-----BEGIN PRIVACY-ENHANCED MESSAGE-----
Proc-Type: 2001,MIC-CLEAR
Originator-Name: webmaster@www.sec.gov
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MIC-Info: RSA-MD5,RSA,
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 YSxKPG/hiXlisObMsPJfhQ==

<SEC-DOCUMENT>0001215302-07-000001.txt : 20071205
<SEC-HEADER>0001215302-07-000001.hdr.sgml : 20071205
<ACCEPTANCE-DATETIME>20071205203644
ACCESSION NUMBER:		0001215302-07-000001
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20071203
FILED AS OF DATE:		20071205
DATE AS OF CHANGE:		20071205

ISSUER:		

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			SUPERTEX INC
		CENTRAL INDEX KEY:			0000730000
		STANDARD INDUSTRIAL CLASSIFICATION:	SEMICONDUCTORS & RELATED DEVICES [3674]
		IRS NUMBER:				942328535
		STATE OF INCORPORATION:			CA
		FISCAL YEAR END:			0331

	BUSINESS ADDRESS:	
		STREET 1:		1235 BORDEAUX DR
		CITY:			SUNNYVALE
		STATE:			CA
		ZIP:			94089
		BUSINESS PHONE:		4087440100

	MAIL ADDRESS:	
		STREET 1:		1235 BORDEAUX DR
		CITY:			SUNNYVALE
		STATE:			CA
		ZIP:			94089

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			LEE MICHAEL JAMES
		CENTRAL INDEX KEY:			0001215302

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	000-12718
		FILM NUMBER:		071287995

	BUSINESS ADDRESS:	
		STREET 1:		1235 BORDEAUX DR
		CITY:			SUNNYVALE
		STATE:			CA
		ZIP:			94089
		BUSINESS PHONE:		4082224887

	MAIL ADDRESS:	
		STREET 1:		1235 BORDEAUX DRIVE
		CITY:			SUNNYVALE
		STATE:			CA
		ZIP:			94089
</SEC-HEADER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>primary_doc.xml
<DESCRIPTION>PRIMARY DOCUMENT
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0202</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2007-12-03</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000730000</issuerCik>
        <issuerName>SUPERTEX INC</issuerName>
        <issuerTradingSymbol>supx</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001215302</rptOwnerCik>
            <rptOwnerName>LEE MICHAEL JAMES</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>1235 BORDEAUX DRIVE</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>SUNNYVALE</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>94089</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>VP Intergrated IC Design</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>NQ stock option (to purchase common stock</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>33.95</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2007-12-03</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>8000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>33.95</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <value>2008-12-03</value>
            </exerciseDate>
            <expirationDate>
                <value>2017-12-03</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>common stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>8000</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>26000</value>
                    <footnoteId id="F1"/>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">ISO Option (to exercise common stock).  Exercise Price: $33.6700.  Shares Granted: 9,820.  Shares Exercisable: 1,940. 1,940 shares exercisable on 3/1/2008. 1,940 shares exercisable on 3/1/2009.  2,000 shares exercisable on 3/1/2010.  2,000 shares exercisable on 3/1/2011.  Expiration date 3/1/2013.  Held direct.</footnote>
        <footnote id="F2">NQ Option (to exercise common stock).  Exercise Price: $33.6700.  Shares Granted: 180.  Shares Exercisable: 60. 60 shares exercisable on 3/1/2008. 60 shares exercisable on 3/1/2009.  Expiration date 3/1/2013.  Held direct.</footnote>
        <footnote id="F3">ISO Option (to exercise common stock).  Exercise Price: $17.3400.  Shares Granted:  10,000.  Shares Exercised: 2,000. Shares Exercisable: 4,000. 2,000 shares exercisable on 12/1/08.  2,000 shares exercisable on 12/1/09.  Expiration Date: 12/1/2010.   Held Direct.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>Michael L. Lee</signatureName>
        <signatureDate>2007-12-05</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
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