[Tex/LaTex] Number/Letter Theorem

Question

Is there any way from the code below that a number letter/number theorem can be generated? I am trying to achieve a theorem labeled 1.1A.

\documentclass{book}
\usepackage{amsthm}

 \newtheorem{theo}{Theorem}[chapter]
 \newcounter{tmp}

 \begin{document}

 \chapter{Test Chapter}
 \begin{theo}
 test
 \end{theo}

 \begingroup
 \setcounter{tmp}{\value{theo}}% store current value of theorem counter
 \setcounter{theo}{0} %assign desired value to theorem counter
  \renewcommand\thetheo{\Alph{theo}}% locally redefine the representation of    the theorem counter
 \begin{theo}
 test
 \end{theo}
  \endgroup


 \end{document}

Answer 1

In my opinion, a 2nd theorem environment which 'depends' on the upper level environment theo is a cleaner solution than to use \begingroup...\endgroup etc. and storing the counter, changing the output etc.

\documentclass{book}
\usepackage{amsthm}

\newtheorem{theo}{Theorem}[chapter]

\newtheorem{subtheo}{Subheorem}[theo] % Use theo as driver counter
\renewcommand{\thesubtheo}{\thetheo\Alph{subtheo}} % Change subtheo counter for alpha output


\begin{document}

\chapter{Test Chapter}
\begin{theo}
test
\end{theo}

\begin{subtheo}
test
\end{subtheo}


\end{document}