Valve replacing

[John]

Hi All, I have just purchased a second hand amp( Peavey valveking 212) I suspect the the three or at least one of the three preamp 12ax7's need changing as it keeps making a popping noise and now has popped the fuse in the back. It has "alegedly" had the four 6L6GC's replaced allready. If the fault is one of the valves how does one go about finding the fauly . They all glow untill the fuse popps that is
cheers John

[John]

Just to add, My next door neighbour has just given me a biscuit tin full of valves of varying sizes many similar in size to my 12AX7's,
for example there are several new mullard PCF802's would these be ok to use?
cheers John also ecc82's ecc85's ef86's

[RayL]

Whew! Hold it right there. Don't plug anything in until you are sure you know what you are doing!

First, some info about the valves that you have.

Valves that start with 'E' use the Mullard convention where the first letter signifies the heater voltage.
So 'E' = 6.3v. The next letters signify the type, so 'C' is a triode, 'F' is a pentode, 'L' is a power pentode. The first of the numbers indicates the size '3' is octal base, '8' is B9A. The last number identifies one valve un a series from another with different characteristics.

So an EL34 is a power pentode on an octal base with a 6.3 v heater, and ECC81, ECC82 and ECC83 are all double-triodes on a B9A base with a 6.3v heater (although because there are two valves in the same envelope, the heaters are wired in series so DO NOT, fo example, plug an EF86 into an ECC83 socket. Beware that PCF80! If I remember correctly 'P' heaters are higher voltage intended for series use in valve TVs - DO NOT USE IT!

The USA system is not quite so detailed, but the first number is the heater voltage, so a 6L6 (a power tetrode) has a 6v heater. A 6L6 is NOT identical to an EL34.

Some USA valves DO, however, have Mullard equivalents.
12AT7 = ECC81
12AU7 = ECC82
12AX7 = ECC83

It is very unusual for preamp valves to go wrong. Their gain reduces as they get older but they VERY rarely cause fuse popping. Power valves can cause fuse popping but there is another VERY common cause of blown fuses and that is caused by arcs ocross the power valveholders. If these are plastic then an initial tiny arc can carbonise the plastic and carbon is a good conductor and bigger arcs then follow and it is these which blow the fuse.

You can guess what I'm going to say, can't you? TAKE IT TO AN EXPERT!

Ray L

[David Martin]

As Ray advises, don't fiddle with unknown quantities...

Here's what Randall Smith of Mesa Boogie has to say about tube fault finding... (p.s. cheap/free tubes are more expensive in the long run...)

TUBE NOISE & MICROPHONICS: You may occasionally experience some form of tube noise or microphonics. Certainly no cause for alarm, this quirky behavior comes with the territory and the Tone. Much like changing a light bulb, you don’t need a technician to cure these types of minor user serviceable annoyances and in fact, you’ll be amazed at how easy it is to cure tube problems...by simply swapping out a pre-amp or power tube!

First may we suggest that you set the amplifier up on something so that you can get to the tubes comfortably without having to bend down. It also helps to have adequate lighting as you will need to see the tube sockets clearly to swap tubes. Use caution and common sense when touching the tubes after the amplifier has been on as they may be extremely hot! If they are hot and you don’t want to wait for them to cool off, try grasping them with a rag and also note that the glass down around the bulbous silvery tip is considerably less hot which makes it easier to handle. Gently rock the tube back and forth as you pull it away from its socket.

DIAGNOSING POWER TUBE FAILURE: There are two main types of tube faults: shorts and noise. Both large and small tubes may fall prey to either of these problems but diagnosis and remedy is usually simple.

If a fuse blows, the problem is most likely a shorted power tube and shorts can either be mild or severe. In a mildly shorted tube the electron flow has overcome the control grid and excess current flows to the plate. You will usually hear the amp become distorted and begin to hum slightly. If this occurs, quickly look at the power tubes as you switch the amp to STANDBY and try to identify one as glowing red hot. It is likely that two of a pair will be glowing since the “shorted” tube will pull down the bias for its adjacent mates, but one tube may be glowing hotter — and that one is the culprit. The other two are often fine — unless they’ve been glowing bright red for several minutes.

Because there is no physical short inside the tube (just electrons rioting out of control) merely switching to STANDBY for a few moments then back to ON will usually cure the problem...at least temporarily. Watch the tubes carefully now. Should the problem recur, the intermittent tube will visibly start to over heat before the others and thus it can be identified. It should be replaced with one from the same color batch, shown on its label. Call us and we will send one out to you. (DM note: this is a Boogie specific service. Change the lot... )
The severe short is not nearly so benign. In the worst cases, a major arcing short occurs between the plate and the cathode with visible lightning inside the glass and a major noise through the speaker. If this is seen to happen, IMMEDIATELY turn the amp to STANDBY. By this time the fuse probably will have blown. Such a short is usually caused by a physical breakdown inside the tube including contaminate coming loose or physical contact (or near contact) between the elements. Replace it and the fuse with the proper slo-blo type and power up the amp using the power up procedure as we described earlier in this manual.

TUBE NOISE: Often caused by contamination within in a tube, the culprit can usually be identified, and by lightly tapping on the glass, you will probably hear the noise change. Hearing some noise through the speakers while tapping on the 12AX7’s is normal however. And the one nearer the INPUT will always sound louder because its output is being further amplified by the second 12AX7. (DM note: 12AX7 and ECC83s are the same...)

The power tubes should be all but quiet when they are tapped. If crackling or hissing changes with the tapping, you have probably found the problem. To confirm a noisy power tube, merely put the amplifier on Standby, remove it from its socket and turn it back on. It will cause no damage to run the amplifier briefly with one power tube missing. You may notice a slight background hum, however, as the push-pull becomes unbalanced. Whenever you are trying to diagnose a suspect tube, keep your other hand on the POWER and STANDBY switches ready to shut them off instantly in the unlikely case you provoke a major short.
If you think you’ve located a problem tube but aren’t sure, we recommend substituting the suspect with a new one just to be sure of your diagnoses. You will be doing yourself and us a big favor by just following the simple guidelines previously mentioned regarding tube replacement. You’ll probably be successful with much less effort than is required to disconnect everything and haul the unit to a technician who will basically perform the same simple tests. If the tubes are still within their six-month warranty period, we will happily send you a replacement. Just note the color designation on the tube label so that we can send you the appropriate match.

DIAGNOSING PRE-AMP TUBE PROBLEMS: Because your amplifier is an all tube design, it is quite possible that you will at some point experience minor pre-amp tube noise. Rest assured - this is no cause for alarm and you can take care of the problem yourself in a matter of minutes by simply swapping tubes.
Let us begin by saying; It is a “very good” idea to keep at least a couple of spare pre-amp tubes on hand at all times to insure uninterrupted performance. These minor pre-amp tube problems can take many forms but can generally be described in two categories: Noise and Microphonics. Noise can be in the form of crackling, sputtering, white noise/hiss and/or hum. Microphonic problems usually appear in the form of a ringing or high pitched squealing that gets worse as the gain or volume is increased thus are more noticeable in the higher gain “HI” modes. Microphonic problems are easily identified because the problem is still present even with the instruments’ volume off or unplugged altogether - unlike pick-up feedback which ceases as the instrument is turned down. Microphonic noise is caused by mechanical vibration and shock: think of banging a microphone around and you’ll understand where the word came from.

The best way to approach a pre-amp tube problem is to see if it occurs only in one specific mode or channel. This should lead you to the tube needing replacement. Then all that remains is to swap the suspect tube for a known good performer. If you cannot narrow down the trouble to a specific mode or channel, the problem may be the small tube that drives the power tubes which is operational in all modes and channels. Though rare, a problem with the driver tube would show up in all aspects of performance - so if you can’t narrow the problem down to being mode or channel specific, you may want to try replacing the driver tube. Driver problems generally show themselves in the form of crackling or hum in all modes of performance and/or weak overall output from the amplifier. Occasionally an anemic driver tube will cause the amplifier to sound flat and lifeless, but this is somewhat uncommon, as worn power tubes are a more likely suspect for this type of problem.

Sometimes making the diagnosis is more trouble than it’s worth and it’s faster and easier to merely replace the small pre-amp tubes ONE AT A TIME with a replacement known to be good. But MAKE SURE you keep returning the tubes to their original socket until you hit the one that cures the problem. You’ll notice that tubes located nearer to the INPUT jack always sound noisier...but this is because they are at the start of the chain and their noise gets amplified over and over by the tubes that follow. The tube that goes into this “input socket” (usually labeled V1) needs to be the least noisy of the bunch. The tube that goes at the end of the preamp chain - just ahead of the power tubes - can be quite noisy without causing any problem at all. The tubes in your amp have already been located in the most appropriate sockets and this is why you should NEVER pull them all out at once and ALWAYS swap them one at a time. ALWAYS return a perfectly good tube to its original socket. Also it’s a good idea to put the amp on STANDBY when swapping tubes to reduce the heat build up in the tubes themselves and to prevent explosive noises (which can still occur even if you are pulling the tubes away from their sockets gently) from coming through the speaker.
Remember, take your time, be patient and chances are real good that you can fix your amp yourself by finding and replacing the bad tube. It kills us to see someone who has shipped their amp back to us...and all it needed was a simple tube replacement!
Pre-amp tubes don’t normally wear out as a rule. Therefore, it is not a good idea to change them just for the sake of changing them. If there isn’t a problem - don’t fix it. If there is no result from your substitutions, it may be possible that you have more than one problematic tube. Though rare, this does happen and though it makes the troubleshooting process a little more intimidating, it is still possible to cure the problem yourself.
NOTE: It is normal to hear a slight metallic ringing sound when tapping on the preamp tubes. As long as the tube does not break into oscillation or start crackling or any other form of bizarre noise, it is considered normal and functional.

If at all worried by all this, go to an expert as Ray suggests... Dennis Cornell is amongst the best in the world and he's just down the road in Southend...

http://www.dc-developments.com/

[John]

Cant believe I asked such a dum question but in my defense I had just got the amp home and was naturally miffed that I couldnt play with . Any way I have ordered three HARMA 12AX7's and will fit them when they arrive but from reading the previous reply it sounds like the 6L6GC's are the culprit as two are definatly hotter than the others and when the fuse blew there was a big flash from one of them. so as I got it cheap any way I can revalve it and still have payed below the odds, providing there is nothing more sinister wrong of course.
Cheers all John

[Amanda]

Hi,

"So an EL34 is a power pentode on an octal base with a 6.3 v heater, and ECC81, ECC82 and ECC83 are all double-triodes on a B9A base with a 6.3v heater (although because there are two valves in the same envelope, the heaters are wired in series so DO NOT, fo example, plug an EF86 into an ECC83 socket. Beware that PCF80! If I remember correctly 'P' heaters are higher voltage intended for series use in valve TVs - DO NOT USE IT!"

ECC8X series valves will work at two different heater voltages, by linking pins 4 and 5 together as in valve amplifiers
and connecting the heater supply across pin 9 and linked 4 and 5 you get a valve tht works at 6.3v at 0.3A.
By wiring across pins 4 and 5 and leaving pin 9 unconnected you get 12.6V at 0.15A (ECC8X series valves were used in TV's wired like this)

As for using second hand valves, care should be exercised trying output valves and rectifiers as they can flash over if faulty!

Valves like ECC series 12A?7 and EF86 are worth hanging on to especially if they are old mullard or other british types, preamp valves tend to take less
of a bashing than output valves and rectifiers.

As for output valves - a lot of the 6L6 etc. series and EL34 have the same pinouts and can be fitted
in place of each other, however they need different Bias settings so it's not a job for novices!

Regards

Amanda

[peter roest]

the problem of plopping fuse cannot be caused by defect preamp valve(s). The fault current is limited by the resistance of the anode resistor (normally 100k ohms or more for an ECC83). In my opinion the problem is caused by the rectifier or an high voltage elco. In this case you need an expert to solve ths problem.

peter

[Amanda]

John,

If a 6L6 valve flashed over when the fuse blew then It's almost certain to be
the 6L6.

I agree with Peter, preamp valves won't blow the fuse.