Ibuilt this Svetlana SV300B stereo amplifier using Plitron’s single-endedtoroidal transformers (designed by Menno van der Veen of Holland). Thedesign includes a high-gain, high-linearity voltage driver circuit, fine-

tuned by simulation (Photo 1).The results showed an excellent power bandwidth: 70kHz at 1W output,

and 50kHz even at 10W out. The overall gain characteristic is extremely flatover the full frequency range. The distortion characteristic is also relativelylow at 0.5% at 1W, and 3.5% at 10W. The sound from the amplifier brings awide, deep presence with a JBL S3100 speaker system, allowing me to enjoyall kinds of music. to page 9

BY SATORU KOBAYASHI

Pete Milletpage 26

WiringTips FromA Pro

Gary Galopage 40

DigitalAudioBreakthrough

Volume 12 Number 6 2000

István Béripage 48

“Ultimate”SE AmpDesign

www.audioXpress.com

$7.00CANADA $10.00

PHOTO 1: The SV300B stereo amplifier.

This article was originally

published in Japan’s pre-

mier high-end tube maga-

zine, MJ Audio Technology,

May 1999.

GLASS AUDIO 6/00 9

(from page 1)

DEFINING OPTIMAL LOADIMPEDANCEAudiophiles have designed many 300Bamplifiers to experience the pleasantsound of that tube. There are too manyexamples published in the audio maga-zines to choose the best one, however.Even Western Electric delivers consider-able 300B data via their website.

Therefore, I examined WE300B operat-ing conditions using statistical analysismethods, with a Microsoft Excel spread-sheet (Table 1). As expected, the follow-ing tendencies appear: 1) the averageoutput power increases by 2W in propor-tion to the plate voltage increment of50V; 2) the distortion stays mostly thesame as the supply voltage varies (Fig. 1).

When you vary voltages through 350V,400V, and 450V and vary the load imped-ance, the power output and distortionwill decrease as the load impedance in-creases. With this information, you candetermine the most preferable load im-pedance from the actual data: 5kΩ is theoptimum load at 450V, 3.5kΩ at 400V, and3kΩ at 350V, respectively (Figs. 2a, b, c).

This allows a tentative load-impedancefix of 5kΩ and a plate voltage of 450V forthe project. The simulation then gives agrid bias of −105V. From this, you can ex-pect approximately 12W maximum out-put power. To drive this output stage prop-erly requires design of a driver circuit thatcan generate 75V RMS (105/√2) with aninput level of 0.5V RMS through 1V RMS.

DRIVER STAGEThe 300B tube needs about −105V gridbias voltage. It requires more attentionthan the conventional tetrode and/or pen-tode power tubes, which need a grid biasof only −20 to −50V. A conventional volt-age driver circuit usually employs a pen-tode at the first stage and a triode at thedriver stage, with negative feedback ad-justing the total gain of the amplifier, aswell as an interstage transformer to get ahigh peak drive voltage.

For this project, I determined that a lin-ear circuit producing a high output ACvoltage is required. Mark Kelly’s “The

Search For Linearity” (GA 6/96, p. 42, 1/97,p. 32) was very useful to me, and allowedme to make a final decision. The articleshows a number of high gain and high lin-earity circuits, such as cascode, mu-fol-lower, SRPP, White cathode follower, andso on. All use a vacuum tube as a currentsource in place of either cathode or plateresistor, enhancing the gain and linearity.

After reading this, instantly I hit uponan idea: If I used one of those, then I coulddrive a 300B easily and simply without aninterstage transformer, and save cost andspace. However, such a high-gain amplifi-er provides a rather high output imped-ance (a few kΩ to tens of kΩ). Thus, animpedance converter is needed to drive a300B with a low impedance. A Whitecathode follower would be a good choice,rather than a standard cathode follower.

Once I tentatively selected the circuittype, then I performed a simulation todetermine the total gain with availabletubes, such as 12AX7, 6189W, 12AU7, and6FQ7. I used TubeCAD software by Glass-Ware to calculate parameters such asgain, maximum output voltage, andother figures very quickly, without need-

ing to draw load lines on characteristiccurves of each tube (Fig. 3). In an SRPPcircuit, the voltage gain of a 12AX7 is more than 90, while 6189W yields a gain of less than 60. I have simulatedthe circuits of the following: standardSRPP, self-biased SRPP (mu-follower),and fixed-bias SRPP. Subsequently, themu-follower circuit showed the best performance in terms of voltage gain and maximum output saturation volt-age (Table 2).

The White cathode follower that I chosefor the driver stage is improved from thestandard cathode follower, by replacingthe cathode resistor with an active currentsource made from a vacuum tube. Thisstructure generates a higher peak voltageoutput than a conventional cathode fol-lower. To choose the best tube out of my personal stock, which includes types6189W, 12AU7, and 6FQ7, I conductedmore simulations using Tube CAD.

The result showed the 6189W to be thelowest output impedance out of threetubes. Also it showed superior power-sup-ply ripple rejection, about 10dB betterthan the other tubes (Table 3).

ABOUT THE AUTHORSatoru Kobayashi is from Tokyo, Japan. He has beeninterested in audio and in ham radio since he was in histeens. After majoring in EE in Tokyo, he joined thesemiconductor industry, designing DRAM circuits for aliving, although he now works in the technical and mar-keting area. His debut as a writer came in the early ‘80sin the form of an article about ham radio for CQ maga-zine. Now he periodically writes on the subject of audiofor a few different magazines.

TABLE 1WE300B OPERATING EXAMPLES

PLATE GRID IDLE LOAD OUTPUT 2ND 3RD

VOLTAGE BIAS CURRENT IMPEDANCE HARMONIC HARMONIC HARMONIC200 −42 30 2000 3 20 31200 −39 40 2500 2.6 26 38200 −37 50 2500 2.5 30 45250 −55 30 2000 4.9 18 27250 −55 30 4500 3.2 27 40250 −52 40 3000 4 26 36250 −50 50 2500 4.4 26 39250 −48 60 2000 4.7 26 38250 −48 60 2700 4.1 30 45250 −45 80 1500 5 26 41300 −65 40 2500 6.7 20 30300 −63 50 2000 7.2 21 29300 −63 50 3000 6.1 26 37300 −61 60 2400 6.6 26 37300 −61 60 3400 5.6 30 44300 −58 80 1700 7.5 26 37350 −76 50 3600 7.8 26 38350 −76 50 5000 6.2 30 45350 −74 60 2000 10.2 21 30350 −74 60 3000 8.3 26 38350 −74 60 4000 7 30 44350 −71 80 2200 9.6 26 39400 −91 40 5000 8.4 26 37400 −89 50 3000 11.5 21 31400 −89 50 4000 9.4 25 38400 −87 60 3500 10.5 26 38400 −87 60 5000 8.3 30 46400 −84 80 2500 12.5 25 37450 −104 40 6000 9.5 26 38450 −102 50 5000 10.7 27 39450 −102 50 6500 9 30 45450 −100 60 4000 12.5 26 38450 −100 60 5500 10.1 30 44450 −97 80 2000 17.8 21 30450 −97 80 3000 14.6 26 37450 −97 80 4500 11.5 31 45Source: Western Electric WE300B Technical Note from WE website.

OPERATING POINTIt is often said that the higher the platevoltage on a tube, the higher the outputvoltage will be…and the better the linear-ity. So, I prefer to increase the plate volt-

age of the driver stage to the highest inthe circuit; 450V might be adequate inthis amplifier. However, this high platevoltage might exceed the heater-to-cath-ode maximum limit of 200V.

In a cascaded pair oftriodes encapsulatedin a single glass tube,the cathode voltage ofthe upper tube isabout one-half of thesupply voltage. Thus, asupply voltage of over400V may cause prob-lems with this circuitstructure. So I chose a350V plate voltage forsimulation purposes,leaving some roombelow the tube’s maxi-mum rating.

It became obviousthat a mu-follower cir-cuit will generate 250V

peak-to-peak (pp) output while holding180V DC at the cathode on this circuitunder a 350V DC supply voltage. Al-though the circuit generates a peak out-put level of 305V, with the output nodeswinging from 55V to 305V, the value ex-ceeds the maximum heater-to-cathodevoltage easily. To prevent damage fromthis excess voltage, the heater of the tubemust be biased to one-half of the peakoutput voltage. A 150V heater bias will beadequate for this purpose.

After this simulation, I built a proto-type circuit whose characteristic I mea-sured with plate supplies of 350V, 400V,and 450V to verify its operation (Figs. 4and 5). Consequently, I found the follow-ing: 1. The maximum output voltage in-creases in proportion to the plate volt-age, and the distortion then decreases. 2.The maximum peak voltage on the 300B,with a 2V RMS signal input, was 325V pp,356V pp, and 390V pp, respectively. 3.The overall voltage gain was approxi-

10 GLASS AUDIO 6/00

FIGURE 1: WE300B output power and distortioncharacteristics.

FIGURE 2A, B, C: WE300B output power and distortioncharacteristics by plate voltage. FIGURE 3: SV300B loading curve by SE Amp CAD simulator.

2A

2B

2C

G-1478-1

G-1478-2a

G-1478-2b

G-1478-2c

G-1478-3

TABLE 2SIMULATION RESULT: MU-FOLLOWER DRIVER

CIRCUIT SRPP SRPP SRPP SRPPFIXED BIAS SELF BIAS SELF BIAS

Tube 12AX7 ← ← ←Plate voltage 350V ← ← 450VPlate current 1.5mA 1mA 0.9mA ←Cathode resistor 10kΩ ← ← ←Plate resistor 10kΩ ← ← ←Input resistor 100Ω ← ← ←Coupling capacitor 1µF ← ← ←Gain 93.49 ← 96.06 ←Cathode voltage 190V 184V 188V 238V(upper unit)Grid voltage 189V 183V 186V 235V(upper unit)Ripple rejection rate −24.1dB ← −28.5dB ←Maximum output −108/ −125/ −124/ −173/ voltage, peak-to-peak +108V +125V +125V +173VGrid bias −1.15V −1.34V −1.3V −1.8VInput impedance 50.1kΩ ← 48.7Ω ←Output impedance 3.99kΩ ← ← ←Frequency 0.16Hz ← ← ←response—lowFrequency >1MHz ← ← ←response—highBias resistor N/A 1.6MΩ/1.8MΩ N/A N/A

GLASS AUDIO 6/00 11

mately 78. 4. The bandwidth at a −3dBlevel was 60kHz or less at 70V RMS out-put. The distortion at this output was ap-proximately 1%.

With the aforementioned simulationand experiment, the lineup of the drivercircuit was fixed, with a 12AX7A for thefirst stage and a 6189W for the driverstage, using a plate voltage of 400V. Ichose the 400V plate voltage because itwill generate the maximum output leveland hold the cathode-to-heater voltageon the driver tubes under the maximumlimit of 200V, given a 150V DC bias at theheater electrode. The maximum outputvoltage was 250V pp with this circuit at3.2V pp input, which is at the onset ofclipping (Fig. 6).

FINAL STAGETo define the final stage operating condi-tion, I used SE Amp CAD software, com-paring three different load impedancesto determine which one would bring thebest performance under simulation. Asof today, Plitron offers three suitablemodels, with primary impedance of2.5kΩ, 3.5kΩ, and 5kΩ—all suitable for a

300B SE amplifier. The simulation pa-rameters assumed a 450V plate voltageand a 70mA idle current, which is withinthe maximum limit with fixed-bias oper-ation for the 300B. Also, the simulatorneeds the transformer parameters, suchas load impedance, secondary wiring re-sistance, and so on (Table 4).

Armed with results from three differ-ent models, I gave first priority to lowerdistortion rather than maximum outputpower, so the final choice for loading was

5kΩ. On the other hand, I had alreadyplaced the order for 2.5kΩ transformersduring the design stage, so I decided tomake the primary impedance 5kΩ byconnecting the 8Ω load to the 4Ω outputtap of the transformer.

Afterwards, I did the simulation again,using fully defined parameters for verifi-cation (Table 5). The result shows 20%headroom below the maximum platedissipation of 40W with 70mA of idlecurrent, giving maximum output powerof approximately 10W with 3.1% har-monic distortion, at 99V input to the300B grid.

TABLE 3SIMULATION RESULT: WHITE CATHODE FOLLOWER

TUBE 6189W 6FQ7 12AU7Plate voltage 350V ← ←Plate current 1.6mA ← ←Cathode resistor 10kΩ ← ←Plate resistor 10kΩ ← ←Input resistor 100Ω ← ←Coupling capacitor 1µF ← ←Gain 0.98 0.95 0.94Cathode voltage (upper unit) 191V ← ←Grid voltage (upper unit) 167V 173V 168VRipple rejection rate −61.6dB −48.6dB −45.9dBMaximum output voltage, ±157V ±152V ±150Vpeak-to-peakGrid bias −2.06V −6.58V −7.62VInput impedance 5.31MΩ 2.15MΩ 5.31MΩOutput impedance 9.99Ω 31.1Ω 42.4ΩFrequency response—low 1.59Hz ← ←Frequency response—high >1MHz ← ←

FIGURE 4: Input versus output characteristics and distor-tion of driver stage.

TABLE 4SIMULATION RESULT: FINAL STAGE LOAD IMPEDANCE

LOAD IMPEDANCE 5kΩΩ 3.5kΩΩ 2.5kΩΩPlate voltage 450V ← ←Idle current 70mA ← ←Primary resistance 80Ω 50Ω 40VSecondary resistance 0.1Ω ← ←Grid bias −98.8V −99.4V −99.5VMaximum output power, RMS 9.53W 11.6W 12.7WMaximum output voltage, RMS 8.73V 9.65V 10.1VMaximum output current 1.09A 1.21A 1.26AGrid voltage (upper unit) 167V 173V 168VOutput impedance 1.8Ω 2.24Ω 2.72ΩDamping factor 4.45 3.58 2.942nd harmonic distortion 2.9% 6% 10.2%3rd harmonic distortion 0.7% 2.1% 4.3%

TABLE 5FINAL SIMULATION RESULT:

OUTPUT STAGE

LOAD IMPEDANCE 4.98kΩΩPlate voltage 450VIdle current 70mAPrimary resistance 40ΩSecondary resistance 0.1ΩGrid bias −98.8VMaximum output power 9.53WMaximum output voltage, RMS 8.73VMaximum output current 1.09AGrid voltage (upper unit) 167VOutput impedance 1.8ΩDamping factor 4.452nd harmonic distortion 2.9%3rd harmonic distortion 0.7%

FIGURE 5: Frequency response of driver stage.

FIGURE 6: Driver stageoutput waveform.

G-1478-4

G-1478-5

G-1478-6

POWER SUPPLYThe power-supply circuit (Fig. 7) doesnot use the traditional choke transformerto save money and lessen the totalweight of the amplifier. Experimental re-sults showed better ripple rejection usinga power MOSFET ripple filter than a con-ventional 10H/200mA choke—the resultwas 9.4mV RMS ripple voltage riding onthe 450V DC supply (Fig. 8).

I also installed the final tube-protec-tion circuit, which consists of a 30-sec-ond timer relay and a 10kΩ/20W seriesresistor in the 340V AC line. After turningthe amplifier on, the plate voltage idles atapproximately 270V DC, holding the300B tubes in a cutoff state with approxi-mately −100V grid bias for 30 seconds.Then the relay eliminates this resistor,raising the plate voltage to the nominalvalue of 450V DC. Also, a 0.3A fuse in theplate supply line ensures additional pro-tection of the 300Bs.

Negative grid bias was generated witha voltage tripler on the 40V AC tap of thepower transformer, plus a 2SC4233 NPNtransistor controlled by an IC voltage reg-ulator, the TL783C by Texas Instruments(TL783C Datasheet. Texas Instruments,SVLS036C—Sept. 1981. Revised April1997). The IC regulator needs only a ref-erence resistor of 82Ω and an extra resis-tor of 6.8kΩ to achieve the desired volt-age, and give a low ripple voltage of only0.3mV RMS. The output voltage of −105Vgoes to a series-connected 68V zenerdiode and 5.6kΩ resistor, so that about 5to 6mA flows through this resistor.

Two potentiometers, connected in par-allel to this bleeder, adjust the grid bias tothe final tubes independently. This struc-ture prevents excessive idle current whenadjusting bias, and it allows use of asmall-size potentiometer (since the cur-rent flow in the potentiometer is only acouple of mA). A 20kΩ pot brings −68 to −105V to the grid of each 300B tube foradjusting the bias level.

I mounted the IC regulator and NPNtransistor onto separate heatsinks forcooling. DC switching regulators, of12V/0.8A and 5V/2A, respectively, powerthe heaters of all the tubes. The powertransformer that I used provides only asingle filament winding of 6.3V/6.8A,which is not adequate for the 300B am-plifier. At the design stage, only thistransformer was available. Plitron has re-cently introduced a new design oftoroidal power transformer for 300Bamps, 6900-X0-00 (Fig. 9). I strongly rec-ommend that anyone who wishes tocopy this amplifier should try to use thisnew model, which provides a couple of

5V/2A windings and6.3V/3A for heaters, acouple of 325V/0.25Afor the plate supply,and even 100V/0.1Afor the grid bias sup-ply. The heater of thedriver circuit is runfrom 12.6V DC biasedabove ground to +150VDC, to lessen the volt-age difference fromcathode to heater.

PARTSFirst of all, I chose theSvetlana SV300B, sincea number of audio magazines evaluatedthis tube and found it to be as good as theWE300B, so the performance of the tubeseems to be superior to its name. Also thistube is much less expensive than theWE300B. Furthermore, Svetlana has a newpackage design for their SV300B that Iprefer, with individual data sheets for eachtube in a pair, as well as a safe and attrac-tive package for the matched pair.

I obtained Plitron transformers via theauthorized dealership, Tec-Sol Inc. inHamamatsu, Japan, who started to carryPlitron in 1999. These transformers weremanufactured in Canada, and were origi-nally designed by van der Veen in Hol-land (Table 6).

The power transformer (#754709) pro-vides 340V-0.7A AC at 280W. The outputtransformer (PAT-3025-SE) features amaximum output power of 13W, definedat the 50% current level over the core-saturation current of 204mA. It implies

12 GLASS AUDIO 6/00

FIGURE 7: Circuit diagram.

FIGURE 9: Plitron 6900-X0-00 trans-former pinout diagram.

FIGURE 8: Ripple filter I-V characteristic.

G-1478-7

G-1478-8

G-1478-9

14 GLASS AUDIO 6/00

that this transformer can handle enoughpower. Thus, the transformer is quiteheavy—5.4kg.

Both transformers need only threeround mounting holes in the chassis,compared to conventional E-I-coredtransformers requiring a large rectangu-lar hole. The package uses a 2mm thickblack polyamide plastic shell. Epoxyplastic encapsulates the bottom. Thisstructure gives a rigid and solid appear-ance as well.

Mounting requires only a ⁵⁄₁₆″ , 3¾″length bolt and nut through a hole in thecenter of the power transformer, whilethe output transformer needs only a 1¾″length, ⁵⁄₁₆″ bolt through a hole in thebottom center of the transformer. Theoutput transformer is encapsulated in ablack spun aluminum can, whose sur-face is black-coated to resemble the tra-ditional Japanese silk fabric for Kimonos,called “Chiri-men.” This gives the trans-former a gorgeous appearance.

The first-stage driver uses one12AX7WA and one 6189W by Philips ECGof the US, which I purchased at a vacu-um-tube shop in Akihabara, Tokyo. Oth-ers come from the parts dealers in Aki-habara, the well-known electronics cen-ter where hundreds of shops are gath-ered on Central Street—like Fifth Avenuein New York City—within a half-mile radius.

ASSEMBLYFirst of all, using my Macintosh G3/266MHz and Claris Draw software, I de-fined the proper chassis size and the partslocations on the chassis (Fig. 10a andPhoto 2). In this project, Plitron trans-formers did not allow a wide variety of lay-out plans because of its big round shape.So I lined up the transformers in a rowalong the long axis of the chassis. Conse-quently, I needed a big chassis: 480 × 240 ×65mm. Then, I chose San-Ei Musen in Ak-ihabara to construct the chassis, whichconsists of a 1.6mm-thick polished stain-less top plate and 1mm-thick steel bottombox, painted with a matching bottom lid,thus giving a good appearance.

I assembled the ripple filter with itsMOSFET on a 10 × 7.5cm PCB, andmounted the other switching power sup-plies on 10 × 15cm and 10 × 11cm PCBswith Teflon-insulated pin electrodes foreasy internal wiring and maintenance. Ialso mounted a couple of potentiome-ters on a PCB to adjust the grid bias.

Each 4-pin socket for an SV300B is as-sembled with a metal plate adapter (Fig.10b), custom-made by San-Ei Musen.This adapter sets the level of the socket

to 30mm beneath the chassis, to makethe height of the tubes and other compo-nents over the chassis more even.

The internal wiring uses a breadboardwith turret terminals, the so-called PTP-board (Fig. 11a) from International AudioGroup in Tex. I custom-ordered a boardto my specs, with terminals placed as Iwished, via e-mail, which was extremelyconvenient. I received my custom boardin less than ten working days. Also, thetoroidal transformers give some extraroom, so this PTP board and the powersupplies (Figs. 11b and 11c) neatly fit intothis custom-made chassis beneath thetransformers.

TABLE 6PLITRON TRANSFORMER CATALOG

PAT-3050-SE PAT-3035-SE PAT-3025-SEPrimary impedance 5060Ω 3490Ω 2490ΩSecondary impedance 4 + 8Ω 4 + 8Ω 4 + 8ΩTurns ratio Np/Ns (4Ω secondary) 35.55 29.52 34.96−0.1dB frequency range 12Hz − 20kHz 16Hz − 21kHz 23Hz − 22kHz−1dB frequency range 5Hz − 45kHz 7Hz − 48kHz 10Hz − 49kHz−3dB frequency range 3Hz − 84kHz 3Hz − 90kHz 5Hz − 91kHzNominal power, RMS 13W 13W 13WFull power bandwidth starting at 20Hz 20Hz 20HzTotal primary inductance 40H 28H 18HPrimary leakage inductance 10mH 7mH 5.5mHEffective primary capacitance 1.2nF 1.1nF 1nFSaturation primary DC current 143.43mA 172.72mA 204.32mATotal primary resistance 80Ω 50Ω 40ΩTotal secondary resistance 0.1Ω 0.1Ω 0.1ΩTube plate resistance (300B) 0.7kΩ 0.7kΩ 0.7kΩInsertion loss 0.17dB 0.17dB 0.18dBQ-factor second-order HF rolloff 0.49 0.49 0.49HF rolloff specific frequency 134.08kHz 142.54kHz 147.21kHzQuality factor (Lp/Lsp) 4100 4100 3273Quality decade factor 3.6 3.6 3.52Tuning factor 7.79 6.38 5.49Tuning decade factor 0.89 0.81 0.74Frequency decade factor (note 4) 4.5 4.41 4.26

FIGURE 10A: Chassis layout.

FIGURE 10B: 300B socket adapter.

G-1478-10a

G-1478-10b

Four 9-pin sockets and all compo-nents for the driver stage are mountedover this board, allowing me to wirethem easily and simply. The parts overthe board are symmetrically placedagainst the centerline of the chassis, to

match the wiring topology elsewhere inthe circuit (Photos 3 and 4).

ADJUSTMENTAfter assembly, I double-checked the internal wiring. After correcting any

errors, I turned the power switch on (before inserting the tubes) and moni-tored the value of grid bias. The gridvoltage should be set to −105V or so, by adjusting the potentiometer controlson the PC board. You can perform theadjustment process with any digitalmultimeter.

Then, I turned the switch off, pluggedthe tubes into their sockets, and turnedthe switch on again. I checked the heaterline voltage, which should be 5V ±0.2V DCand 12.6V ±0.4V DC, respectively. Afterseveral minutes warm-up time, I checkedthe idle current of the output tubes bymeasuring the voltage drop across a series1Ω resistor placed between the plate sup-ply and the output transformer. The valuemust be around 70mV or so. That com-pletes the adjustment.

FINE-TUNING AND MEASUREMENTThe key area of fine-tuning was in thepower supply (Fig. 12). No further fine-tuning was required on the amplifierpart, since the design was verified by ex-periment and simulation, optimizedwith the SV300B and Plitron outputtransformer.

After completing the assembly, whileworking on the aforementioned adjust-ment process, I turned the amplifier onwith a speaker system attached. A loudhum came out of the speaker. Originally Idesigned a semiconductor DC regulatorfor the filament/heater supply of the300B and its driver tubes by using a sin-gle 6.3V AC winding. I was confident

(to page 20)

16 GLASS AUDIO 6/00

PHOTO 2: Overhead view inside the chassis.

FIGURE 11A: PTP terminal board layout.

FIGURE 11B: 450V regulated power supply. FIGURE 11C: 12.6V DC, grid bias regulated power supply.

G-1478-11a

G-1478-11b G-1478-11c

(from page 16)about making a low-ripple DC supplywith various IC regulators, thanks to mysemiconductor design background.

However, using only one 6.3V ACsource did not allow for a low-ripple DC-regulated supply for the SV300B fila-ments. I was able to confirm a 5V DClevel monitored by a multimeter, whichis an adequate level to run SV300Bs, yetthe ripple value was about 60mV RMS—even though I added more than27,000µF in electrolytic capacitors. Itwas difficult for me to eliminate the rip-ple voltage. I have checked it out usingan oscilloscope and found the ripplevalue was more than 150mV peak topeak.

Thus, I experimented by running theSV300B tubes with my home-brew regu-lated power supply (variable from 4 to18V, 10A maximum, 3mV ripple voltage).The experiment showed that residual AChum voltage at the secondary of the out-put transformer dropped to 0.2mV. Thisresult changed my mind about using aswitching power supply. It was tempting,since these supplies are available in Aki-habara at very low cost.

After this improvement, the residualnoise voltage at the speaker terminals de-

20 GLASS AUDIO 6/00

FIGURE 12: Modified power supply circuit for upgrade.

PHOTO 3: Inside view front to rear. PHOTO 4: Rear of the unit.

G-1478-12

FIGURE 13: Input-output characteristic.G-1478-13 FIGURE 14: Distortion. G-1478-14

creased to 0.5mV in one channel, 0.6mVin the other. Obviously I prefer to use thisripple-less switching power supply, dri-ving 300Bs as well as other tubes.

After this tuning, I measured the fol-lowing characteristics:

• Input versus output and distortion(Figs. 13 and 14)The result shows 10W output with3.5% distortion at an input level of 0.9VRMS. This result corresponds with thesimulation results. The sound is verygood for a non-NFB amplifier.

• Frequency response (Fig. 15)The results showed a good powerbandwidth of 70kHz at 1W and 50kHzat 10W. The overall gain characteristicis extremely flat over the full frequencyrange.

• Damping factor (Fig. 16)I used the on-off method at the load-ing of 8Ω, and applied a 1V RMS out-put level. The calculated result shows adamping factor of 6, a good value for anon-NFB amplifier.

• Waveforms (Fig. 17)The waveforms show a good powerbandwidth, with a better square-waveresponse than that of many conven-tional amplifiers. The waveform at100Hz shows less distortion than thatof E-I-cored conventional transform-ers. I observed no overshoot at thetransition edges at 1kHz and 10kHz,since the toroidal transformers arewell-tuned and well-damped by theirinherent design.

LISTENING IMPRESSIONI brought this amplifier to my old friendwho owns JBL S3100 speaker systems, tolisten to music and get his impression aswell as my own. First of all, I was im-pressed with the clarity of the soundcoming from the speaker. The sound ofmusical strings, such as cello, violin, gui-

tar and so on, impressed me so muchthat I can draw a picture in my mind ofthe artists playing their instruments infront of me. The vocal recordings, includ-ing Joan Baez, Ella Fitzgerald, and NatalieCole, sounded light and pure. I could heareven their breathing during each song,thanks to accurate middle-range tone.

I checked the low-frequency responsewith a CD by the Oscar Peterson Trio, WeGet Requests, recorded in 1964. I chose“You Look Good To Me” for testing. At the

beginning, the sound of a triangle wasincredibly clear, and subsequent basslines by Ray Brown came out with great

22 GLASS AUDIO 6/00

MEASUREMENT EQUIPMENTHP 334A audio analyzerKenwood AG-204D audio generatorTrio RA-920 attenuator8Ω 50W 2 ch. homemade dummy loadHP 1746A oscilloscopeFluke 8020A digital multimeter

FIGURE 16: Damping factor. G-1478-16

REFERENCES“300B Power Amplifier Kessaku-sen (Design Pool of300B power amplifier),” Seibun-do Shinkosha (MJAudio Technology Magazine).WE300 Technical Note, Western Electric/Westrex Co.,Atlanta, Ga.Svetlana SV300B Datasheet, Svetlana Electron De-vices Inc., Huntsville, Ala.

SOURCESTec-Sol Inc.Hamamatsu-shi, Wada-cho 514Shizuoka 435-0016 Japan+81-53-468-1201FAX +81-53-468-1202http://www.tec-sol.com/http://www.plitron.com/e-mail: sales@tec-sol.comSvetlana vacuum tubes and Plitron toroidal transformersSan-Ei Musen (This business closed as of August 2000.)Chiyoda-ku, Soto-Kanda 1-15-16Tokyo 101 Japan+81-3-3251-7985 FAX +81-3-3251-2343Custom made chassisInternational Audio Group Inc.PO Box 10096Killeen, TX 76547-8702Phone/FAX (254) 699-8702e-mail: hiag@n-link.comPTP boardAngela Instruments10830 Guilford Rd., Suite 309Annapolis Junction, MD 20701(301) 725-0451FAX (301) 725-8823http://www.angela.com/Angela capacitors

FIGURE 17: Output waveforms(100Hz, 1kHz, 10kHz).

G-1478-17

FIGURE 15: Frequency response.G-1478-15

power and control. Also, I could hear Raywhispering as he was swinging.

I was really impressed by the soundfrom this amplifier, my first exposure tothe SV300B and to Plitron toroidal trans-formers. The sound brought me a morerealistic presence than I had heard be-fore, with a wider and deeper atmos-phere than conventional amplifiers Ihave built in the past.

I have compared the sound ofWE300Bs installed in this amp—though Icould not find or hear any better im-provement on what we got with theSV300Bs.

24 GLASS AUDIO 6/00

TABLE 7PARTS LIST

AMPLIFIERVacuum tube SV300B (Svetlana) 2Vacuum tube 12AX7 WA (Philips—NOS) 2Vacuum tube 6189W (Philips—NOS) 2Power transformer Plitron 754709 or 6900-X0-00 1Output transformer Plitron PAT-3025-SE or PAT-3050-SE 2Resistor 1Ω 1W 2Resistor 100Ω ½W 2Resistor 1kΩ ½W 2Resistor 10kΩ ½W 10Resistor 82kΩ 1W 2Resistor 100kΩ ½W 4Resistor 1MΩ ½W 4Zener diode 47V, 3W Toshiba, 3Z47 2Capacitor 0.1µF 630V Angela (Solen Equivalent) 4Capacitor 1µF 630V Solen 2Electrolytic capacitor 47µF 16V OS capacitor by Sanyo 4Electrolytic capacitor 47µF 160V 2Electrolytic capacitor 47µF 450V tubular 2

POWER SUPPLYSwitching regulator 12V—0.8A, 5V—0.7A, 100V AC input 1Switching regulator 5V—2A, 100V AC input 2Potentiometer 20kΩ (B) Bourns #3296 2Power MOSFET 2SK719 (NEC) or IRFPC40 (VGS = 600V, 1

Id = 6.8A, Pd = 150W, RDS = 1.2Ω)Transistor 2SC4233 (VCBO = 1200V, VCEO = 800V, Ic = 3A, Pc = 60W) 1IC regulator TL783C (Texas Instruments) 1Zener diode Z6150, 150V 2W, Ishizuka Denshi 4Zener diode Z6120, 120V 2W, Ishizuka Denshi 1Zener diode Z668, 68V 2W, Ishizuka Denshi 1Zener diode 6V 1W, NEC RD-6F 2Diode 1000V 1A RG4C Shindengen 4Diode 600V 1A, RH1S Shindengen 3Resistor 82Ω ½W 1Resistor 1kΩ ½W 3Resistor 5.1kΩ 2W 1Resistor 5.6kΩ 2W 1Resistor 6.8kΩ ½W 1Resistor 10kΩ ½W 1Resistor 10kΩ 20W w/bracket 1Resistor 56kΩ 1W 1Resistor 75kΩ 5W 1Resistor 100kΩ ½W 5Capacitor 0.1µF 600V, ceramic 8Electrolytic capacitor 47µF 160V 2Electrolytic capacitor 47µF 200V 2Electrolytic capacitor 220µF 350V, Nippon Chemicon 2Electrolytic capacitor 100µF + 100µF 500V, Elna Cerafine 1ZNR 14K431U Matsushita, purchased at Akizuki Denshi 1Timer relay OMRON MY-2V 30 sec. w/socket 1Heatsink Yoshikawa Kinzoku 17P23 (for 2SC4233) 1Heatsink Mizutani 20P25-25 (for TL783C) 1Heatsink Mizutani (for MOSFET) 1Fuse holder w/0.3A fuse 1Pin terminal Teflon insulatedPCB 1 ea. 100 × 150mm,

2 ea. 100 × 75mm, Copper clad epoxy glass t = 1.6mm 3

OTHERSChassis 480 × 240 × 65mm stainless-steel top custom made by San-Ei Musen 1Socket Svetlana SK-4A 2Socket adapter custom made by San-Ei Musen 2Socket 9 pin Chuoh-Musen 4RCA socket San-Ei Musen 2Power switch 125V 10A 1AC inlet w/fuse 1Speaker terminal San-Ei Musen 1Pilot lamp 12V 150mA 1Spacer 10mm long, 3mm dia, brass base, nickel plated 12Spacer 15mm long, 3mm dia, brass base, nickel plated 8Spacer 20mm long, 3mm dia, brass base, nickel plated 4Spacer 30mm long, 3mm dia, brass base, nickel plated 8⁵⁄₁₆″ bolt & nut 90mm long 1⁵⁄₁₆″ bolt 32mm long 2

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