-----BEGIN PRIVACY-ENHANCED MESSAGE-----
Proc-Type: 2001,MIC-CLEAR
Originator-Name: webmaster@www.sec.gov
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 TWSM7vrzLADbmYQaionwg5sDW3P6oaM5D3tdezXMm7z1T+B+twIDAQAB
MIC-Info: RSA-MD5,RSA,
 OxV8ZH3l1vCpKQ+UhsfmmfwNzdlanW/nISmgDQEiM3p3zrltrTsLGTIuVRUKw3TV
 kC/LUHXhAepN0iwyEsyGGg==

<SEC-DOCUMENT>0001195897-08-000003.txt : 20081211
<SEC-HEADER>0001195897-08-000003.hdr.sgml : 20081211
<ACCEPTANCE-DATETIME>20081211181535
ACCESSION NUMBER:		0001195897-08-000003
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20080902
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:			LOVELESS W MARK
		CENTRAL INDEX KEY:			0001195897

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	000-12718
		FILM NUMBER:		081244476

	BUSINESS ADDRESS:	
		STREET 1:		1235 BORDEAUX DRIVE
		CITY:			SUNNYVALE
		STATE:			CA
		ZIP:			94089

	MAIL ADDRESS:	
		STREET 1:		1235 BORDEAUX DR.
		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>2008-09-02</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001195897</rptOwnerCik>
            <rptOwnerName>LOVELESS W MARK</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>1235 BORDEAUX DR.</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>SUNNYVALE</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>94089</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>NQ stock option (to exercise common stock)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>29.7</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2008-09-02</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>5000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>29.7</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <value>2008-09-02</value>
            </exerciseDate>
            <expirationDate>
                <value>2018-09-02</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>common stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>5000</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>20000</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: $17.06.  Shares granted: 10,000.  Shares exercisable: 8,000.  2,000 shares exercisable 9/1/2009.  Expiration date: 9/1/2011.  Held Direct.</footnote>
        <footnote id="F2">NQ Option (to exercise common stock).  Exercise Price: $35.86.  Shares granted:  5,000.  Shares Exercisable: 2,000. 1,000 shares exercisable on 9/4/2009.  1,000 shares exercisable on 9/4/2010.  1,000 shares exercisable on 9/4/2011.  Expiration Date:  9/4/2017.   Held Direct.</footnote>
        <footnote id="F3">NQ Option (to exercise common stock).  Exercise Price: $29.70.  Shares granted:  5,000.  Shares Exercisable: 1,000. 1,000 shares exercisable on 9/2/2009.  1,000 shares exercisable on 9/2/2010.  1,000 shares exercisable on 9/2/2011.  1,000 shares exercisable on 9/2/2012. Expiration Date:  9/2/2018.   Held Direct.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>W. Mark Loveless</signatureName>
        <signatureDate>2008-12-11</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
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