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Proc-Type: 2001,MIC-CLEAR
Originator-Name: webmaster@www.sec.gov
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MIC-Info: RSA-MD5,RSA,
 Nx5arV+yWbCwqwtQNiCSi3Zx3JwIMpN8PphqQHkKvxbPw+88EaKRAjjNeHQlRIjA
 8YmGv9l/JZNFUoAUAMF0Cg==

<SEC-DOCUMENT>0001379961-08-000003.txt : 20081211
<SEC-HEADER>0001379961-08-000003.hdr.sgml : 20081211
<ACCEPTANCE-DATETIME>20081211203115
ACCESSION NUMBER:		0001379961-08-000003
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20081201
FILED AS OF DATE:		20081211
DATE AS OF CHANGE:		20081211

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:			1221

	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:			Kagel Phil
		CENTRAL INDEX KEY:			0001379961

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	000-12718
		FILM NUMBER:		081244679

	BUSINESS ADDRESS:	
		BUSINESS PHONE:		408-222-4827

	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>X0303</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2008-12-01</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001379961</rptOwnerCik>
            <rptOwnerName>Kagel Phil</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, Finance and CFO</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>NQ stock option (to exercise common stock)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>20.85</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2008-12-01</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>35000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>20.85</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <value>2009-12-01</value>
            </exerciseDate>
            <expirationDate>
                <value>2018-12-01</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>common stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>35000</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>65000</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">NQ Option (to exercise common stock).  Exercise Price: $40.5600.  Shares Granted: 23,000.    Shares Exercisable: 9,200.  4,600 shares exercisable on 6/1/2009.  4,600 shares exercisable on 6/1/2010. 4,600 shares exercisable on 6/1/2011. Expiration Date: 6/1/2013.  Held Direct.</footnote>
        <footnote id="F2">NQ Option (to exercise common stock).  Exercise Price: $46.9200.  Shares Granted:  7,000.  Shares Exercisable: 2,800.  1,400 shares exercisable on 12/1/2009.  1,400 shares exercisable on 12/1/2010. 1,400 shares exercisable on 12/1/2011.  Expiration Date: 12/1/2013.   Held Direct.</footnote>
        <footnote id="F3">NQ Option (to exercise common stock).  Exercise Price: $20.8500.  Shares Granted:  35,000.  Shares Exercisable: 0. 7,000 shares exercisable on 12/1/2009.  7,000 shares exercisable on 12/1/2010.  7,000 shares exercisable on 12/1/2011.  7,000 shares exercisable on 12/1/2012. 7,000 shares exercisable on 12/1/2013.  Expiration Date: 12/1/2018.   Held Direct.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>Phillip A. Kagel</signatureName>
        <signatureDate>2008-12-11</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
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