capnjuan said:
Default said:
Isolation Transformer. Don't want to lose any of my virtual friends, they're tough enough to get. Heck, the only reason cj comes over to play is because Mom tied a GZ34 around my neck....
We need to recitfy that.....In the Harmony schematic up top, that's a filament transformer below the 12AU6, no? Are you suggesting an isolation transformer can isolate our man from bzzt up the arm?
Yeah, the problem is down at the plug. You plug it in the wrong way and the hot on the ac is connected directly to the chassis. The switch and
added fuse for safety (schnikies!) and the hots from the from the iso-tranny are soldered into the same place as the old leads. Now for some blatantly stolen info from CJ on the Valco/Maggie group:
Written by CJ, Sat 1/19/2002
(Corrections in text, editing, are the responsibility of the author)
OK, in an earlier post I said I was gonna do this.......
Since we were talking about so-called "AC/DC" or transformerless amps (such as the Harmony H400) recently, I thought I'd try and give y'all a quick run-down, mostly for the "newbies" to the group. In my opinion, such amps represent extremely poor value for the money, and frankly, I wouldn't recommend even bothering with one unless you are able to to purchase it locally (so there's no shipping costs involved) and for very cheap, which to me means $20 or so, give or take a little......here are the reasons why I believe that such amps are mostly a waste of time and money:
1) extremely low power output, typically about 2 watts or less !!
2) high danger of lethal electrical shock !!
3) they tend to be noisy in operation, with lots of humming and buzzing
4) poor tone
5) overall cheesy build quality, with tiny little output transformers, and no power transformer whatsoever !!
These amps were originally designed in such a way as to be as cheap to build as possible, and since power transformers are expensive (and take up space and weigh a lot, which increases shipping costs), the manufacturers left out the power transformer entirely (this came about first with table-radio manufacturing in the late 1930's, and the concept was later extended to guitar amps at the lower end of the quality spectrum). In a high-quality amp, the power transformer would have been used to accomplish several goals: 1) step up the incoming 110-120 volt AC wall voltage to 300 or 400 volts or more, so that you can get high power from the output tubes; 2) step down the 110-120 volt wall voltage (with several additional transformer secondary-windings) to 5 volts and 6 volts for heating rectifier tubes, preamp tubes and power output tubes; 3) affording a degree of electrical isolation from shock; and 4) lower hum and buzz through the speaker.
All the "good" quality amps that I know of use 6 volt or 12 volt tubes: 6V6, 6L6, 6CA7/EL34, 6BQ5/EL84, 6550 etc. are all 6 volt tubes----by which I mean that it takes 6 volts to the filament ("heaters" is the correct term, actually) of each tube to heat them up, but the actual power-output of the tubes is done with much higher voltages, typically 300 volts or more to the "plates" of the tubes. 6EU7 and 6CG7/6FQ7 are examples of preamp tubes that have 6 volt heaters; 12AX7's, 12AT7's and 12AU7's would then appear to be 12 volt tubes, but in actuality they contain two six-volt heaters (one per each "half" of the tube, since they are "dual triode" tubes) connected together in series, with a "tap" between the two heaters, enabling these tubes to be run off of either 6 volts or 12 volts !! They actually run quieter (in sort of a "humbucking mode") at 6 volts than they do at 12 volts, so this is what most guitar amps do, running the 12AX7's etc at 6 volts (otherwise the power tranny would also need a 12 volt secondary as well, whch would add further cost at the manufacturing end). By the way, in a "typical" quality-built guitar amp, all the preamp tubes AND power tubes are run in parallel off of the same 6 volt secondary, and if the amp uses a tube rectifier such as the 5U4, 5Y3 etc then there will also be a separate 5 volt transformer winding just to heat that tube (some smaller amps use a 6 volt rectifier such as a 6X4, 6X5, 6CA4 etc, in which case the rectifier is often heated from the same transformer secondary winding as all the other preamp tubes and output tubes). Another BTW: one advantage of wiring tubes in parallel is that in certain parts of the circuitry (such as the tremolo or reverb in some amps, or in an amp with two channels), if one tube has a burned out filament and doesn't light up any more (or if you remove one of the tubes to test it etc), the rest of the tubes still stay lit up and the amp might still be usable & playable, on the "other" channel or whatever...........
However, the cheapo transformerless AC/DC amps have no power transformer whatsoever, therefore there is no way to step up the voltages, so the output tube(s) can only be run on about 130-140 volts maximum (rectifying 110-120 volts AC into DC gives you about 130-something volts of DC voltage), which translates to very low audio output power being deliverd to the speaker....and since a transformerless amp cannot step the 110-120 volts from the wall socket down to 6 volts, there is only one way to heat all the tubes, and that is to wire all the tube heaters together in Series (not parallel), and using tubes that were specifically designed for that purpose; if you look at the tubes used in one of these amps, you will note that all the tube heaters added together equals at least a hundred volts or more. As an Example: you might see a small cheap amp with a 35W4 rectifier, 50L6 output tube, and two 12AX7's----which adds up to 107 volts, close enough to the 110 volts or so from the wall, so that you can run all the tubes together in series and each tube will be getting roughly the proper voltage (35 volts to the 35W4, 50 volts to the 50L6, 12 volts to the 12AX7's etc). Series wiring, unfortunately, is how cheap Xmas lights are wired, and if one tube burns out (if it's heater goes "open") or if you remove one tube from the amp, or if corrosion or a bad solder joint prevent one of the tube heater-pins from making a connection, then all the tubes will appear to go "dead" and will not light up, and the amp will not play. Troubleshooting a transformerless amp to find the bad tube or open connection can be a bit of a pain sometimes......anyway, if the amp in question (on e-bay or whatever) uses anything other than 6 volt or 12 volt tubes (if there is a 25, 35, 50 or whatever attached to one or more of the tube types), you can be almost certain that it is a transformerless amp !! Choose your maximum bid accordingly......
Other disadvantages to such transformerlass amps: 1) the rectifier tube in a transformerless amp (such as the 35W4 mentioned above) is nearly always a half-wave rectifier, which delivers highly "dirty" and noisy DC voltage for the amp to operate on, as opposed to amps with full-wave rectifiers (5Y3, 5U4, 5V4, 5AR4, etc) which deliver cleaner quieter voltage; 2) the series-wired tube heater circuit is also very noisy in operation and cannot usually be wired so as to take advantage of the 12AX7's center-tap; 3) while parallel-wired heater circuits usually use a pair of wires that are tightly twisted together to prevent radiating hum and buzz into other parts of the circuitry, this is virtually impossible to do in a series-strung heater circuit ; 5) good quality amps use a power trannny that has a center-tapped heater-voltage secondary,or you can add a couple of resisitors to the trannys' filament secondary as an artificial center-tap, and both techniques lower noise, but you can't do this with transformerless amp !! 4) the metal amp chassis is used as a "ground" or return" for the 110-120 volts from the wall socket (that heats the tubes and provides the mediocre high-voltage to the output tube), so depending which way the power cord is plugged into the wall you have a 50/50 chance of having 110-120 volts "live" or "hot" AC on the amp chassis !! Your guitar strings are hooked up to the amp chassis as well (via the guitar cord) , so if you were to simultaneously touch a grounded object while you are touching your amp or the guitar strings, you may get electrocuted !! Ths style of dangerous electrical-appliance manufacturing would never be allowed nowadays die to underwriters laboratories guidelins and electrical codes, but back then nobody cared, I guess........
Using an isolation transformer (has a 1 to 1 voltage ratio, so that 110 volts AC in equals 110 volts AC out) between the amp and the wall socket will alleviate the chance of electrical shock, but isolation trannies are expensive and hard to find, and you are still left with the problems of low power, poor tone and noisy operation.....as far as I am concerned, the only reasons to buy such cheapo amps are 1) you already have a plain amp chassis without a case and you intend to gut the cheap amp and use it for it's wood chassis, mounting your raw chassis into the combo cab; 2) you are going to gut the amp circuit and build yourself a whole new amp into the existing case and metal chassis, with a proper power tranny and output tranny----(perhaps you already have a Bogen or other small mono tube amp that you can rape for the necessary parts, etc); 3) perhpas the amp has a small Jensen speaker already, in which case it may be worth $20 or so for the amp just to get the speaker !! Otherwise, such amps rarely have any valuable parts----the output tranny is usually too cheesy to be worth bothering with, and there ain't no power tranny that you can steal from it----perhaps you can get a couple of nice knobs too, but that's about it. Of course, if you like the way the amp looks, you can simply put it up on a shelf for display !!
Just my own two cents worth........
CJ
PS The reason these are sometimes called AC/DC amps is because they can run on 110-120 volts DC as well as on 110-120 volts AC, although it has been at least 60 years since DC was piped directly into peoples' homes---that system of power transmission was abandoned a long time ago as being highly inefficient and impractical.........
INSTALLATION OF AN ISOLATION TRANSFORMER IN AC/DC AMPS
An isolation transformer is basically a small power transformer that has a 1 to 1 ratio of primary to secondary. In other words 110 volts AC in gives you 110 volts AC output. These transformers can be found from most on-line or mail order suppliers, and are rated in wattage-----whatever the wall-wattage power consumption that is marked on your amp (30 to 50 watts is fairly standard for an AC-CD amp), you want to get an iso-tranny of at least that same wattage rating or a little more.
Even Antique Electronic Supply, the most expensive catalog house I know of, should have suitable trannies for $35 or less as I recall.
(I wonder if an electrical-supply house has them or you can order them, it’s worth asking! I bought a couple at a ham-radio fest last year for about $6 as I recall.)
You can either install the transformer directly to the amp chassis, or mount it to the inside of the amp's cabinet or on a small plastic or metal project box. The power cord gets wired to the transformer’s primary and the transformer’s secondary gets wired to the amps' existing power cord, or to whatever terminals inside the amp that the power cord originally went to (usually the on/off switch and the fuse holder).
That's basically it!! It's also a very good idea to add a ground wire directly to the amp chassis (use a 3-wire cord to the iso-tranny and ground the amp to the wall socket with the green wire). If you elect to use a separate project box to hold the iso-tranny (better to put the transformer on the outside of the box to lessen heat build-up, but inside the box would still work), you could get fancy and build a GFI device into the little box that holds the new iso-tranny and/or add an on-off switch, fuse-holder, polarity reversal switch (to lower hum), and neon pilot light........but none of those are strictly necessary.
Aside from installing fresh caps and tubes, try a few different speakers--especially small, lightweight, flimsy speakers. Sometimes you can get a killer tone from a low-power amp by trying a speaker that you would not otherwise have thought to consider. As an example, Mike E has a Torres Tiny Tone amp that he says sounds absolutely killer with a cheesy little Zenith 6" speaker that he had gotten from me!!
Try cannibalizing some 50's/60's grade-school portable phonographs (especially Voice Of Music phono & tape players), or old table radios, or really cheap "hi-fi" consoles, you might find a good speaker there for this particular application. Sometimes a really cheap-looking small-magnet 6x9 from an old car or from a portable phonograph or tape deck works well for low-powered guitar.