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DUSTY FILES PART 1 :EVEN DUSTIER!
Note that due to the age, condition, and occasional faint and/or small type in the originals, they have been posted here in full 300 dpi printer resolution format, as many would not be legible if reduced. Just load the document, click on the right mouse button to save, and print thru a graphics program. Average diagram size is approx 125 kbytes.
You can download a ZIP file of all the diagrams (3 megabytes) by clicking here<
Wurlitzer schematic courtesy Bill Auxier
Bogen schematic reproduced permission Bogen Communications Inc.
RCA MI-12246, Muzak 975 thanks to Paul Dorobialski
RCA MI-9377, Simplex AM-1026 courtesy Don Helgeson
WE-86A and French language schematic appear thanks to kind help of Atila Balaton.
Altec 260A, 1570 provided courtesy Altec Lansing Corp.
Gotham schematic courtesy Gene Kijowski
McGohan M1001 thanks to Jim McGohan- McGohan Electronics Inc.
And of course, thanks to our customers who were amiable enough-to wait for this!
The point of Dusty Files Part 1 unlike Part 1 wasn't and isn't to actually give people plans for building amplifiers (though it could be done), but rather to let people look at schematics they might otherwise rarely see and look at unusual & different ways that amplifiers might be built.
Starting from Page One, in order-
This is the way your Grandpa did it, the WURLITZER model 680 (300K) utilizes a pair each of type '30, '45, and 2A3 triodes to produce maybe-if-you're-lucky 18 watts- 12 was probably more like it by our experience given the usual line- voltage variation, lousy transformers, tiny power supply capacity and usual reluctance of operators to change tubes until they put out more power in the form of light than acoustics. Most of the gain was in the transformers-for good reason tranformers are low noise, non microphonic, a big consideration in a jukebox where the 15" speaker playing bass- heavy music, sat less than 18" from the amplifier) and the tubes had minimal gain (mu factor only from 3 to 9). The filament cathode tubes used were already obsolete by the time the amp was made and were used principally for their fast warmup characteristics so the amplifier would be operating at full volume before the massive osmium-tipped pickup (tracking at record-grinding 70 grams!) hit the record. Now this system may seem archaic to those accustomed to todays' massive transistor-bespeckled-and- regulated tube pieces but there are those whose advocate the use of filament-cathode tubes at least for output stages, witness the small but strong revival of tubes such as 845's, 6B4-G's and 300B's) claiming improvements in distortion and linearity and using tube rectifiers-logical since it is documented and demonstratable that changes in rectifiers make measurable changes in sonic performance, distortion and damping factors, -although the question of what's better is the subject of considerable debate- the sonic effects of using 1930's style rectifier tubes heavily choked supplies and relatively sloppy regulation may surprise those who had thought that the development of "high-speed" regulated transistor supplies had settled the whole power supply issue. The last and perhaps most controversial advocation regarding this style of amplifier is the use of transformers for amplification and phase-splitting. This idea too carries some logic to it-use the tubes for what they're good at, converting voltage to current, and transformers to convert current back to voltage to match the high input impedance to the succeeding tube. The problems, though, are numerous- first finding transformers of sufficient quality- only really good transformers have the bandwidth or degrade the signal less than a tube secondly , they are are not cheap and thirdly, picking up such amplifiers after they're built. There are, however, numerous citations that early audio engineers considered transformer coupling superior to other forms, so much early studio and high fidelity equipment was heavily transformer coupled (usually always on the input and output at least and often push-pull balanced and transformer coupled throughout, even on preamps and line amps as well as power) considerable argument could be made that most designs strayed away from this configuration solely for cost reasons and many circuits were developed which were intended to duplicate desirable transformer characteristics with tubes. Those who consider transformers completely abhorrent may consider that most high quality recordings made prior to the early 1980's contained at least one and usually several transformers in the audio chain. The key is of course, high quality units to make them relatively transparent.
The swan song of the great triode amps of the 1930's is the Western Electric 86A. This amp, still considered a near deity in some circles, was in today's dollars perhaps the most expensive amplifier per watt (and perhaps per kilogram!) ever built- although the massive 1920's WE theater sound amps probably would take the prize in that category. Since they were only leased when new (Ma Bell's'policy) the actual cost is not known but converted for inflation the figure would likely put cost-no-object" current production amplifiers into a different perspective. The 86A benefits extensively from pioneering Bell Labs' research-the first audio tubes designed specifically for low noise and hum (262B) and low output distortion (300B, maybe with the lowest inherent distortion of any audio output tube ever, certainly much better than any beam tetrode such as 6550 or 6L6), and audio transformer which incorporated the high grade steel and winding techniques WE and Bell Labs developed. The amplifier features negative feedback around each voltage amplifier stage, transformer phase splitting (no tube, of course, can naturally split phase as precisely as a transformer) and well regulated choke input (generally de reguier for early commercial equipment) power supply. It's low maintenance, too-the WE 3008's required replacement perhaps only every 10-20,000 hours, the 274B 20-40,000 hours, the 262B's every 50,000 hours or more. (After spending all this money on equipment, Ma Bell wanted to make it up by not having to send employees out to maintain it!) There's even a built in tube tester and of course it's "pure Class A triode".
Cost accountants and sales managers both rejoiced over the arrival of beam tetrodes (6L6-G) and multi-section tubes (6SN7-GT) in the mid 30's, which combined to finally make real public address and the first traces of "hi-fi" equipment cost effective, for the average beam tetrode did away with the extra voltage-boosting transformers, bias networks, and poor efficiency which added much cost and weight to the old triode amps, and multi section tubes saved valuable chassis space and sockets. By WWII the cost of PA amps had dropped to not much more than a good radio. A typical design is Western Electric 142A public address amplifier (WE still believed in triodes for its theater amps, however), using the 5 tube lineup that was the mode du jour until the mid 50's The 6L6 and 6V6 put PA and music systems in the hands of millions (if you didn't mind 8-10% IMD without feedback!) and with the typical 95-100 db efficiency of the usual electrodynamic speakers (permanent magnets at reasonable cost not being available yet) a rack of even low powered amps like the 86A and 142A had enough punch to fill even pretty large theaters.
But some applications, particularly drive-ins and outdoor stadiums needed more power, and prior to the appearance of popularly priced, higher powered tubes such as the 6550 and 6CA7/EL34 , small disposable transmitter tubes were often favored. (The other approach, push-pull parallel, such as in the RCA MI-9377, had its own limitations, although the multiple pair approach has lately proved quite popular). These tubes were considerably cheaper and smaller than the older style thoriated tungsten filament tubes that WE had used earlier (211A,284,242,845), and worked on relatively benign voltages-as low as 550 volts on the plate. The tubes (811A,809, etc.) operated in highly efficient zero-bias Class B triode, cutting costly power supply requirement stand eliminating bias supplies, the tradeoff being the necessity of well regulated power supplies due to the wide variations in plate current, and the need for a power driver circuit ahead of the output tubes, the output tubes needed their grids driven up to 160 volts positive, and needed 5 to 8 watts of power input for full power output. The distortion, however. was less than comparable tetrodes or pentodes, generally less than 1 or 2% THD up to full output. Those aghast at the thought of using Class B in audiophile applications might consider that besides the fairly low overall distortion, Class B distortion generally tends to be mostly second harmonic-as effect which many designers try to engineer into current production by overloading the tubes slightly. It will probably surprise many some well regarded current production amplifiers are mostly Class B, although the manufacturers either gloss over or fail to mention it due the audiophile stigma of "Class B" operation. Any amplifier operating with very low or close to cutoff no-signal current is Class B, whether the amp maker mentions this or not. Amps which derive 200 watts from four EL34 or 6550 tubes are either entirely or mostly-Class B beyond very small signal levels. Thus it may be concluded that those advocating "pure Class A triode" operation without some statistical justification for it are often merely repeating 1930's dogma and/or , simply want the psychological satisfaction of having a "pure Class A" amplification. The fact that many hard-core triode enthusiasts are moving towards single ended (and thus by definition high second harmonic due to lack of push pull harmonic cancellation) triode operation tends to support this conclusion, since a Class B amplifier is sort of (well, not exactly, but..) two single-ended amplifiers back to back. Nuff said. The MI-12246 RCA amp pictured here is typical of 1940's Class B amplifiers.
The compromise approach to producing higher power prior to the appearance of 6550 and EL34 tubes is typified by the IPC Simplex AM-1026, which cranked out (hopefully) 100 watts out of a pair of 807's by driving the grids positive on signal peaks (as opposed to continuously as in the case of Class B triodes, thus Class AB2) makes the 807's produce higher wattage while keeping plate voltages lower and without exceeding plate dissipation ratings, it's also more efficient than a class AB1 100 watt amps although would be best considered a 50 watt Class AB1 amp with 50 watts of headroom, since continuous use at 100 watts probably exceeds the continuous duty ratings of 807's. Of note here is the use of a neat floating paraphase- cascode phase splitter arrangement, thus demonstrating there's very little new under the sun in tube amps....
The "Maestro" is one of the first attempts to build a high power amplifier class AB1 using conventional cheap relieving tubes. The 6146 tubes (an 807 derivative) had drawbacks as audio power tubes-the very short bulbs and leads, while desirable for the VHF short-duty cycle service had a tendency to build up excessive heat, and thus loose bases, when used as continuous duty audio tubes. Many manufacturers who built amps using 6146's eventually issued retrofits to convert them to 6550's.
In lieu of the yet to be introduced 6550, some manufacturers turned to the readily available TV "sweep tubes''. While not necessarily low distortion or particularly stable, (improved versions for audio and RF amplifier service, such as 7867, M2057, and 8908 appeared later)the low cost, high output and good longevity made them popular in many audio and PA as the Dukane lA475. Note the RC network between the plates and ground, probably to protect the output transformer and tubes when "hot switching" loads.
Finally in 1954 the Tung-Sol company introduced the first truly high power Class AB1 audio tube in the USA, the now ubiquitous 6550. About the same time GEC (AKA Genelex) in the UK introduced the KT88 tube, which is functionally identical (in fact many USA 6550's of the period had plate assemblies which appeared quite similar to the KT88's, thus making one curious as to whether there was collusion). The next year Altec Lansing introduced the first consumer unit with them, the Altec A-340A with a pair loafing along producing 35 watts. This amp also used the then new RCA 12AY7 which answered calls by some for a medium-gain tube designed for audio without having to resort to the 12AT7 which had characteristics then considered undesirable for audio use:small plates and lack of a twisted, hum rejecting filament. (later 12AT7's apparently incorporated the latter improvement) However the tube never quite caught on in latter day equipment as the 12AT7 did. Note the voltage regulator tube-also present in the RCA MI-9377 and the Dukane amp, testifying to the necessity of stabilizing screen voltages in high gain amplifiers using beam tetrodes when other solutions to the problems of instability and oscillation with beam tetrodes (generally cathode feedback, ultra linear and "unity coupled" arrangements) weren't used. This additional design consideration helped popularize the "true pentode" tubes such as the 6CA7/EL34 and 6BQ5/EL84 in the fifties. These types generally have lower inherent IMD as well.
The Williamson amplifier design dominated early 50's home hi-fi amp design and numerous variations were available. The Bogen DO-30A is one of these Williamson clones with just enough trick changes to impress the customers, like phase compensation, cathode feedback, damping factor controls, 12AT7 and 12AU7 in lieu of the usual 6SN7's, and of course genuine British KT66 tubes to ice the cake.
While pentodes and tetrodes dominated the scene, the triode amp fans had their last hurrah for many years in the mid 50's, triodes being lost in the rush to miniaturize and yet build more powerful amplifiers. The 15 watt all direct coupled amp is one of the last efforts. The problem with this amp is that it probably won't work as shown as every 6A5 the author has seen had the cathode internally coupled to the filament, thus this amp in practice would require an additional filament transformer ,note, 2A3, 6A3,6B4G, 300B, as well as heater cathode types such as 6AS7 could be substituted here.
In the 1950's it was still considered that real "industrial-strength" amplifiers should use transmitting tubes. The Altec 1570B and 260A are good examples, The 260A incorporates design features generally associated with high powered audio amps and modulators using transmitting tube types to assure stability-perhaps made more necessary with the somewhat touchy 813 tubes. These features include impedance coupling (choke in grids of 813's) stopper grid resistors, shielded coupling capacitors, choke input power supply, and full push-pull operation with push-pull feedback. Altec went out of their way to warranty unconditional stability, but design notes indicate changes made either due to field problems or simply corporate nervousness about it.
The Gotham PFB-150A recording amplifier and it was unique for its toroid output-and its use of 811A Class B triodes again tends to negate the contention that Class B operation is an evil to be avoided at all costs- particularly since many fine recordings were made with this amplifier.
The power supply for the PFB-150A is a good example of use of TV damper tubes as rectifiers.
The appearance in the 1950's of the patented Hafler "ultra-linear" and McIntosh "unity- coupled" circuits and their wide popularity sent other manufacturers scrambling to invent competing arrangements-if only to avoid royalty payments. Two of the more unique examples are the National Horizon 20 and the Electrovoice "Circlotron" A-30. The Electrovoice circuit had the supposed advantage of zero DC primary current-thus no "gap" in the transformer core (conventional transformers use bidirectional current flow to cancel out the saturating effects of DC in the primary-and a core gap to boot just in case of unbalance between the output tubes). But output tube balancing and AC balancing would seem to be just as important here as in other amplifiers.
The National company was famous for its line of shortwave recievers, but in the mid-50's decided to take a flyer in the field with a line of tuners and amplifiers. The "Horizon" amplifier was mainly distinguished by its shunt-series connected output tubes whose arrangement which avoided using the transformer to couple between the output tubes. This single-ended push pull type of circuit hand variations thereof, was widely promoted but never really caught fire with the general public or the manufacturers.
Those wishing to build high-powered triode amplifiers never quite give up and are forever battling the evils of the design considerations, the high cost of suitable tubes, and avoiding such possibly undesirable shortcuts as interstage transformers or Class B operation. Then often the complexity of overcoming the obstacles sometimes becomes a problem unto itself. The "Purple Cow" homebrew of 1959 demonstrates some of the cures for terminal triode-itis, multiple pairs of expensive industrial voltage regulator tubes, a totem-pile type driver configuration using 6SN7's to swing the requisite hundreds of volts of driving voltage, with 795 volts on the top tubes!) and a wild arrangement consisting of no less than five transformers to supply the required voltages. The lack of suitable parts is also evident in the output transformer as the UTC LS-58 unit had the 8 ohm speaker hooked to the 30 ohm tap to give the proper reflected impedance to the primary. The multiple filament transformers were necessary to maintain the heater-to-cathode voltage differences within acceptable limits. The other two unusual aspects of this unit are the Fletcher-Munson loudness curve compensated volume control (which would be considered bad form by most audiophiles nowadays), and the very high gain stage following the control to make up for the control's losses. This stage uses a current-starved pentode with a 4.7 meg plate load, which those familiar with pentode design will recognize as having very high gain but also very limited frequency bandwidth. The designer counters the bandwidth problem by coupling the plate of the pentode directly to the grid of the suceeding cathode follower and then employing considerable negative feedback. This is an interesting circuit: the use of a 6U8 tube is questionable, there are better and lower noise pentodes, also this circuit could be done using a cascoded double triode in lieu of the pentode. The use of 5R4 rectifiers is curious, silicon diodes or lower impudence tube rectifiers (5V4, 5AR4.or 5U4 for be used at these low voltages This circuit overall is awfully complex (but probably no worse than some Audio Research designs) although the designer certainly thought that the 48 watts pure Class A triode he extracted from the completed amp was worth it.
Through the 1950's the frequency bandwidth of driver circuits was not generally not believed to be a major design consideration, so long as it was flat through the audio region. Then in 1959 Harmon Kardon tried hooking up a sideband (out to 2 Mhz fiat) oscilloscope vertical driver circuit using 12BY7's to a pair of KT88's, and the result was an instant classic, the Citation II. While considered then and now to be one of the finest designs ever, apparently it broke too many rules of convention as no popular amplifier since-even H-K's- ever used this type of approach.
Tube amps remained popular with commercial sound users for years after they had fallen out of favor in the consumer market-apparently the reliability of early transistor amps was so poor that when improved models appeared Muzak Corporation and others were almost begging sound contractors to use them. But the familiarity, easy serviceability and abuse resistance of tube amplifiers kept them on production lines of some PA amp and jukebox manufacturers until about 1980. The Muzak 975 75 watt amp was typical of late model PA amps- 2 rectifier and 4 output tubes (so that failure of any one tube would not shut the whole amp down), tertiary feedback winding, choke input power supply (the .1 uF cap is solely to prevent the choke's inductive kick from shorting SS plugin rectifier replacements ) then plate chokes and grid stopper resistors for stability. A unique feature of the 975 was the zener diode in the cathode of the 7027A's giving pseudo-fixed bias operation (and eliminating the expense of lever and parts for a bias supply).
The last major entry in the audio power tube market was the 8417-a tube with 6550 type ratings but twice the gM, thus allowing large power output stages to operate with only 15 to 20 volts or so of negative bias, thus simplifying driver stage requirements. The tube was an immediate hit with the commercial sound people, immediately being put to use by Dukane, Bogen, McGohan, and others.The problem with the 8417 was quality control-the tube had be well built otherwise the high sensitivity is a one-way ticket to instability and early tube death. The McGohan M1001 100W amp was taken out of production largely due to lack of adequate quality output tubes. This amplifier uses a second power transformer winding to supply a stable screen voltage in lieu of regulators, the driver circuit is very similar to present-day VTL amplifiers, although VTL uses a balance control in lieu of fixed unbalance in driver plate load resistors.
This brings us to present day and our last diagram is that of a latter-day European high-end amp which uses a "long tail pair" modified Mullard Circuit long popular there. the output tubes are EL34's, running the screens close to the plate voltage tends to reduce IM distortion in pentodes, The tubes are slightly overloaded to produce a harmonic profile biased towards more tolerable, lower order even harmonics. The input tube is a 6SJ7, although any good low noise audio pent ode could be used here, 5693, 1620, 6J7, 5879, EF37A, EF86, EF806, 310A, 328A, and 348A all have similar electrical characteristics although bulbs, basings and filament voltages differ. The driver tube in this instance is a 6SN7, however again numerous other similar tubes could be used, there's quite a selection of audio dual triodes that could be substituted, 5692, ECC33, ECC32, 12AU7, 12BH7, 6GU7, 6FQ7/6CG7, 5687, 7119 (just to name a few) or two single triodes such as The screen voltage on this amp is hooked to the regulator shown alto it's difficult to ascertain that from the diagram. Several other European power amplifiers use driver circuits similar to the one shown here-the Americans are considerably less enamored of pentode input tubes altho there are derivatives using triode inputs on the market. This circuit appeared in its original form in the early 50's.
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