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                                                                    EXHIBIT 10.4




                            As of September 28, 1999

Advanta Revolving Home
Equity Loan Trust 1999-B
c/o Bankers Trust Company of California, N.A.
Three Park Plaza, 16th Floor
Irvine, California  92714

Ambac Assurance Corporation
One State Street Plaza
New York, New York 10004

         Re:      Advanta Revolving Home Equity Loan Trust 1999-B
                  Sale and Servicing Agreement

Ladies and Gentlemen:

                  Pursuant to the Sale and Servicing Agreement dated as of
September 1, 1999 (the "Agreement") among Advanta Conduit Receivables, Inc. as
Sponsor, Advanta Mortgage Corp. USA, as Master Servicer ("AMCUSA"), Advanta
Revolving Home Equity Loan Trust 1999-B (the "Trust"), Advanta Holding Trust
1999-B ("Holding") and Bankers Trust Company of California, N.A. as Indenture
Trustee (the "Indenture Trustee"), AMCUSA in its capacity as Master Servicer,
has undertaken certain financial obligations with respect to its servicing of
the Mortgage Loans, including, but not limited to, the making of Servicing
Advances. In addition, the Sponsor has, in the Agreement undertaken certain
financial obligations, including, but not limited to, the payment of the Loan
Reacquisition Price relating to the repurchase of non-qualifying Mortgage Loans,
the payment of Substitution Amounts in connection with the substitution of
Qualified Replacement Mortgage Loans and the payment of certain expenses of the
Trust. Any financial obligations of AMCUSA or the Sponsor under the Agreement,
whether or not specifically enumerated in this paragraph, are hereinafter
referred to as the "Joint and Several Obligations"; provided, however, that
"Joint and Several Obligations" shall mean only the financial obligations of
AMCUSA and the Sponsor under the Agreement (including the payment of money
damages for a breach of any of AMCUSA's or the Sponsor's obligations under the
Agreement, whether financial or otherwise) but shall not include any obligations
not relating to the payment of money (e.g., the obligation to service the
Mortgage Loans).

                  The Insurer has required the undersigned, Advanta Mortgage
Holding Company ("AMHC"), the parent corporation of AMCUSA and the indirect
corporate parent of the Sponsor, to acknowledge its joint-and-several liability
with AMCUSA and the Sponsor for the payment of the Joint and Several Obligations
under the Agreement.



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                  Now, therefore, the Trust, the Insurer and AMHC do hereby
agree that:

                  (i)      AMHC hereby agrees to be absolutely and
                           unconditionally jointly and severally liable with
                           AMCUSA and the Sponsor to the Trust and the Insurer
                           for the payment of the Joint and Several Obligations
                           under the Agreement.

                  (ii)     AMHC may honor its obligations hereunder either by
                           direct payment of any Joint and Several Obligations
                           or by causing any Joint and Several Obligations to be
                           paid to the Trust and the Insurer by AMCUSA, the
                           Sponsor, or another affiliate of AMHC.

                  This letter and the respective obligations and rights
hereunder and thereunder shall not be delegated or assigned by you without the
prior written consent of the Insurer. This letter may not be amended or
otherwise modified except pursuant to a writing signed by each of the parties
hereto. This letter may be executed by the signatories hereto in several
counterparts, each of which shall be deemed to be an original and all of which
shall constitute one and the same letter.

                  THIS LETTER SHALL BE GOVERNED BY, AND CONSTRUED IN ACCORDANCE
WITH, THE LAWS OF THE STATE OF NEW YORK. EACH OF THE UNDERSIGNED PARTIES HEREBY
KNOWINGLY, VOLUNTARILY AND INTENTIONALLY WAIVES ANY RIGHTS IT MAY HAVE TO A
TRIAL BY JURY IN RESPECT OF ANY LITIGATION BASED HEREON, OR ARISING OUT OF OR IN
CONNECTION WITH, THIS LETTER, AND ANY OTHER COURSE OF CONDUCT, COURSE OF
DEALING, STATEMENTS (WHETHER ORAL OR WRITTEN) OR ACTIONS OF ANY OF THE
UNDERSIGNED PARTIES IN CONNECTION HEREWITH OR THEREWITH.




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                  Capitalized terms used herein and not defined herein shall
have their respective meanings as set forth in the Agreement.

                              Very truly yours,

                              ADVANTA MORTGAGE HOLDING COMPANY

                              By:_______________________________________

                              Authorized Signatory

ACKNOWLEDGED AND AGREED:
ADVANTA REVOLVING HOME EQUITY
LOAN TRUST 1999-B
By:  WILMINGTON TRUST COMPANY
        as Owner Trustee


By:_______________________________________
      Authorized Signatory

ACKNOWLEDGED:
AMBAC ASSURANCE CORPORATION





By:_______________________________________
      Authorized Signatory




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