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(@ricochet)
Illustrious Member
Joined: 21 years ago
Posts: 7833
 

"Impact and attack" will be lost if your filter caps have dried up and lost capacity. Bass response, too. They don't get talked about much, but electrolytic cathode bypass caps dry up and quit bypassing, and will have a bigger effect on your tone than the filter caps. There's a negative feedback in operation for all frequencies below the rolloff point of the high-pass filter represented by that cap. Let a bypass cap dry up (or replace it with a smaller value or take it out of the system) and you'll definitely hear the loss of bass, impact and attack. Put it back and you'll hear things tighten up. You don't want to build in a lot more bass response than you need, though, or you run the risk of "motorboating," or low frequency oscillations. But you can generally get away with big cathode bypass caps if you want them and caps much bigger than theoretically indicated are often used.

Don't do that with coupling caps, or you'll run into that unpleasant blocking distortion causing your sound to cut in and out like I mentioned earlier. There it's best to keep 'em down in size to roll off at your bottom frequency of interest (about 80 Hz for guitar.) Don't go overboard on grid load resistor values, either. You won't gain much gain by doing so, and you not only increase the time constant for blocking, you increase the chance of a tube "running away" from ionized gas shifting the grid bias progressively more positive as the tube heats up, making it heat up further, etc. That's why the values in the tube manuals for maximum grid resistance are there, and why it's different (lower) for fixed bias than for cathode bias, which gives a negative feedback to increasing idle current.

"A cheerful heart is good medicine."


   
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(@oldiron)
Estimable Member
Joined: 20 years ago
Posts: 111
 

Some of the Eastern European tube manufacturers have quality control issues. I'll try to find some of the links for the antique radio sites that still have NOS GE and Phillips tubes available. I'm trying to hold on to my stash.

The caps in your early 70's vintage amp are about twenty years past their prime. It wouldn't hurt a bit to go though and re-cap the whole thing. I play with a lot of antique radios and that is usually the first thing I do when restoring an old radio. Doesn't hurt the value a bit as long as you find exact replacements. You can't substitute ceramics for electrolitics.

Another thing a blue glow from a tube is something to be careful of. That blue glow is generating x-rays so the tube should have a shield if it is operating within specs. If there is no shield you don't want to sit and watch the pretty blue glow. It ain't healthy. The case of the amp is enough to shield you from the small dose of x-rays but sitting and looking at it isn't a good idea. If the same tube starts emitting an orange or yellow glow it's getting "gassy" and should be replaced.

I've been an electronics tech for 27 years and learned my trade on tube radar sets. I may not be the best picker in the world but I know tubes.

I may be going to hell in a bucket but at least I'm enjoying the ride. (Jerry Garcea)


   
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(@ricochet)
Illustrious Member
Joined: 21 years ago
Posts: 7833
 

Tubes are always generating X-rays when operating, blue glow or not, but at the ~300-400V operating voltages of guitar amps, they're too "soft" (long wavelength, low energy) to penetrate the normal lime glass envelope of a tube. Don't worry about that.

Blue glow in a power tube is fluorescence on the inside of the glass from accelerated electrons that miss the plate and hit the glass, making it fluoresce in exactly the same way as the screen of a CRT does. Blue glow can only be sustained when enough secondary electrons are knocked out of the glass by those electrons to maintain the glass at a positive charge near plate potential. If the accelerating voltage dies down while the cathode is still emitting electrons, as when you turn off the standby switch with the tubes hot, slower electrons will hit the glass with insufficient force to knock out secondary electrons, the glass will accumulate enough electrons to repel new ones when the plate voltage is reapplied (what CRT engineers call a "sticking potential"), and the glow will cease until the amp's turned off and the charge slowly bleeds away. The blue glow is normal for power tubes and not a sign of malfunction in the tube or equipment. Ionized gas glowing in a tube is all inside the plate structure, of a different (usually more purplish) color, and if you can really see it glowing, the plate is also likely to be glowing red from the increased plate current caused by the positive ions bleeding off grid bias.
:D

"A cheerful heart is good medicine."


   
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(@oldiron)
Estimable Member
Joined: 20 years ago
Posts: 111
 

That was just a safety thing we always observed. The thyratrons we used in power supplies were running at 400 volts but with a lot more current than what is used in guitar amps. It doesn't hurt to be careful. I've seen guys who would pull the shields off tubes because they thought it looked "cool". Not necessarily a good idea.

BTW if you run across any old Grundig radios let me know.

I may be going to hell in a bucket but at least I'm enjoying the ride. (Jerry Garcea)


   
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(@ricochet)
Illustrious Member
Joined: 21 years ago
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No, it certainly doesn't hurt to be careful!

The shields in guitar amps are on preamp tubes, to keep electrostatic interference out and to reduce electrostatic cross-communication between the two halves of tubes like the 12AX7.

Putting shields on power tubes that weren't meant to run them will cause tube overheating and premature failure.

There are several issues with big high power tubes used in broadcasting and similar uses that we don't need to worry about in guitar amps, most notably "cathode stripping." It's never been demonstrated in small receiving-type vacuum tubes such as we use, and other than guitar amps, consumer equipment never had standby switches or delayed voltage application in startup. Doesn't hurt to be careful as you said, but it's really not a matter of urgent importance to warm up a guitar amp with standby off, or to even have a standby switch on the amp. Power up a mercury vapor rectifier without first thoroughly warming it up, though, and you've got a dead tube.

On the other hand, small receiving type tubes that are left hot without plate voltage applied for extended periods, as in old tube computers and standby communications equipment, most definitely can and do suffer from "sleeping sickness," where the cathode coating loses it's emissive capacity and the tube is either dead or malfunctioning from weak emission when put back into service. If not too far gone, it may recover after being put into use, much as brand new cathodes don't emit well until they go through a "burn-in" process. Special tubes with cathode sleeves of very low-silicon nickel were put into tubes intended for standby use in the early '50s when it was discovered that silicon in the nickel was reacting with the oxide coating to form an insulating interface layer between the cathode and its coating. That's what some of those specially numbered equivalents of common types are, ones with low-silicon cathodes. Don't know why the interface doesn't form or is delayed when cathode current is flowing, but that's what was reported after researching the problem.

The other thing that happens is that microscopic globules of barium metal, liberated electrolytically from the oxide as cathode current flows through it, account for most of the electron emission from the oxide coating. That free metal is slowly lost from evaporation into the vacuum from a hot cathode, as well as reoxidized from stray gas in a tube (whether it's hot or just in long storage,) and has to be replaced by electrolysis. That's why NOS tubes or used tubes that have been out of service for years often measure low on emission when tested straight out of the box. Put 'em to work for an hour and check 'em again; they're often A-OK. That may work to clean up slightly gassy tubes, too, but it's risky to try that with power tubes in a fixed bias amp. They can "run away" from positive gas ions de-biasing the grids, the resulting increased plate current overheating the tube and "cooking out" more air molecules to get ionized and bombard the grid, and so on...

"A cheerful heart is good medicine."


   
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(@oldiron)
Estimable Member
Joined: 20 years ago
Posts: 111
 

All we need is another hand!

I've been building a collection of NOS tubes. I have an old Tectronics scope, Marconi spec-an, H-P VTVM and several signal generators that are tube sets besides futzing around with old ham gear and radios. I have had mixed results using Sovetec and other modern repo tubes. The old GE and Phillips tubes seem to be more reliable in the test equipment. It usualy takes a couple hours for things to stabilize before alignment and calibration. After that the NOS stuff dosen't drift nearly as bad as the Sovetech stuff does.

Not that guitar amps have to be as tight as that type of gear.

Most every manufactuer I've ever seen will shield just about everything but finals and rectifiers. At least in low power applications (for anybody who might slide in on this a 200W guitar amp is low power compaired to the amplifiers used in broadcast TV, radio or the radars that I worked on for years). If the engineers put a shield on something it should be left on if at all possible. Pulling shields so your amp will look cool can induce cross talk and all sorts of noise. If the socket had a shield collar it should have a shield.

If your enough of a nutcase like I am or Ricochet here and are building your own gear from the junk box then I hope you have enough background in tube theroy to keep yourself and others safe.

I may be going to hell in a bucket but at least I'm enjoying the ride. (Jerry Garcea)


   
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(@ricochet)
Illustrious Member
Joined: 21 years ago
Posts: 7833
 

Right. If it started off with a shield, keep it shielded.

Hey OldIron, my handle on some other boards is OldIronMan. :D

"A cheerful heart is good medicine."


   
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(@oldiron)
Estimable Member
Joined: 20 years ago
Posts: 111
 

The Oldiron comes from my collecting and restoring old engines. Family members would give me a hard time about my "rusty junk". That was till I had a guy show up one day when the family was visiting and bought several restored engines for a pretty sum. From that day forward it was classic old iron.

I may be going to hell in a bucket but at least I'm enjoying the ride. (Jerry Garcea)


   
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(@ricochet)
Illustrious Member
Joined: 21 years ago
Posts: 7833
 

Same with mine, from old engines.

"A cheerful heart is good medicine."


   
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