power but 6L6s aren't the best tubes for high power.

 

They're popular only because there's lot of them around, not bad for a tube designed back in 1936 (the original metal shell 6L6)!

 

Some amps, like the Fender Twin, run them at their absolute maximum plate voltage (500 V), which gives them unique distortion characteristics, but certainly doesn't do any good for longevity and only promotes gassing over time, from the exposure to these relatively high voltages.

 

KT88's are much better tubes for higher power audio in this configuration, but to take advantage of them, you'd end up having to do considerable changes to the amplifier components (power transformer, power supply filter, and output transformer changes). Even then, for that expense, you're only gaining maybe a 3 db increase over a pair of 6L6's, so it may not be worth it.

 

Anything of a higher power rating and you're really talking about using RF transmitter tubes for the output, like 807's (80 watts per pair, run in class AB2 at 600 volts on the plate) or 811's (225 watts per pair, run in pure class B at 1500 volts on the plate). Both the 807 and the 811 are still commonly available in the amateur radio community.

 

If you REALLY want some power, see if you can scare up a pair of 833-A tubes (they might be hard to find). Run them in pure class B at 4000 volts on the plate, and you'll be rewarded with 2700 watts per pair!

 

YUM!!!

 

Also, one fine day I was playing really loud rock and roll with the thing, and I noticed it was starting to smoke...I looked at the tubes and they were glowing bluewhite hot! I was seconds away from committing a terrorist act. So I shut the thing down. After it cooled off, I fired it up again, and never had another problem with it. I use it from time to time...but I'm frightened now.

 

Blue glow isn't uncommon at all. In fact it's very common in new tubes as well. But if the glow was blue with predominantly white, if the glow seemed to consume the entire volume of the tube and was exceedingly bright, and now especially if there was smoke, it sounds to me like at least one of the tubes was VERY gassy.

 

So gassy in fact that it was acting like a Thyratron (tube version of an SCR) and just always drawing huge amounts of DC.

 

When that happens, the tubes are really not trustworthy any longer and you really have to change them because the more you use them, the worse it will get, until it will finally blow a fuse immediately upon warm up.

 

Hope some of this helps?

Beware the lollipop of mediocrity; one lick and you suck forever.
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Yeah, that was some pretty impressive tech info- worthy of going into the archive bin. Hear that, DBB?

 

I'll crack it open again and recheck those solder joints with a fine-tooth comb just to see if there's something I've missed. If I had to take a guess, I'd say that it's a 120Hz hum, and the mention of a "power suply filter" seems right, as least on a superficial level.

 

In a few weeks, when the studio computer is up and running, I'll use a refrence mic and a digital RTA and hope that this hum is realted to a single freq. (which I think it is). After I find this frequency, I'll add a fine-tuned parametric EQ with an extremely narrow Q to the mix and see if I can't alleviate this problem. If not, I'll just have to live with it until I can repair it.

 

By the by- through several openings of the case, I've never seen any evidence of "hack" repair jobs, and rest assured my repairs wouldn't be hacks.

 

Thnaks for the ideas guys!

...think funky thoughts... :freak:
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