DRBILL

That's the way we used to do it! Variacs were far too expensive for the casual experimenter out in a shed. I still have mine under the bench somewhere. It has 10W, 25, 60, and 100. All we were trying to do was to "make up" the electrolytic caps (form a thin aluminum oxide dielectric between sheets of aluminum). It actually is pretty effective. You could also use it in the "smoke test" when you completed a kit or made questionable repairs or in "I wonder what would happen if...". That brings back good memories. Thanks! DR BILL

Fellows, I have been out of town for over a week and upon returning found my desk about to collapse with ASAP nonsense. I think most of your questions have been answered by our colleagues. I'll re-read everything in the next day or two and see if any stones have been left unturned. I got my KT99's from Ned at Triode. From my field-notes I show 560v on the plates and -65v bias (which equals 1.56v across the 11.2ohm shunt). This is for a MARK III. RE: Isotap. Far less expensive than a variac and makes far less a fashion statement if you factor a wife into the equation. As an added bonus, you can choose an output that is isolated from the line voltage, or if that is not deemed necessary, you can choose an output from an autotransformer primary at much greater current. They show up on E-BAY with regularity. I gave $15 for mine, but the shipping was murder. Craig put me on to these. There's a good man! By fooling with it, I can get exactly 110vac with 128vac in. It doesn't do bad things to the shape of the sinewave in either mode. I'm on the edge of my chair waiting to see if any of you guys are going to be as enthusiastic about the KT99 as I. I was in Atlanta inspecting grandchildren. They passed. DR BILL

"How much can you replace before it isn't what the name says?" Ask a vintage auto restorer that question! DR BILL

Paul could be a Bulldog. But I never saw him angry. He would smile crookedly, and then painstakingly take your clock apart. DR BILL

KT90's (Electro-Harmonix) Im really enthusiastic about this tube. Im using a pair in a MARK-III and it is astounding. I listen to a lot of pipe organ music, and the heavy pedal notes (16-32Hz) are often just an expensive draft. Not so with the KT90. Plus, there is no flutter in the highest pitches (5000+Hz) that I have been experiencing with the KT88s when playing together with heavy bass notes. I had almost convinced myself that the power supply was running out of steam. I put the KT90s in the MK-IIIs for a bench test initially. On the scope there wasnt a great deal of difference ---maybe the square wave was a little sharper than on the KT88s in the lower frequencies. But on the Klipschorns, they were something else altogether. The only downside is that they really stink the first time you turn them on! I think it may be the adhesive that holds the base on. I was frantic looking for a component about to fry, then I realized it was the tubes themselves. A half-hour resolved the matter. One of the biggest items is going to be the added headroom for voltages. When you run a lot of the Dynakit stuff on modern house current (mine is often 128!) you can see an occasional red plate or (worse) a glowing grid. I think the KT90 is going to solve a lot of this. I now run my personal stuff through a vintage RCA Isotap. I wish you could still get em. The KT90 is a direct replacement for the 6550 and the KT88. I expect it is going to be around. Try them if you get a chance. DR BILL

Yo, Tony, "well, I am enjoying the amp immensely now and have to accept that tube amps can and will fail more frequently than SS gear, it's the price we pay for the tube sound. one can be lucky or not, the tubes can last virtually forever or not..." PWK was antsy about the introduction of SS amplifiers. He told me that tube amps seldom caused any trouble with his speakers except for "clipping". Low power amplifiers took out more Klipsch tweeters than the brute tube gear. What he dreaded was the failure of high powered SS gear with no output transformers. These were deadly to woofers. When they shorted, the voice coil could actually catch fire! Hundreds of watts of DC rushing into an eight ohm load. Not pretty. "On the whole, I'd rather be in Philadelphia" as said WCF. DR BILL

Yes, sometimes they just fail because the filiment burns out, like in an incandescent lamp. Actually, this is fairly rare. Internal shorts and "gas" are the most frequent failures. The cathode coating can flake off like dandruf and short things. You know the mirror coating on the inside of a tube? The "getters" are supposed to suck up the last molecules of gas in the tube, and they also are supposed to direct the stuff that boils off the incandescent filiment to a safe place away from the elements in a tube. Thus, the silvering on the top and sometimes the side of the tube. However, after many hours of service, some of this stuff begins to plate out down where the wires come through the glass envelope. Shorts. There is no such thing as a perfect vacuum, so stuff boils off all of the tube elements and sometimes gets so rich that it ionizes. That's a gassy tube. The most catastrophic failure is a "runaway tube". This is probably caused by gas. Current flows unimpeded to the plate which turns bright red-orange. This can become so hot that the glass envelope shatters. It can take out an output transformer. This is not likely to happen if you have protected your device with a proper fuse. I can think of no tube failure that would deliver DC to your speakers. I haven't thought about this in perhaps fifty years. I may have left something out. in any case, I hope this helps. DR BILL

Tony, I really wanted to hear that you got your amp back up and running! Yes? DR BILL

GLEN, For my birthday my son-in-law gave me a FURMAN RP-8 POWER CONDITIONER. I don't know how one would rate its effectiveness. Nothing has failed since August 10. But then, again, nothing failed due to power surges before August 10! I guess I rest better at night knowing that it is in place. I have never had to collect from my fire insurance on my house. But I wouldn't do without it. I'll bet you have had some particular bad experience. Maybe if you shared that with the FORUM we could get some ideas going. DR BILL

Has anybody tried the KT90 as a substitue for the KT88? Were you able to set the bias? What did it sound like? Any general comments? THANKS! DR BILL

I have in my hand a letter written by PWK and dated 31 January 1996 in which he was lamenting the changes that were being made in the organization as his sun was setting. I will only quote one line inasmuch as the letter contains some personal material. "The (few) Klipschorns still produced have been redesigned to resemble an imitation liquor cabinet." That just about sums it up! DR BILL

Yo, Oscar, Try this theory. What if every time a vendor put returned tubes back in the bin. The time would come when one out of four picks would be a dud. Eventually, every pick would be a dud! As you say, we have to depend on the integrity of the people we deal with. How do you like the KT90's? I've heard mixed reports. They have a reputation for being very accepting of high plate and screen-grid punishment. I thought one "authority" was reviewing wine instead of vaccum tubes. Something like "a warm and smooth tube that quickly cloyed the ear with its monotonous sameness!" Me? I never met a tube I didn't like! DR BILL

Dear JJ, I have a fairly decent bench of test equipment, all in proper calibration. I always have a schematic in front of me on a clipboard in case my mind wanders. You must understand that when the good tubes were in place, everything worked according to specs. By substituting a bad tube for a good tube, the plate glowed in the bad tube. You can do that in any socket position. I presently have three tubes in two amplifiers that work perfectly. I'm hoping to get a fourth good tube to complete the set. And for the record, I don't own any "Radio Store" test equipment! Thanks for your interest. DR BILL

It all depends. I have a reconditioned B&K 700 that I use several times a week. I have a few bands that depend on me to keep their tube gear playing. Within my immediate family there are nine tube amps hooked to Klipsch speakers! Something is on the bench at any moment. I don't know if I could justify owning one if I was only looking after my own gear. A unit that is out of calibration from E-Bay et al would probably do more harm than good. Check with Michael Marx www.vacuumtubes.com. He usually has several to offer. Michael doesn't let anything leave the warehouse that isn't in top shape. Take a look at his NOS tube offerings while you are there. DR BILL

How does the symphony compensate for the tuba in the back row and the violin in the front row? They do nothing. The ear is very forgiving. "Try listening to the music instead of listening to the speakers". That was from the mouth of PWK! DR BILL

I have been working fitfully on a pair of MARK III's for over a year. Most of the time was spent waiting on out-of-stock parts. Also, a lot of time was spent returning defective or mistaken orders. Unfortunately, this litany included some of the oldest and most trusted names in the HAM and audio trade. My most vexing incident happened during the last three weeks. I ordered four KT88's for which I paid a premium in order to get the "prefered" imported brand. All four tested perfect on my B&K-700. No shorts or grid emmission and with expected meter readings. I popped them in the MARK III's, set the bias and checked the under-chassis voltages, and one tube in each amplifier exhibited a runaway rosy plate. The red plate followed them to other sockets. They were bad tubes. No questions asked, the vendor swapped them for two more. Both passed the tube tester, but one was also a runaway in the amplifier. I have ordered ONE KT-88 from another vendor with the hope that the tubes in his bin are from a better lot. I do a fair amount of business with a guy that specializes in NOS tubes. I have never had to send anything back. The reason that I don't do all of my tube orders from him is "$$$$!" Perhaps I am being penny-wise and pound-foolish. I am about to the point that the higher price might offset the frustration. Some words have formed in my mouth, recently, that surprised me! Are any of you having similar experiences? DR BILL (aka DR GLOW in the local band business!)

Your best bet will be e-bay. They weren't the best idea Klipsch and Associates ever had. I bought a pair in a pawnshop for $50. I was robbed. DR BILL

---------------- On 12/11/2004 10:00:16 PM jlossint wrote: It was stated by PWK that the back chamber should be air tight. This "tightness" should have a time constant of several seconds. If I remember right the procedure for checking this 1/ open a hole into the back chamber. 2/ Next apply a dc voltage to the driver, which will offset the cone ( i.e move it). 3/ Close the hole in 1/ 4/ Remove the dc voltage and instantly measure the dc voltage over the voice coil and measure the time it takes before the voltage is zero. (There will be voltage since the cone/voice coil is now moving towrd its resting position. Take my word, if the chamber/box is properly air tight makes a difference and this is actually valid for any type of loudspeaker. ---------------- Mmmm. What did you say we would be measuring? My guess would be the "spike" caused by the collapsing magnetic field. I believe you could get similar results with the driver sitting on the bench in free air. Just a thought. DR BILL

RETIRED PRIEST. Now it is time for some full-time FUN! There is a remarkable similarity between the smell of incense and the smell of rosin core solder. The picture is a MARK III that started with a blank chassis --brand new for all purposes. DR BILL

All of the above having been said, here is a tip from a really old f**t. Use a dab of KESTER SOLDERING PASTE. It is parrafin-based and non-acidic. Touch a q-tip to the paste and then do a very light dab on the joint. With this stuff, you could solder a snowball to hades. DO NOT use this on PC boards. For really heavy jobs, put up the iron and get out the propane torch. DR BILL

---------------- On 11/7/2004 9:15:14 PM djk wrote: In case someone here can't read the above: DO NOT 'Y' THE STEREO OUTPUTS TOGETHER. If you are too cheap to spend $8 on the correct part, then 'Y' the stereo outputs together through a pair of 5K resistors and feed to a third 5K from that point to ground. Look at the PWK note on deriving a center channel feed if you can't visualize this. --------------- Dee: I was going to suggest the same thing. I believe you told me you had one. Or did I just send you a schematic? DR BILL

Take a look at WWW.VACUUMTUBES.COM Michael has supplied me with tubes for at least 10 years. He is not always the least expensive but he can always manage to get EXACTLY what you want. DR BILL

Thanks, Gill, I loaned my copy of EIGHT CARDINAL POINTS several years ago and it never came back. Rereading it was just like a visit from an old friend. DR BILL

I have my Heresies "stacked" side by side as a center channel (parallel into 4 ohms) and there is a great improvement in sound geometry and bass response. DR BILL

Some days I feel like I could start a fight at a Quaker picnic. DR BILL