tube fanatic

Greetings! You obviously didn't hang out in the Rockaways around 1965. Crestlynn Sales Co. on B. 116th Street was a tube only vendor!!! They sold to the public, many of the local TV repair guys (of which I was one!!!), and also serviced the huge "drug store" tube tester market. Maynard

I assume that the other filaments are lighting (if not, of course check the fuse)? If that's the case try a new rectifier, but as SF mentioned, use a variac to start out with a lower than usual line voltage (say 95-100) until you have an idea about what the bias is doing. As I recall, the LK-72 does not have any way to adjust the bias as it was generated at a fixed value by the DC supply for the front end filament string, so I assume that you installed bias adjustment pots. I'll be glad to talk you through the whole troubleshooting process if you send me an email. Regards-- Maynard

Good advice. The hum was probably from the power transformer in response to excessive current draw. Is the filament of the rectifier tube still lighting and did you note any arcing within the tube when you first powered up the amp with the new tubes?

What kind of amplifier?

In your picture of the underside it looks like you installed a toggle switch at the lower left corner to serve as the on/off switch. Leaving the switch wired in, take it out from its location and fold it back so that the wiring is nowhere near the output tube sockets (fold it over the side of the chassis) and see if the hum persists (with the 7199s removed also.)

It would be helpful to find out where the hum is originating. Pull the 7199s leaving only the output stages active. If you still hear hum, it suggests that the problem is in the power supply. It's very unlikely that the hum is originating in the output stages as the push-pull operation should cancel it, and particularly since the the midpoint of the heater supply has +20 volts DC strapped to it (and be sure that R61 and R62 are measuring very closely in value.) If the hum disappears with this experiment, plug the 7199 of one channel back in and see if the hum returns. If it does, you now have a place to start looking for potential causes. If not, plug in the other 7199 and the hum should then return. Email if you want to discuss the troubleshooting process in more detail. I didn't have time to read everything posted, so apologies if I've restated anything mentioned before. Maynard

Some concerns have been raised about exposure to E6000. Worth keeping in mind if used on a more frequent basis: http://www.tapplastics.com/uploads/pdf/MSDS%20E-6000.pdf

There's no answer to this question. New equipment may sound different, but not necessarily better. One simply has to listen and choose the preferred equipment. The KT 600 is a wonderful preamp though and, if properly restored, will hold its own against many modern units. It was designed by a friend of mine and has remained very much in demand even to this day. Too bad he got out of the field years ago as he has one of the sharpest minds that I've known!

you will regret it as much as you will regret not getting all the wires, terminals, screws replaced with silver, platinum, or iridium wires, terminals, screws and all the tubes replaced with cryo tubes, all the solder removed and replaced with silver solder, power transformer replaced with medical grade shielded transformer, yada, yada, yada Finally, someone has articulated the exact nature of the whole restoration and construction debate!!! If we accept the fact that EVERYTHING in the signal chain has the potential to exert a sonic influence, then all of what Speakerfritz mentioned must be considered, as well as replacing all of the tube sockets with newly manufactured, corrosion free sockets (Deoxit can only do so much.) Wafer switches need to be totally disassembled and cleaned (or in some cases replated.) The carbon resistors used in many vintage amplifiers and other components, while usually of decent quality, are not necessarily the best from a noise standpoint and should be replaced. In restoring vintage gear, EVERY solder joint should be redone (in the shop that was standard procedure for us) as there are many which are "cold" in spite of looking well on the surface (I encounter this every day, particularly with equipment which may have had some moisture exposure at some time in its life.) Every riveted chassis ground connection should be soldered too. So, those who only promote the use of "audio grade" capacitors while ignoring everything else are not offering you the best sonic performance which is available. One must also consider that you can only get so much performance out of a given piece of equipment. Tubes are inherently noisy devices due to the electron flow and other factors. How many hundreds of tubes are you going to try before deciding on "the perfect one?" And, once the tube has some hours of use, it won't be so wonderful any longer! So, trust what Norman tells you. He is obviously speaking from experience. And, as I've stated many times before, NEVER negate the skills of local technicians, many of whom do exemplary work.

Norman gave you good advice. Modern, high quality, caps are vastly superior to those of even 30-40 years ago. There's no need to spend a fortune on the so-called "audio grade" caps, even for amplifier service. I've been involved in many blind listening tests which compared high quality modern film caps with many of the popular "audio grade" caps which some promote for use in amplifiers, and the results were totally inconclusive at best.

By listening at low levels not only is the distortion of the speakers and amplifier lower, but also the interactions between the speakers and the room are decreased as well which improves imaging and soundstaging. You didn't mention the distance from the speakers at which you listen, but if you are in the near field (basically the distance equal to the speaker separation, or even slightly less), all the better. Even your "normal" listening level of 75-80 db is very low compared to the preferred level of some forum members. It's not surprising that you are finding such listening to be so enjoyable! I have a friend who listens to all kinds of music (using La Scalas) at levels of 106-108 db (I can't stay in the room at those levels) and there's basically no imaging, and the soundstage is non-existent as the room interaction results in a massive wall of sound which is far from enjoyable imo.

Since we're getting a bit nostalgic here, don't forget Burstein-Applebee: http://www.daveswebshop.com/bursteinapplebee.shtml

Lafayette Radio turned out some of the best sounding tube equipment that was made during the 50s and early 60s. In fact their KT600 preamp, which was designed by a friend of mine who was their chief engineer at the time, is still known for being an amazing piece of equipment. And Erik is absolutely correct in saying that the size of the OPTs has no bearing on the resultant sound. Definitely restore it Bruce, and if you decide to get rid of it, and Erik doesn't want it, I may be interested myself.

Yes, finding fuses wrapped in foil would raise my eyebrows also; but, I've seen worse over the years! Since you have the service manual, and seem familiar with taking measurements, I'd turn on the amp and start verifying voltages after it has been on for an hour. Check the DC offset again (as you are doing it, with no load), verify the bias and adjust it accordingly, and check the power supply rail voltages. If all is well, I'd just enjoy the amp.

When checking DC offset, are you doing it with the speakers disconnected? In my experience it's not always as low as desired, but below 50 mv I wouldn't be overly concerned. How much does it vary between initial turn-on and, say, an hour later? Btw, I'm also in Hunterdon County in case you ever need any help. Regards-- Maynard

Craig, that's downright insulting to the highly trained, and highly competent "local technicians" whose work is exemplary in every respect. As with any endeavor, one can always find all levels of competence. To imply that such technicians are incompetent (because they prefer to work on a local basis?) is a disservice to your colleagues in the audio restoration, design, and fabrication business. It should be beneath your dignity as a respected member of that community to even make such a statement! Listen all I can do is share my expereince. What I stated is absoluely true.... If you don't like it then pass the post by. I said "about zero" which means not "all" but "most".... the fact of the matter is "most" local techinians perform the "fix what is broke" technique with gear. Which is simply put "a recipe for disaster" with 50+ year old tube gear. Also "most" local technicians use bargin bin components that do not do this gear justice from a sonic stand point. Well, no point arguing about this. But, there are an awful lot of local techs across this country and I find it hard to believe that you have seen the work of so many of them that you can qualify your opinion by saying that "most" are below par. I just want to make it clear to our fellow forum members that there are "many" local techs who do magnificent work; to give the impression that consulting with them is a disservice to their equipment is not only misleading, but totally untrue. If you want to discuss this further, email me and we can trade "war stories."

Crosstalk between the channels from sharing a common power supply often reduces imaging quite significantly when compared with a pair of mono amps.

Interesting looking internals. Not sure why so much filtering is used. I'm in favor of as little ripple as possible from the p/s, but that looks like massive overkill (not that it's necessarily bad!) By chance, have you taken any measurements off of the B+ rail using a 'scope?

Get hold of Speakerfritz. I think he had a really nice Jolida for sale a while ago at a very fair price. The Jolida amps offer good value for the money imo.

You raise an interesting point. In my experience over the years with the "local technicians" involved in audio (as well as antique radio restoration), it is they who are the true craftsmen who also possess incredible technical knowledge. Because they create, or restore, for the pure love of what they do (and not because they derive their income from it) their work has often been at a level which no one could touch. They don't care if they invest 100+ hours in designing/building an amp for example, as it's the end result that they are after. Beautiful to look at above and below the chassis, and beautiful to listen to. They are known strictly by reputation, and work for those who just happen to hear about them or are recommended by someone for whom they did some work. So, my advice to people following this discussion is to seek out local talent. Ask around and you will be amazed at the number of highly skilled local people who work in audio (and just about anything else.)

Craig, that's downright insulting to the highly trained, and highly competent "local technicians" whose work is exemplary in every respect. As with any endeavor, one can always find all levels of competence. To imply that such technicians are incompetent (because they prefer to work on a local basis?) is a disservice to your colleagues in the audio restoration, design, and fabrication business. It should be beneath your dignity as a respected member of that community to even make such a statement!

\ please do Check out the online used book sellers for a copy of Practical Radio Servicing (2nd edition or later) by Marcus & Levy. It will give you a really nice basic overview of how tubes, power supplies, audio amplifiers and much more work. After getting that under your belt, try to find a copy of Essentials of Radio by Slurzberg & Osterheld. It will go way beyond the first book and will truly give you an understanding about what's going on in tube type audio amplifiers, among other things. I found both to be extremely useful when I taught this stuff more years ago than I want to admit!

Unfortunately, restoring an amp like this is not a project for someone without experience. If you want to get into it, a better alternative would be to find an old transformer operated tube radio on which to experiment (keeping in mind that tube equipment uses potentially lethal voltages- reading some good online tutorials on electrical safety when testing such equipment would be useful.) I can recommend some books if you decide to explore this. The restoration of the Scott could be done in stages. All electrolytic, coupling (a leaky cap here could be causing the red hot plate), and many bypass caps must be replaced all at once (even caps which are not problematic at the moment can quickly become a problem with use.) The caps in the tone control circuitry can be dealt with at leisure. Unless you see an obviously burnt resistor, those can be dealt with over time. The carbon resistors used in amps like this usually age in the upward value direction which doesn't usually cause any serious issues. Parts cost, using high quality replacements, might run $50-$85. Tubes, using modern replacements, aren't overly expensive. So, I hope this helps you out a bit. Email or post if you have other questions. Maynard

the 7591/6L6 amps were the later/higher output scott integrated amps? correct me if im wrong. wouldnt more than 17-20wpc be a bit overkill with 100db+ efficient speakers? or is there a significant difference in the sound signature of the later amps vs the earlier 7189/el84 amps The amount of power you need depends on room size, how loudly you listen, etc. But, in most cases, the power available from push-pull 7189s is plenty with high efficiency speakers. The sonic signature of various tubes can be different (much depends on the output transformers used), but there's nothing to complain about with the 7189s. You do need to factor in the cost of restoration, though. It wouldn't be out of line to pay around $500 for a total (i.e. replacement of every capacitor, out of spec resistor, tubes as needed) restoration of an amp like this, so it's worth discussing things with your tech before committing to purchase. And, absolutely, give the amp a listen before you make your decision (not for too long though given the tube with the red hot plate.) Wish you were closer or I'd help you out for the parts cost only; shipping would be prohibitively expensive. Plenty of good advice is available here, so post any questions that you have as you go along.