SCOTT MODEL 99-B
Herman Hosmer Scott, (1909-1975) aka H.H.Scott
|I found this on the bay and couldn't
resist. It dates to 1954. It's rated at 22 watts but the
best I can get is about 9 watts to 17 watts between 30hz and 20khz, the
lower power at the lower frequencies.. All tubes were replaced as
well as power supply caps and several others in the circuit, notably the
black "bumblebee'" caps which, as I read on a few audio sites,
will fail internally. According to some, the ESR can rise so high
as to make the cap behave much like a resistor. Some did have very
high ESR and their capacitances varied widely from specification.
For example, C25 (0.022uf) measured 0.041uf. C22 (0.0047uf)
and C17 (0.047uf) measured 0.073.
Once the caps were opened on one end and the tubes removed, several resistors were easily checked and found to be OK. The others will have to be checked by lifting one end to isolate it. Resistors R42 to R47 measure well within a 10% tolrance.
The power supply delivers 388vdc at pin 2 of the 5U4. The voltages present at the capacitors moving left from the tube and through the resistors are 378, 359, 321 and 229 vdc. All are well below the 500 volt rating of the new filter caps, perhaps too low. The transformer delivers 360-0-360 vac with 120vac line.
Building another supply and using a variable transformer, higher voltages can be obtained. The original filter caps are rated at 475vdc which seems very high for the dc voltages currently present. The caps ratings are 25% higher than the highest available dc voltage. It is my feeling that the 378vdc should be more like 15% higher, like 434vdc, which is still 8.6% below the 475vdc rating.
It was suggested to use a variable transformer at the line and crank the 120vac but that doesn't seem advisable considering the age of the original power transformer. The line would have to be taken to around 135vac to get the dc voltage at the 5U4 pin 2 higher. There's more on this at the bottom of this page, photos 5 & 6.
Doubling the power supply capacitors made little to no improvement on power output. Going higher would increase the inrush current and load the rectifier tube. While the tube can take it, the power transformer may not.
I built the wood cabinet. Nothing fancy but at least the amp isn't naked.
The seller said that the LEVEL control on the left didn't work but it does. One has to see the schematic to see why. Refer to FIG. 1
Two of the "bumblebee" caps have yet to be replaced as I couldn't obtain the correct values. When those are obtained, I'll probably replace all the resistors also
The brown cap in the center, C32 will also have to be replaced but couldn't find a 50uf/100v or close anywhere. There's also what appears to be a 50k bias pot, called a balance in the schematic. It seems to balance tubes V3a and V3b which, in turn, drive the output tubes, V4 and V5. An imbalance here may cause a loss of power output and/or an unsymmetrical waveform at the output. The voltages on each end of that balance pot are 170vdc. On the outer ends of R35 and R39, the voltages are 167vdc and 177vdc, resp. This might indicate either a bad R35, R39 and/or bad C27, C31. The voltages at differ considerably but not knowing specification, I can only guess. I could be normal as there isn't any non-symmetry in the waveform at low frequencies. (See photo 6) In short, all components associated directly with the 6L6 output tubes will have to be checked. This will be done when I have replacements.
Not being an engineer or experienced tech, I have to proceed with caution. The four blue caps are those of the power supply. 20uf each at 500vdc
The LEVEL control is at C. It works only in the PHONO mode. Positions 1 thru 6 are for EUR, LON, AES, RIAA, COL and NARTB TAPE. (National Association of Radio and Television Broadcasters). In positions 7, 8 and 9, all the circuitry in the upper left corner and the LEVEL control are bypassed. The level control is to adjust/compensate for the different output voltages of various phonograph cartridges.
This widget is a dynamic noise suppressor.
It suppressed record noises, nics and pops from playing 78 rpm records. It was connected to the jacks A and B on top of the 99-B chassis.
Note that the jack at B; it's a switch. When a phono jack is plugged in, the signal is routed through the noise suppressor. It seems that Mr. Scott was a couple of decades ahead of his time.
A link to a better description is here.
I don't have this unit but after the amplifier is working satisfactorily, I just might look for one.
Light pink, lower trace: Tuner input, bass & treble controls@ -1, Volume/Loudness switch @ Volume, Rumble & Scratch filters OFF.
The black and darker pink are obtained u8nder identical conditions but 9 months apart. Bass @ 0.25, treble @ -1.75, rumble filter ON, Scratch filter OFF
Trace 8 same as in fig.3.
The orange trace shows the effect of the rumble and scratch filters.
The various recording compensation EQ's all of which have been standardised to the RIAA curve. How much these deviate from the ideal is unknown but who cares?
Trace #14, the EUR 78 is the exception but fixing it will be purely for the sake of fixing a problem.
An explanation of the AES recording EQ curve.
According to the third curve in the following link, the EUR EQ circuit isn't working right.
Equipment used. Tektronix AA501 distortion analyser and Tektronix FG-502 function genny which has 0.5% THD, so subtract that from the figures below.
|THD + noise relative to 2w @ 1khz into 16 ohms (resistive)|
|f (hz)||THD+noise (%)||Output Relative to 1khz (dB)|
|THD + noise @ full power into 16 ohms (resistive)|
|f(hz)||THD + noise (%)||Output relative to 1khz (dB)||Power (W)|
|at the visible point of clipping seen on scope Tektronix 465, an oldie|
This is a 50hz signal with controls set as follows and the same as per Fig. 3. Bass @ 0.25, treble @ -1.75, rumble filter ON, Scratch filter OFF.
The line voltage has been taken to 135vac. The output of the power transformer is 400-0-400vac and that of the 5U4 pin 2 is 441vdc. This produced 17.41vrms across the 16 ohms resistive load for a power output of 19 watts.
This photo shows the waveform at the load resistor after the AC line was dropped to 120vac. Nothing else was changed. the waveform is highly distorted. When the amplifier volume control was lowered to produce a clean waveform, the output power dropped to 15.8 watts.
Not knowing the conditions under which the 22 watts was achieved and not knowing the voltages at various points in the original design leaves one in the dark, so to speak.
At least the waveform is symmetrical.
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