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DON'T EVER TAP YOUR TUBE AMPLIFIER OR TUBES!
I just destroyed about $150 worth of tubes. With a single light tap. The victims were a true NOS NIB TungSol round plate 6SN7GT and a hard-to-find NOS NIB bottom getter RCA 6AS7G.
The tap likely shorted the 6SN7, which dragged the bias of the 6AS7 off-kilter and damaged it as well. Now there is only a massive hum or nothing in the L channel when either tube is in. :(
For FM transmitter levels above 20kW output, tubes are more efficient. You can parallel 2 20kW solid state FM transmitters together to get higher output, but costs become a factor. This combination would cost about $2-300,000.00, where as a tubed transmitter of the same output would be about $80-100,000.00
My amp's topology:
- Class A1 (the output tube is never driven into A2)
- Plate loaded transformer coupled
- No feedback topology
Tapping on tubes does indeed take the audio enthusiast to new places.
When my Audio Research SP10 needed new gain tubes after about five years- it uses 12 6DJ8 / ECC88 family tubes, I didn't then know the different ratings for the family and that the SP10 drives the tubes in a way that only E188CC / 7308 and the Russian 6H23p's can do for very long.
And the high gain of the SP10 phono section- 72dB is demanding of very quiet tubes. As well, they have to be non-microphonic, and the 6DJ8s have this nearly microscopic frame grid- imagine a picture frame with an amazingly fine screen on it, and these tiny wires seem to love to vibrate. It seem the little frame acts much as a microphone diaphragm- if it moves it modulates the signal.
Shopping for SP10 tubes, I decided to try the bottom and see what the whole range is like. I bought 10 6H23p's for $3.95 each (1992). Putting them in the toughest spot- the four phono tubes and without dampers, it was possible with these tubes to shout at them and have that sound at the speakers. At that point they were actualmicrophones ! Tapping them made a metallic sound that would ring such that if I turned up the gain, the sound it would start a feedback loop to the tubes.
With these results, I bought Dutch Amperex E188CC (7308s)! The choices today are about NOS 7308 and the Sovtek and EH 6H23p-EB, the "EB" important as it takes much higher currents. Even tough E88CC/6922 like the Siemens will lead brilliant but abbreviated lives. I had a white labelled Amperex 6DJ8 become noisy in about 5 hours.
Would that in 1992 I bought 50- Amperex E188CCs / 7308 - the best, tested ones were $20-25.
My mad, Nicola Tesla side: I still have the microphone/6H23p's and I was thinking as an experiment, rigging up a tapered paper megaphone so it bears on one of these tubes- perhaps the cone is attached to a collar around the base of the tube. The tube itself would be extended into the air by connecting the pins by wires to a separate base so the tube is suspended by the wires and freer to vibrate. My daftest concept is to put the tube on a little raft and float it on a pint of mineral oil! When shouting down the cone it would mechanically vibrate the tube. Now that would be a PURE tube mechano-electric floating microphone! -Call Royer- it's a "ribbon" microphone too! Frequency response? The other idea is to put a long steel rod into the ground and connect the tube to that- being mechanical, it could record the song of the Earthquake!
I've also destroyed tubes by tapping on woofer cones with the tube amp on. This tapping sends a large reverse voltage through the amp that the feedback trys to correct. My dog was trying this experiment yesterday with her tail. She wasn't as good as me at blowing things up.
IMHO this is just a coincidence.
I've been tapping tubes (using them also) for over 28+yrs (since 7) and none of that you mentioned happen to me before, after servicing over several hundreds of tube gears, radios, etc.
In fact, even if you carefully pamper your tubes from now on, what makes you think that your tube supplier would handle the tubes and/or the shipper, courier, manufacturer of the tubes, etc. would "pamper" them exactly they way you are trying to do right now?
good lord-- i've been trucking tube amps around for years in vans.. if tube tapping was so dangerous, all of my guitar amps'dve met their demises years ago-- but none of them has.
i'd look for an issue outside of the tubes-- i seriously doubt that tapping a 6sn7.. one of the TOUGHEST tubes i've met- would kill an amp. you may've snapped a solder joint, lost a capacitor, or had another bad component! check those out-- you might even find your sn7s are still tickin'!
Agree with all the above - I lightly tap DHTs with the end of a fingernail to check how microphonic they are. Haven't had any problems, and as said, a tube that blows up was probably going that way anyhow.
Well....I can tell you that tapping on tubes was a common practice in TV stations back when there were plenty of tubes in the transmitting and video gear.
If you remember back then, there were plenty of times when the on-air transmissions were interrupted with some sort of a message saying, "please stand by" or maybe a test pattern. Behind the scenes, there were frantic engineers diagnosing the video chain. To do this, a common technique was to take the back of a screwdriver and run it down a line of tubes in a rack. The offending tube would be quickly found if that was the culprit. Sure, there may have been a occasional breakage but good engineers knew how to apply the pressure.
As for your tragedy, I'm very sorry to hear that some more well-thought-of tubes no longer grace this earth. I still use the tapping technique when I'm looking for a noisy tube and have had one that had sufficient shedding of some sort inside that it arced in a spectacular blaze of light. Perhaps I did myself a favor....
Transmitter tubes were a different animal. Most transmitting amps were located outside the main TV station in a separate room - for obovious reasons. The story I related had to do with all the small-signal processing that went on in the studio itself. There were plenty of racks of gear for switching and managing the network feed as well as inhouse production. This is where the technique I mentioned was applied. If you had Walter Cronkite on a 7PM doing the news and you lost your video to the transmitter, what did you do? Remember this was live!
All radio and TV transmitters had regular maintenance schedules and the FCC in those days required that all meter readings and maintenance logs be kept up to date. Failure resulted in loss of operating license. The FCC was mainly concerned with poor transmitting performance and didn't really care if the feed was lost. In fact, part of the records required that an engineer go out into the field and measure the amount of RF from the broadcast station to ensure that the radiating power was correct.
You didn't go near those transmitting tubes without first shutting it down - many small animals found out why the hard way. What we now know as the Darwin Awards took out those engineers who didn't pay attention.
I have experence with very large Navy missle fire control radars from the '60s. But were talking about much more power at much higher frequencies.
My Dad was a Radar Officer on a carrier in WWII and he told stories about the birds that were perched on the radar antenna would get fried when they fired up the radar... Early example of microwave cooking I guess :)
I always thought it was dangerous to mix extremely high current and water, but many transmitting tubes were/are water cooled- they have a water jacket around the base with inlet and outlet.
A friend gave me what is really a modest transmitting tube, marked as a Mazda. The tube appears to have never been used. It's a two pin- the pins are perhaps 12mm, envelope is about 50cm long and 10cm diameter- and air cooled. It interesting in that it doesn't have any kind of getter, there is a small handful of granular metal chips and I'm told this material soaks up any outgassing of the materials. It's very odd to have a tube with all this heavy, free grains in inside- it must be extremely non-conductive. There are also interesting manufacturing notations both inside and between the pins by several hands, suggesting a lot of hand work and individual testing.
It seems an attractive idea to build 4000W monoblock amplifers with a couple of tranmitting tubes. Besides the power, I imagine they're very linear and dynamic too, but imagine having to call the plumber when you need to move your amplifier to another room!
Imagine a humongousoid triode in parallel with an elctrostatic speaker...cathode connects to one surface, plate connects to the other surface, and the dynamic voltage across the triode is the voltage across the 'stat. Talk about SET OTL! The tube would simultaneously bias and power the 'stat. Cooling and safety? Minor issues, right?
I like your idea very much- it has a kind of purity to it and we all know how delightful are tubes and electrostatics!. -I'd keep the power supply in the garage next to the blower for my home pipe organ!
For more tube-use perversity, see my post above: "I once had singing 6H23p's (6DJ8)- Plus, an idea for a homemade ribbon microphone".
I don't know about TV stations, but my understanding is that transmitter tubes are still being used at radio stations - apparently for signal transmision there are some jobs that tubes can do more efficiently and cheaper than transistors.
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