Document:

<PAGE>   1
                                                                 Exhibit 10.1(b)

     SECOND AMENDMENT, dated as of December 14, 1999 (this "Second Amendment"),
to the CREDIT AGREEMENT, dated as of March 18, 1999, as amended by the First
Amendment dated as of June 28, 1999 (the "Credit Agreement"), among CHARTER
COMMUNICATIONS OPERATING, LLC (the "Borrower"), CHARTER COMMUNICATIONS HOLDINGS
LLC, the Lenders parties to the Credit Agreement, the Documentation Agents and
Syndication Agents named therein and THE CHASE MANHATTAN BANK and BANK OF
AMERICA, N.A., as Administrative Agents (in such capacity, the "Administrative
Agents"). Terms defined in the Credit Agreement shall be used in this Second
Amendment with their defined meanings unless otherwise defined herein.

                             W I T N E S S E T H :

     WHEREAS, the Borrower wishes to amend the Credit Agreement in the manner
set forth herein; and

     WHEREAS, each of the parties hereto is willing to enter into this Second
Amendment on the terms and subject to the conditions set forth herein;

     NOW, THEREFORE, in consideration of the premises and for other good and
valuable consideration the receipt and sufficiency of which is hereby
acknowledged, the parties hereto hereby agree as follows:

  SECTION 1. AMENDMENTS TO CREDIT AGREEMENT.

     1. Section 1.1 -- Amended Definition. The defined term "Specified Senior
Notes" is amended and restated in its entirety as follows:

     "Specified Senior Notes": the collective reference to (a) any Senior Notes
having a maturity approximately eight years after the Stage One Closing Date
and (b) any other Indebtedness incurred pursuant to Section 7.2(j) having a
maturity on or prior to March 17, 2009, provided, that no more than
$900,000,000 of Specified Senior Notes shall be outstanding.

     2. Section 7.2(i). Clause (ii) of Section 7.2(j) of the Credit Agreement
is hereby amended and restated in its entirety as follows:

  "(ii) except in the case of the Senior Notes, 100% of any Net Cash Proceeds
  thereof (other than any such Net Cash Proceeds that are applied to refinance
  other Indebtedness of Holdings to the extent permitted by Section 7.8) shall
  be used by Charter Holdings (or shall have been used) to make Investments in
  one more of Charter Holdings' Affiliates primarily involved (either directly
  or through Subsidiaries) in businesses of the type described in Section
  7.14(a)"

  SECTION II. MISCELLANEOUS.

     1. No Change. Except as expressly provided herein, no term or provision of
the Credit Agreement shall be amended, modified or supplemented, and each term
and provision of the Credit Agreement shall remain in full force and effect.

     2. Effectiveness. This Second Amendment shall become effective as of the
date hereof upon receipt by the Administrative Agents of (a) counterparts
hereof duly executed by Holdings and the Borrower and (b) consent letters
authorizing the Administrative Agents to enter into this Second Amendment from
the Required Lenders.

<PAGE>   2

Borrower and (b) consent letters authorizing the Administrative Agents to enter
into this Second Amendment from the Required Lenders.

     3.   Counterparts.  This Second Amendment may be executed by the parties
hereto in any number of separate counterparts, and all of said counterparts
taken together shall be deemed to constitute one and the same instrument.

     4.   Governing Law.  THIS SECOND AMENDMENT AND THE RIGHTS AND OBLIGATIONS
OF THE PARTIES UNDER THIS SECOND AMENDMENT SHALL BE GOVERNED BY, AND CONSTRUED
AND INTERPRETED IN ACCORDANCE WITH, THE LAW OF THE STATE OF NEW YORK.

     IN WITNESS WHEREOF, the parties hereto have caused this Second Amendment
to be duly executed and delivered as of the day and year first above written.

                         CHARTER COMMUNICATIONS HOLDINGS LLC

                         By: /s/ Eloise A. Engman
                             ------------------------------------
                             Name:   Eloise A. Engman
                             Title:  Vice President

                         CHARTER COMMUNICATIONS OPERATING, LLC

                         By: /s/ Eloise A. Engman
                             ------------------------------------
                             Name:   Eloise A. Engman
                             Title:  Vice President

                         THE CHASE MANHATTAN BANK,
                           as an Administrative Agent

                         By: /s/ Edmund DeForest
                             ------------------------------------
                             Name:   Edmund DeForest
                             Title:  Vice President

                         BANK OF AMERICA, N.A., as an Administrative
                           Agent

                         By: /s/ Jennifer Zydnay
                             ------------------------------------
                             Name:   Jennifer Zydnay
                             Title:  Managing Director<PAGE>   1
                                                                Exhibit 10.26(a)

                                                                       EXECUTION

                       FIRST AMENDMENT TO CREDIT AGREEMENT

                                December 21, 1999

           Reference is made to that certain Credit Agreement dated as of
November 15, 1999 (as heretofore amended, supplemented or otherwise modified,
the "Credit Agreement"), by and among CC Michigan, LLC and CC New England, LLC,
as borrowers ("Borrowers"), CC V Holdings, LLC (formerly known as Avalon Cable
LLC) ("Holdings"), as a guarantor, the financial institutions listed on the
signature pages thereof, Bank of Montreal, Chicago Branch, as Administrative
Agent, and the Co-Arrangers, Syndication Agents, and Co-Documentation Agents
named therein. Capitalized terms used herein without definition herein shall
have the meanings assigned to such terms in the Credit Agreement.

           Borrowers, Holdings and the undersigned Lenders hereby agree as
follows:

           (1) Section 1.1 of the Credit Agreement is hereby amended by deleting
the definition of "Qualified Indebtedness" contained therein in its entirety
therefrom and substituting therefor the following:

           "Qualified Indebtedness": (a) with respect to a Qualified Parent
       Company, any Indebtedness (i) which is issued in a Rule 144A private
       placement or registered public offering, (ii) which is not held by any
       Affiliate of a Borrower and (iii) as to which 100% of the Net Cash
       Proceeds thereof are used by such Qualified Parent Company to make
       Investments in one or more of its Subsidiaries engaged substantially in
       businesses of the type described in Section 6.14(a) and/or to refinance
       other Qualified Indebtedness or Indebtedness of a Borrower and (b) with
       respect to an Affiliate of any of the Borrowers, any Indebtedness as to
       which 100% of the Net Cash Proceeds thereof were contributed to any of
       the Borrowers.

           (2) Schedule 6.2(d) to the Credit Agreement is hereby amended by
adding at the end thereof "The Existing Senior Subordinated Debt."

           On and after the First Amendment Effective Date (as defined below),
each reference in the Credit Agreement to "this Agreement", "hereunder",
"hereof", "herein" or words of like import referring to the Credit Agreement,
and each reference in the other Loan Documents to the "Credit Agreement",
"thereunder", "thereof" or words of like import referring to the Credit
Agreement shall mean and be a reference to the Credit Agreement as amended by
this Amendment (the Credit Agreement, as so amended, being the "Amended
Agreement"). Except as specifically amended by this Amendment, the Credit
Agreement and such other Loan Documents shall remain in full force and effect
and are hereby ratified and confirmed. The execution, delivery and performance
of this Amendment shall not, except as expressly provided herein, constitute a
waiver of any provision of, or operate as a waiver of any right, power or remedy
of any Agent or any Lender under, the Credit Agreement or any of such other Loan
Documents.

           This Amendment may be executed in any number of counterparts and by
different parties hereto in separate counterparts, each of which when so
executed and delivered shall be deemed an original, but all such counterparts
together shall constitute but one and the same instrument. This Amendment shall
become effective (the date of such effectiveness being the "First Amendment
Effective Date") with respect to the Credit Agreement upon the execution of a
counterpart hereof by the Borrowers, Holdings and Required Lenders and receipt
by the Borrowers and Administrative Agent of written or telephonic notification
of such execution and authorization of delivery thereof.

           THIS AMENDMENT AND THE RIGHTS AND OBLIGATIONS OF THE PARTIES
HEREUNDER SHALL BE GOVERNED BY, AND SHALL BE CONSTRUED AND ENFORCED IN
ACCORDANCE WITH, THE INTERNAL LAWS OF THE STATE OF NEW YORK (INCLUDING

                                       1

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WITHOUT LIMITATION SECTION 5-1401 OF THE GENERAL OBLIGATIONS LAW OF THE STATE OF
NEW YORK), WITHOUT REGARD TO CONFLICTS OF LAWS PRINCIPLES.

AGENTS AND LENDERS:                     BANK OF MONTREAL, CHICAGO BRANCH,
                                        individually and as Administrative Agent

                                        By: /s/ Michael Silverman
                                           -------------------------------------
                                            Name:  Michael Silverman
                                            Title: Managing Director

                                        FIRST UNION NATIONAL BANK, individually
                                        and as a Syndication Agent

                                        By: /s/ Chris Kalmbach
                                           -------------------------------------
                                            Name:  Chris Kalmbach
                                            Title: Vice President

                                        PNC BANK, NATIONAL ASSOCIATION,
                                        individually and as a Syndication Agent

                                        By: /s/ Jeffrey E. Hauser
                                           -------------------------------------
                                            Name:  Jeffrey E. Hauser
                                            Title: Vice President

                                        MERCANTILE BANK NATIONAL ASSOCIATION,
                                        individually and as a Co-Documentation
                                        Agent

                                        By: /s/ Jennifer L. Schaefer
                                           -------------------------------------
                                            Name:  Jennifer L. Schaefer
                                            Title: Banking Officer

                                        THE CHASE MANHATTAN BANK

                                        By: /s/ Edmund DeForest
                                           -------------------------------------
                                            Name:  Edmund DeForest
                                            Title: Vice President

                                        BANK ONE, N.A.

                                        By:  /s/ Ronna Bury Prince
                                           -------------------------------------
                                            Name:  Ronna Bury Prince
                                            Title: First Vice President

                                       2
<PAGE>   3
                                        PARIBAS CAPITAL FUNDING LLC

                                        By:
                                           -------------------------------------
                                            Name:
                                            Title:

                                        TORONTO DOMINION (TEXAS), INC.

                                        By:  /s/ Lynn Chasin
                                           -------------------------------------
                                            Name:  Lynn Chasin
                                            Title: Vice President

                                        FLOATING RATE PORTFOLIO

                                        By: INVESCO Senior Secured Management,
                                            Inc., as attorney in fact

                                        By:  /s/ Gregory Stoeckle
                                           -------------------------------------
                                            Name:  Gregory Stoeckle
                                            Title: Authorized Signatory

                                        FREMONT INVESTMENT & LOAN

                                        By:
                                           -------------------------------------
                                            Name:
                                            Title:

                                        THE BANK OF NEW YORK

                                        By:  /s/ Debra M. Ritchie
                                           -------------------------------------
                                            Name:  Debra M. Ritchie
                                            Title: Assistant Vice President

                                        U.S. BANK NATIONAL ASSOCIATION

                                        By:  /s/ Thomas G. Gunder
                                           -------------------------------------
                                            Name:  Thomas G. Gunder
                                            Title: Vice President

                                      3
<PAGE>   4
                                        COOPERATIVE CENTRALE RAIFFEISEN-
                                        BOERENLEENBANK B.A., "RABOBANK
                                        INTERNATIONAL", NEW YORK BRANCH

                                        By: /s/ Alan E. McLintock
                                           -------------------------------------
                                            Name:  Alan E. McLintock
                                            Title: Vice President

                                        By: /s/ Nancy J. O'Conner
                                            ------------------------------------
                                             Name:  Nancy J. O'Conner
                                             Title: Vice President

                                        ABN AMRO BANK N.V.

                                        By: /s/ Thomas M. Toerpe
                                           -------------------------------------
                                            Name:  Thomas M. Toerpe
                                            Title: Vice President

                                        By: /s/ Roxana Sopola
                                           -------------------------------------
                                            Name:  Roxana Sopola
                                            Title: Vice President

                                        CITIBANK, N.A.

                                        By: /s/ Maureen Marona
                                           -------------------------------------
                                            Name:  Maureen Marona
                                            Title: Vice President

                                        THE BANK OF NOVA SCOTIA

                                        By: /s/ Ian A. Hodgort
                                           -------------------------------------
                                            Name:  Ian A. Hodgort
                                            Title: Authorized Signatory

                                        FRANKLIN FLOATING RATE TRUST

                                        By: /s/ Chauncey Lufkin
                                           -------------------------------------
                                            Name:  Chauncey Lufkin
                                            Title: Vice President

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<PAGE>   5
                                      EATON VANCE INSTITUTIONAL SENIOR LOAN FUND

                                        By:   Eaton Vance Management, as
                                              Investment Advisor

                                        By: /s/ Barbara Campbell
                                            ________________________________
                                              Name:   Barbara Campbell
                                              Title:  Vice President

                                        EATON VANCE SENIOR INCOME TRUST

                                        By:   Eaton Vance Management, as
                                              Investment Advisor

                                        By: /s/ Barbara Campbell
                                            ________________________________
                                              Name:   Barbara Campbell
                                              Title:  Vice President

                                        DELANO COMPANY

                                        By:  PACIFIC INVESTMENT MANAGEMENT
                                             COMPANY, as its Investment Advisor

                                        By:
                                            ________________________________
                                             Name:
                                             Title:

                                        CAPTIVA III FINANCE LTD, as advised by
                                        PACIFIC INVESTMENT MANAGEMENT COMPANY

                                        By:
                                            _________________________________
                                             Name:
                                             Title:

                                        CATALINA CDO LTD.

                                        By: PACIFIC INVESTMENT
                                             MANAGEMENT COMPANY, as its
                                             Investment Advisor

                                        By:
                                            ________________________________
                                             Name:
                                             Title:

                                       5
<PAGE>   6
                                        AERIES FINANCE-II LTD.

                                        By: INVESCO Senior Secured Management,
                                            Inc., as Sub-Managing Agent

                                        By: /s/ Gregory Stoeckle
                                            ________________________________
                                             Name: Gregory Stoeckle
                                             Title: Authorized Signatory

                                        AVALON CAPITAL LTD.

                                        By: INVESCO Senior Secured Management,
                                            Inc., as Portfolio Advisor

                                        By: /s/ Gregory Stoeckle
                                            _______________________________
                                             Name: Gregory Stoeckle
                                             Title: Authorized Signatory

                                        SENIOR DEBT PORTFOLIO

                                        By: Boston Management and Research, as
                                            Investment Advisor

                                        By: /s/ Barbara Campbell
                                            _______________________________
                                             Name: Barbara Campbell
                                             Title: Vice President

                                        OXFORD STRATEGIC INCOME FUND

                                        By: Eaton Vance Management, as
                                            Investment Advisor

                                        By: /s/ Barbara Campbell
                                            _______________________________
                                             Name: Barbara Campbell
                                             Title: Vice President

                                        ARES III CLO LTD.

                                        By:    ARES CLO Management LLC

                                        By: /s/ J. M. Moore
                                            _______________________________
                                             Name: J. M. Moore
                                             Title: Principal

                                       6
<PAGE>   7
BORROWERS:                             CC MICHIGAN, LLC

                                       By: /s/ Eloise A. Engman
                                          -------------------------------------
                                          Name:   Eloise A. Engman
                                          Title:  Vice President

                                       CC NEW ENGLAND, LLC

                                       By: /s/ Eloise A. Engman
                                          -------------------------------------
                                           Name:   Eloise A. Engman
                                           Title:  Vice President

HOLDINGS:                              CC V HOLDINGS, LLC (formerly known as
                                       Avalon Cable LLC)

                                       By: /s/ Eloise A. Engman
                                          -------------------------------------
                                           Name:   Eloise A. Engman
                                           Title:  Vice President

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