I spend much of my time hanging around Fisher gear, but that doesn't mean I don't appreciate quality pieces from other manufacturers as well. Besides Fisher -- Eico, Heath, Mac, Dynaco and Scott pieces all populate my listening room. Within my Scott stable, I have a very nice LC-21, LT-110 (brown face), LK-48B, LK-72B, original LK-72 brown face (some may remember my efforts with that one here):
http://audiokarma.org/forums/index....vintage-classic-scotts-original-lk-72.783989/
and I even have a very nice 335 Multiplex Adapter I can press into service when needed. But one piece of Scott equipment I didn't have and always wanted was a 208.
To my mind, for the type of service the 208 was primarily intended for (console power amplifier duty), it stands without equal where 7591 power packs are concerned. Yes, Mac made the well revered MC225, but it was hardly a console piece, meaning that for me, and other than the Mac, all other 7591 basic power amplifiers bow to the 208.
If memory serves, neither Heath nor Eico produced a 7591 based basic power amplifier model, and Dynaco never even broached the 7591. Pilot produced one, and while I have nothing against Pilot, their output transformers often lack the performance from similar pieces by other manufacturers. Fisher produced a couple of 7591 based console destined basic power amplifiers. But they require modification (more than just bridging the power switch terminals) to operate in stand alone mode, and the output transformers used are a clear step down from the superb transformers used in their 7591 integrated and receiver offerings. One version is even an 8Ω output only.
Against this field of contenders, Hermon produced the 208. A ready made fully stand alone basic stereo power amplifier, the 208 sports his very best 7591 output transformers (TRA-11-2), as used in the best versions of his 299 and LK-72 integrated models. It was designed to be hidden away, but is plenty good looking just as it is, and always draws an eye. By the end of the vacuum tube era, I know of no product in its service class that rose to its equal.
With that backdrop then, I recently acquired a very nice looking example of the 208, of the older(?) three can variety without center chassis support bracket underneath. It had new JJ power tubes, and a previous owner had painted the transformers. Underneath, one of the coupling caps had been replaced, as had the power resistor coming off the rectifier tube, and two of the 400K output tube grid resistors as well. Oh yeah. The leads to the power transformer were rather long, not at all in keeping with how the factory would have installed it, so the power transformer that was installed had clearly been done so after the unit left the factory. Hummmmm. But the transformer was of the TR-13 variety (-2-1) which was the transformer class used in Scott's 7591 offerings, so with it being a bonafide Scott transformer, I moved on to trying this puppy out. It was offered for sale as good to go........
A precheck of the (original) power supply caps showed that in fact they were well formed and drew no significant leakage current. When I plugged this baby in then everything acted normally -- until the rectifier tube warmed up. No sparks or fireworks -- just a power transformer growling in protest. The B+ to the output tubes was pretty much dead on 470 vdc as called for, so the growling was investigated. The rectifier tube tested very good so no issues there, so the rectifier tube wiring was investigated. That revealed what surely was a "fix" installed somewhere along the line with this unit for a hardly unheard of Scott complaint in some of their models -- excessive B+ voltage: The pin 6 HV lead at the rectifier tube socket had been attached to pin 5 (effectively connecting it to nothing), and pin 6 cleaned up so that everything looked quite normal to the casual glance (other than the long transformer leads). Connecting this lead to its proper pin 6 connection point eliminated the transformer protest (the whole amplifier had effectively been running off of 1/2 of the power transformer and 1/2 of the rectifier tube), but produced a new problem: B+ at the rectifier tube cathode was now nearly 520 vdc, and the output tubes were clearly showing protest with visible plate color in a well lit room. The chalk screen grid bleeder resistor was none too happy about the matter either, reaching roasting temperature in short order. There is little doubt then that the miss-connected transformer lead had probably been miss-connected on purpose somewhere along the line as a remedy to the excessively high B+ voltage produced when connected properly.
So what to do? Had the transformer that's now installed been so because the previous transformer also produced excessive voltage and this was an attempt to fix it? Had the previous transformer simply gone bad or was it used to fix another unit (maybe with high voltage!) and this was the result of the swap or the only available Scott replacement? The unit had multiple previous owners, so its doubtful that the answer to that question will ever be known -- although it's always interesting to know how a unit ends up in a given condition. In any event, what was known was that it was apparently the correct transformer for the unit (by part number anyway), but simply produced too much juice. But there was another factor.
About 10 volts of the 520 volt B+ was due in part to operating the unit from a 121 vac line -- except that at this line voltage, the heater voltage to the output tubes was 6.40 vac -- meaning that if the line voltage was reduced by a variac or multiple current limiters to tame the B+, then the output tube heaters get short changed. So while there was room for a little AC line voltage adjustment, there was nowhere near enough to bring the B+ in line.
So next, makeshift output tube cathode resistors were installed to get a better picture of what was going on. The tubes were well matched, and drawing just over 40 mA on average, so with about 38 mA of plate current and 495 volts of plate voltage, the color in the plate was certainly understandable -- the tubes were dissipating right at their rated plate dissipation level of 19 watts -- and these were JJ tubes! The bottom line then was that if the quiescent current was backed down to save the tubes, then the B+ would go even higher, and clearly overshoot the rated voltage of the power supply electrolytic caps.
With little downward movement in AC line voltage possible, and reducing the output tube quiescent current to sane levels guaranteed to make things worse, something had to be done if any kind of long term dependability was to be expected without this sucker reaching critical mass in my listening room. And yet, there was still one other factor that would add further insult to injury.
Next time. For now, a couple quick pics of the patient.
Dave
Below: One of the better looking 208s I've seen. They aren't all that common, and some look pretty pathetic. This one took a smack on the power transformer, indenting both end bells, and the chassis under it as well. Some creative coaxing undid most of the chassis indentation, while the end bells were deemed acceptable enough.
Below: Other than my temporary correction of the HV lead to its proper connecting point at pin 6, this is the unit as received.
http://audiokarma.org/forums/index....vintage-classic-scotts-original-lk-72.783989/
and I even have a very nice 335 Multiplex Adapter I can press into service when needed. But one piece of Scott equipment I didn't have and always wanted was a 208.
To my mind, for the type of service the 208 was primarily intended for (console power amplifier duty), it stands without equal where 7591 power packs are concerned. Yes, Mac made the well revered MC225, but it was hardly a console piece, meaning that for me, and other than the Mac, all other 7591 basic power amplifiers bow to the 208.
If memory serves, neither Heath nor Eico produced a 7591 based basic power amplifier model, and Dynaco never even broached the 7591. Pilot produced one, and while I have nothing against Pilot, their output transformers often lack the performance from similar pieces by other manufacturers. Fisher produced a couple of 7591 based console destined basic power amplifiers. But they require modification (more than just bridging the power switch terminals) to operate in stand alone mode, and the output transformers used are a clear step down from the superb transformers used in their 7591 integrated and receiver offerings. One version is even an 8Ω output only.
Against this field of contenders, Hermon produced the 208. A ready made fully stand alone basic stereo power amplifier, the 208 sports his very best 7591 output transformers (TRA-11-2), as used in the best versions of his 299 and LK-72 integrated models. It was designed to be hidden away, but is plenty good looking just as it is, and always draws an eye. By the end of the vacuum tube era, I know of no product in its service class that rose to its equal.
With that backdrop then, I recently acquired a very nice looking example of the 208, of the older(?) three can variety without center chassis support bracket underneath. It had new JJ power tubes, and a previous owner had painted the transformers. Underneath, one of the coupling caps had been replaced, as had the power resistor coming off the rectifier tube, and two of the 400K output tube grid resistors as well. Oh yeah. The leads to the power transformer were rather long, not at all in keeping with how the factory would have installed it, so the power transformer that was installed had clearly been done so after the unit left the factory. Hummmmm. But the transformer was of the TR-13 variety (-2-1) which was the transformer class used in Scott's 7591 offerings, so with it being a bonafide Scott transformer, I moved on to trying this puppy out. It was offered for sale as good to go........
A precheck of the (original) power supply caps showed that in fact they were well formed and drew no significant leakage current. When I plugged this baby in then everything acted normally -- until the rectifier tube warmed up. No sparks or fireworks -- just a power transformer growling in protest. The B+ to the output tubes was pretty much dead on 470 vdc as called for, so the growling was investigated. The rectifier tube tested very good so no issues there, so the rectifier tube wiring was investigated. That revealed what surely was a "fix" installed somewhere along the line with this unit for a hardly unheard of Scott complaint in some of their models -- excessive B+ voltage: The pin 6 HV lead at the rectifier tube socket had been attached to pin 5 (effectively connecting it to nothing), and pin 6 cleaned up so that everything looked quite normal to the casual glance (other than the long transformer leads). Connecting this lead to its proper pin 6 connection point eliminated the transformer protest (the whole amplifier had effectively been running off of 1/2 of the power transformer and 1/2 of the rectifier tube), but produced a new problem: B+ at the rectifier tube cathode was now nearly 520 vdc, and the output tubes were clearly showing protest with visible plate color in a well lit room. The chalk screen grid bleeder resistor was none too happy about the matter either, reaching roasting temperature in short order. There is little doubt then that the miss-connected transformer lead had probably been miss-connected on purpose somewhere along the line as a remedy to the excessively high B+ voltage produced when connected properly.
So what to do? Had the transformer that's now installed been so because the previous transformer also produced excessive voltage and this was an attempt to fix it? Had the previous transformer simply gone bad or was it used to fix another unit (maybe with high voltage!) and this was the result of the swap or the only available Scott replacement? The unit had multiple previous owners, so its doubtful that the answer to that question will ever be known -- although it's always interesting to know how a unit ends up in a given condition. In any event, what was known was that it was apparently the correct transformer for the unit (by part number anyway), but simply produced too much juice. But there was another factor.
About 10 volts of the 520 volt B+ was due in part to operating the unit from a 121 vac line -- except that at this line voltage, the heater voltage to the output tubes was 6.40 vac -- meaning that if the line voltage was reduced by a variac or multiple current limiters to tame the B+, then the output tube heaters get short changed. So while there was room for a little AC line voltage adjustment, there was nowhere near enough to bring the B+ in line.
So next, makeshift output tube cathode resistors were installed to get a better picture of what was going on. The tubes were well matched, and drawing just over 40 mA on average, so with about 38 mA of plate current and 495 volts of plate voltage, the color in the plate was certainly understandable -- the tubes were dissipating right at their rated plate dissipation level of 19 watts -- and these were JJ tubes! The bottom line then was that if the quiescent current was backed down to save the tubes, then the B+ would go even higher, and clearly overshoot the rated voltage of the power supply electrolytic caps.
With little downward movement in AC line voltage possible, and reducing the output tube quiescent current to sane levels guaranteed to make things worse, something had to be done if any kind of long term dependability was to be expected without this sucker reaching critical mass in my listening room. And yet, there was still one other factor that would add further insult to injury.
Next time. For now, a couple quick pics of the patient.
Dave
Below: One of the better looking 208s I've seen. They aren't all that common, and some look pretty pathetic. This one took a smack on the power transformer, indenting both end bells, and the chassis under it as well. Some creative coaxing undid most of the chassis indentation, while the end bells were deemed acceptable enough.
Below: Other than my temporary correction of the HV lead to its proper connecting point at pin 6, this is the unit as received.
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