A few years ago, an unemployed vacuum cleaner salesman with a fetish for "low DC resistance" power supplies decided that you could use common, fairly inexpensive, industrial control transformers for plate supplies for tube audio amps. And, you can. Hams did that for years, as did laser experimenters and people building worm and nightcrawler collecting shockers.
There are upsides and downsides for this. The upside is price, they are not that expensive new, and they are often available for free or very cheap as pulls from building salvage and old air conditioning equipment. They are ruggedly potted and insulated.
The downsides?
They are very heavy and awkward. They usually have the connections on their topsides which in an amp build need to be covered over for safety and long leads have to come through the chassis. The size they come in usually starts at 250VA, meaning they are too big to fit most amp chassis and still have room for other things.
The voltages they put out are usually direct multiples of 120 volts, meaning that you get 120, 240, or 480 volts out. If you need a center tap for a bi-phase half wave rectifier (i.e. you want to run rectifier tube(s))this means you are usually stuck with 240-0-240.
There is only one secondary, in effect, you need another transformer for filament voltage, and a third for bias voltage unless you have one wound. You are already pushing the space and weight budget. This makes it worse.
What this means is that ICTs are better utilized for bench supplies or for providing "foreign" voltages to imported equipment than incorporated into actual amp builds in most cases. But yes, they are a tool in the armamentarium of the DIY builder.
In my opinion, the best source of cost effective power transformers for homebrew and small production tube gear is to use guitar amp replacement units-IF you can get wholesale pricing on these. The music industry works on a pricing scale where "A Mark" (a phrase never to be uttered in a music store unless you want to be kicked out) is fifty percent. Most replacement guitar amp transformers from the bigger distributors are A Mark, sometimes B Mark, items. (B Mark is variously 40, 35 or 30 percent, depending.)
Tuesday, January 3, 2012
Don't mess with the Stereo 70...pass it on, or pass on it.
The first really popular component stereo amplifier was the Dynaco Stereo 70. Something like a quarter of a million were made over a roughly fifteen year period, and many still exist. Every day, people find them in closets, garages, basements and other places and put them up for sale or decide to fix them up (or have it done for them) and use them.
Approximately twenty firms offer upgrades, replacement parts and services related to the venerable little amp. In my opinion, investing good time and money in them is a bad idea. If you find one, my advice is pass it on to someone else and start from scratch, unless you have a fixation on assembling an early-sixties-era college dorm setup as a nostalgia trip.
Why is something so popular something I think should be avoided? Because the Stereo 70 isn't a jewel in the rough, but rather a case of a product that was designed to shave costs so thoroughly that making a good unit out of it means replacing everything. It makes little sense to replace everything and still wind up with a undersized, crummy old chassis and a product that you can't resell very easily for nearly what you have in it anyway.
Let's look at any effort to upgrade the amplifier. Inevitably, the original circuit board has to be replaced, either with a recreation of the original circuit or one of a dozen others on new board material. We also must replace the filter capacitors, and usually all the wiring and connectors. This means we are reusing the chassis, the three transformers and the choke, and that's about it.
But let's look at those transformers. They're between 35 and 50 years old, and insulated with old materials that have a habit of breaking down. In addition, both the power and output transformers have flaws that mean we'd rather have new ones anyway. The power transformer is undersized, for one thing, and runs quite hot. It also has a primary meant for 115 volts when the line voltage in the US has increased by five to ten volts in most areas over the past half century. If we replace the tube rectifier with a solid state diode pair, as one might want to to reduce load on the transformer, both the HT primary and the heater voltage will increase. We will have to increase the voltage rating on the filter caps, which is a good idea anyway, and we might put the 5 volt heater winding in series with the primary to drop the voltages a little, but still, the HT voltage will be more than we really want.
Replacing the power transformer definitely helps: we want one with more lams, more copper, designed for current utility voltages, and perhaps with a primary set up for solid state rectifiers. But then we still have the old output transformers, which are both arguably the best thing about the old warhorse and the limiting factor on what can be done with it. They were wound according to a Hafler patent, which allows for perfect AC balance at the expense of DC symmetry in the primary. Why suppliers so proudly offer "perfect reproductions" of these compromised parts is beyond me.
Replacing the output transformers on the Dyna chassis is problematic due to the space available.
The bottom line is that if you find an old Dyna Stereo 70, the best move you can make is to pass it on to someone else and put the money into a new clean build. Buy or fabricate a decent chassis and a good pair of output transformers and go from there.
Approximately twenty firms offer upgrades, replacement parts and services related to the venerable little amp. In my opinion, investing good time and money in them is a bad idea. If you find one, my advice is pass it on to someone else and start from scratch, unless you have a fixation on assembling an early-sixties-era college dorm setup as a nostalgia trip.
Why is something so popular something I think should be avoided? Because the Stereo 70 isn't a jewel in the rough, but rather a case of a product that was designed to shave costs so thoroughly that making a good unit out of it means replacing everything. It makes little sense to replace everything and still wind up with a undersized, crummy old chassis and a product that you can't resell very easily for nearly what you have in it anyway.
Let's look at any effort to upgrade the amplifier. Inevitably, the original circuit board has to be replaced, either with a recreation of the original circuit or one of a dozen others on new board material. We also must replace the filter capacitors, and usually all the wiring and connectors. This means we are reusing the chassis, the three transformers and the choke, and that's about it.
But let's look at those transformers. They're between 35 and 50 years old, and insulated with old materials that have a habit of breaking down. In addition, both the power and output transformers have flaws that mean we'd rather have new ones anyway. The power transformer is undersized, for one thing, and runs quite hot. It also has a primary meant for 115 volts when the line voltage in the US has increased by five to ten volts in most areas over the past half century. If we replace the tube rectifier with a solid state diode pair, as one might want to to reduce load on the transformer, both the HT primary and the heater voltage will increase. We will have to increase the voltage rating on the filter caps, which is a good idea anyway, and we might put the 5 volt heater winding in series with the primary to drop the voltages a little, but still, the HT voltage will be more than we really want.
Replacing the power transformer definitely helps: we want one with more lams, more copper, designed for current utility voltages, and perhaps with a primary set up for solid state rectifiers. But then we still have the old output transformers, which are both arguably the best thing about the old warhorse and the limiting factor on what can be done with it. They were wound according to a Hafler patent, which allows for perfect AC balance at the expense of DC symmetry in the primary. Why suppliers so proudly offer "perfect reproductions" of these compromised parts is beyond me.
Replacing the output transformers on the Dyna chassis is problematic due to the space available.
The bottom line is that if you find an old Dyna Stereo 70, the best move you can make is to pass it on to someone else and put the money into a new clean build. Buy or fabricate a decent chassis and a good pair of output transformers and go from there.
Sunday, January 1, 2012
Can't Solder?
So often I run into people who "can't solder".
My sister can solder. In fact, she's a rework operator at a big electronics plant which shall go nameless and she teaches the new hire class for assemblers. Her pass rate is well over 99% and except for techs and engineers going through, all her pupils are women who have never soldered before. Guys, if a bunch of GIRLS can do it, why can't YOU?
Proper soldering is not that tough. I learned to do it out of books, and later went through a solder school or two while working in electronic plants while in college. The only thing that ever gave me trouble was the smaller surface mount stuff. Yet, so many people in audio and ham radio have a huge problem making acceptable solder joints and others refuse to try. I refuse to believe anyone who can play any musical instrument at all-except maybe a trombone- can not learn to solder correctly and in a short time.
The biggest mistake people make is not understanding what is required to make good solder joints. You need a soldering iron with a clean, tinned tip, a clean pair of metal items-wires, leads, tags, pads, whatever-and some solder and flux.
For chassis and panel soldering, a regular electric iron-NOT a gun, an iron-will work, but if you want to solder circuit boards, it HAS to be a temperature controlled iron. You HAVE to be able to CLEAN and TIN the tip, EACH USE. You need to have the surfaces clean, so you will want a small ACID BRUSH and some SOLVENT, usually mineral spirits or methanol.
Tinning the tip means you need a SPONGE with some WATER on it. You can use real natural sponge or the kind sold just for soldering, (but not the cheap kitchen type) and DI or distilled water is better but not mandatory. You also need some SOLDER.
The ONLY kind of solder to use until you are highly experienced is FRESH, ROSIN CORE solder of 63/37 lead-tin composition. It has a special name, EUTECTIC solder. That is because it goes from liquid to solid in one instant as it cools instead of going through a mushy stage. It has to be ROSIN CORE, you must NEVER use acid core solder on electronic items. It is for plumbing and sheet metal work.
You will also want some FLUX. The kind you want is called RMA Flux. That stands for Rosin Mildly Activated. You must carefully clean each joint you make after using it. You will also want a little solvent for the flux and a small needle bottle for dispensing it. You should use little flux. There is also a paste flux you can get that is non-acidic that is good mostly for tinning old or chewed up soldering iron tips. Rarely you might use a little on a particularly big solder joint. Flux goes bad which is why you want to start with a fresh roll of solder.
You will also want some tools such as a couple of picks, needlenose pliers and other miscellany.
If you have acid core or fluxless tin/lead solder, unless you do tinsmithing or plumbing, get rid of it. If you have a soldering gun, put it aside for other purposes or get rid of it. If you have old cheap unregulated irons, they can be used for chassis and panel (point to point) soldering if you can get the tips cleaned and tinned, otherwise get rid of them.
If you buy everything you need at one time from a good supplier like Stanley Supply and Services, (the former Jensen Tool people) you
are going to lay out a couple hundred bucks. If you do some hamfest trading you can get everything but the solder, sponge, holder, and flux for less.
Chassis and panel soldering is well described in the old military electronics books. The thing to do is get some wire and supplies and practice. Practice a lot.
I have been through a couple of company solder schools, but the best I have been through was as a guest with the old GTE Telephone craft class for inplant repairers in the early eighties. One of the exercises we did was to take a piece of wire-regular solid copper wire, perhaps 12 or 14 gauge, and cut it into twelve exactly equal pieces. We then had to build a square of four pieces and then a cube using all twelve. The only tools we were allowed were the soldering iron, solder, a pair of phone man needlenose pliers and a pair of surgical forceps like needle clamps or hemostats. We also had a steel plate about 3x5 inches which had a surface that looked as if it had been rusted and then oiled and buffed.
When we were done with our perfect cubes-which took half the day-they crushed them. The solder joints had to hold.
The first cube took hours. By the end of the day most of us were on our fourth or fifth. The instructors then showed us how they did it-they could do it in five minutes flat, starting with the untrimmed, unstripped wire.
Today, phone companies do very little soldering-it's all punchdown blocks. But in the old days, they got really good at it.
My sister can solder. In fact, she's a rework operator at a big electronics plant which shall go nameless and she teaches the new hire class for assemblers. Her pass rate is well over 99% and except for techs and engineers going through, all her pupils are women who have never soldered before. Guys, if a bunch of GIRLS can do it, why can't YOU?
Proper soldering is not that tough. I learned to do it out of books, and later went through a solder school or two while working in electronic plants while in college. The only thing that ever gave me trouble was the smaller surface mount stuff. Yet, so many people in audio and ham radio have a huge problem making acceptable solder joints and others refuse to try. I refuse to believe anyone who can play any musical instrument at all-except maybe a trombone- can not learn to solder correctly and in a short time.
The biggest mistake people make is not understanding what is required to make good solder joints. You need a soldering iron with a clean, tinned tip, a clean pair of metal items-wires, leads, tags, pads, whatever-and some solder and flux.
For chassis and panel soldering, a regular electric iron-NOT a gun, an iron-will work, but if you want to solder circuit boards, it HAS to be a temperature controlled iron. You HAVE to be able to CLEAN and TIN the tip, EACH USE. You need to have the surfaces clean, so you will want a small ACID BRUSH and some SOLVENT, usually mineral spirits or methanol.
Tinning the tip means you need a SPONGE with some WATER on it. You can use real natural sponge or the kind sold just for soldering, (but not the cheap kitchen type) and DI or distilled water is better but not mandatory. You also need some SOLDER.
The ONLY kind of solder to use until you are highly experienced is FRESH, ROSIN CORE solder of 63/37 lead-tin composition. It has a special name, EUTECTIC solder. That is because it goes from liquid to solid in one instant as it cools instead of going through a mushy stage. It has to be ROSIN CORE, you must NEVER use acid core solder on electronic items. It is for plumbing and sheet metal work.
You will also want some FLUX. The kind you want is called RMA Flux. That stands for Rosin Mildly Activated. You must carefully clean each joint you make after using it. You will also want a little solvent for the flux and a small needle bottle for dispensing it. You should use little flux. There is also a paste flux you can get that is non-acidic that is good mostly for tinning old or chewed up soldering iron tips. Rarely you might use a little on a particularly big solder joint. Flux goes bad which is why you want to start with a fresh roll of solder.
You will also want some tools such as a couple of picks, needlenose pliers and other miscellany.
If you have acid core or fluxless tin/lead solder, unless you do tinsmithing or plumbing, get rid of it. If you have a soldering gun, put it aside for other purposes or get rid of it. If you have old cheap unregulated irons, they can be used for chassis and panel (point to point) soldering if you can get the tips cleaned and tinned, otherwise get rid of them.
If you buy everything you need at one time from a good supplier like Stanley Supply and Services, (the former Jensen Tool people) you
are going to lay out a couple hundred bucks. If you do some hamfest trading you can get everything but the solder, sponge, holder, and flux for less.
Chassis and panel soldering is well described in the old military electronics books. The thing to do is get some wire and supplies and practice. Practice a lot.
I have been through a couple of company solder schools, but the best I have been through was as a guest with the old GTE Telephone craft class for inplant repairers in the early eighties. One of the exercises we did was to take a piece of wire-regular solid copper wire, perhaps 12 or 14 gauge, and cut it into twelve exactly equal pieces. We then had to build a square of four pieces and then a cube using all twelve. The only tools we were allowed were the soldering iron, solder, a pair of phone man needlenose pliers and a pair of surgical forceps like needle clamps or hemostats. We also had a steel plate about 3x5 inches which had a surface that looked as if it had been rusted and then oiled and buffed.
When we were done with our perfect cubes-which took half the day-they crushed them. The solder joints had to hold.
The first cube took hours. By the end of the day most of us were on our fourth or fifth. The instructors then showed us how they did it-they could do it in five minutes flat, starting with the untrimmed, unstripped wire.
Today, phone companies do very little soldering-it's all punchdown blocks. But in the old days, they got really good at it.
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